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\begin{document}
\title{USA TSTST 2021 Solutions}
\subtitle{United States of America --- TST Selection Test}
\author{Andrew Gu and Evan Chen}
\date{63\ts{rd} IMO 2022 Norway and 11\ts{th} EGMO 2022 Hungary}
\maketitle
\tableofcontents
\newpage
\addtocounter{section}{-1}
\section{Problems}
\begin{enumerate}[\bfseries 1.]
\item %% Problem 1
Let $ABCD$ be a quadrilateral inscribed in a circle with center $O$.
Points $X$ and $Y$ lie on sides $AB$ and $CD$, respectively.
Suppose the circumcircles of $ADX$ and $BCY$
meet line $XY$ again at $P$ and $Q$, respectively.
Show that $OP=OQ$.
\item %% Problem 2
Let $a_1 < a_2 < a_3 < a_4 < \cdots$ be an
infinite sequence of real numbers in the interval $(0,1)$.
Show that there exists a number that occurs
exactly once in the sequence
\[ \frac{a_1}{1}, \; \frac{a_2}{2}, \;
\frac{a_3}{3}, \; \frac{a_4}{4}, \; \ldots. \]
\item %% Problem 3
Find all positive integers $k > 1$ for which there exists a positive integer
$n$ such that $\binom{n}{k}$ is divisible by $n$, and $\binom{n}{m}$ is not
divisible by $n$ for $2\leq m < k$.
\item %% Problem 4
Let $a$ and $b$ be positive integers.
Suppose that there are infinitely many pairs of positive integers $(m, n)$
for which $m^2+an+b$ and $n^2+am+b$ are both perfect squares.
Prove that $a$ divides $2b$.
\item %% Problem 5
Let $T$ be a tree on $n$ vertices with exactly $k$ leaves.
Suppose that there exists a subset of
at least $\frac{n+k-1}{2}$ vertices of $T$,
no two of which are adjacent.
Show that the longest path in $T$ contains
an even number of edges.
\item %% Problem 6
Triangles $ABC$ and $DEF$ share circumcircle $\Omega$ and incircle $\omega$
so that points $A$, $F$, $B$, $D$, $C$, and $E$ occur in this order along $\Omega$.
Let $\Delta_A$ be the triangle formed by lines $AB$, $AC$, and $EF$,
and define triangles $\Delta_B$, $\Delta_C, \dotsc, \Delta_F$ similarly.
Furthermore, let $\Omega_A$ and $\omega_A$ be the circumcircle and incircle
of triangle $\Delta_A$, respectively, and define circles
$\Omega_B$, $\omega_B, \dotsc, \Omega_F$, $\omega_F$ similarly.
\begin{enumerate}[(a)]
\item Prove that the two common external tangents to circles $\Omega_A$ and $\Omega_D$
and the two common external tangents to circles $\omega_A$ and $\omega_D$
are either concurrent or pairwise parallel.
\item Suppose that these four lines meet at point $T_A$,
and define points $T_B$ and $T_C$ similarly.
Prove that points $T_A$, $T_B$, and $T_C$ are collinear.
\end{enumerate}
\item %% Problem 7
Let $M$ be a finite set of lattice points
and $n$ be a positive integer.
A \emph{mine-avoiding path} is a path of lattice points with length $n$,
beginning at $(0,0)$ and ending at a point on the line $x+y=n$,
that does not contain any point in $M$.
Prove that if there exists a mine-avoiding path,
then there exist at least $2^{n-|M|}$ mine-avoiding paths.
\item %% Problem 8
Let $ABC$ be a scalene triangle.
Points $A_1$, $B_1$ and $C_1$ are chosen
on segments $BC$, $CA$, and $AB$, respectively,
such that $\triangle A_1B_1C_1$ and $\triangle ABC$
are similar.
Let $A_2$ be the unique point on line $B_1C_1$
such that $AA_2 = A_1A_2$.
Points $B_2$ and $C_2$ are defined similarly.
Prove that $\triangle A_2B_2C_2$ and $\triangle ABC$ are similar.
\item %% Problem 9
Let $q=p^r$ for a prime number $p$ and positive integer $r$.
Let $\zeta = e^{\frac{2\pi i}{q}}$.
Find the least positive integer $n$ such that
\[
\sum_{\substack{1 \le k \le q \\ \gcd(k,p) = 1}}
\frac{1}{(1 - \zeta^k)^n}
\]
is not an integer.
(The sum is over all $1\leq k\leq q$ with $p$ not dividing $k$.)
\end{enumerate}
\pagebreak
\section{Solutions to Day 1}
\subsection{TSTST 2021/1, proposed by Holden Mui}
\textsl{Available online at \url{https://aops.com/community/p23586650}.}
\begin{mdframed}[style=mdpurplebox,frametitle={Problem statement}]
Let $ABCD$ be a quadrilateral inscribed in a circle with center $O$.
Points $X$ and $Y$ lie on sides $AB$ and $CD$, respectively.
Suppose the circumcircles of $ADX$ and $BCY$
meet line $XY$ again at $P$ and $Q$, respectively.
Show that $OP=OQ$.
\end{mdframed}
We present many solutions.
\paragraph{First solution, angle chasing only (Ankit Bisain)}
Let lines $BQ$ and $DP$ meet $(ABCD)$ again at $D'$ and $B'$, respectively.
\begin{center}
\begin{asy}[width = 0.5\textwidth]
path omega = circle((0, 0), 1);
pair A = dir(70);
pair B = dir(180);
pair C = dir(200);
pair D = dir(340);
pair O = (0, 0);
real x = 0.2;
real y = 0.3;
pair X = (1-x) * A + x * B;
pair Y = (1-y) * C + y * D;
pair P = 2 * foot(circumcenter(A, D, X), X, Y) - X;
pair Q = 2 * foot(circumcenter(B, C, Y), X, Y) - Y;
dot("$A$", A, A);
dot("$B$", B, B);
dot("$C$", C, C);
dot("$D$", D, D);
dot("$O$", O, -dir(A+B+C+D));
dot("$X$", X, dir(A+B));
dot("$Y$", Y, dir(C+D));
dot("$P$", P, dir(30));
dot("$Q$", Q, dir(-60));
draw(omega);
draw(A -- B -- C -- D -- cycle);
draw(circumcircle(A, D, X));
draw(circumcircle(B, C, Y));
draw(X -- Y);
draw(P -- O -- Q, dotted);
pair Bp = 2 * foot(B, O, foot(O, X, Y)) - B;
dot("$B'$", Bp, dir(Bp));
pair Dp = 2 * foot(D, O, foot(O, X, Y)) - D;
dot("$D'$", Dp, dir(300));
draw(B -- Dp -- D -- Bp -- cycle, dashed);
\end{asy}
\end{center}
Then $BB' \parallel PX$ and $DD' \parallel QY$ by Reim's theorem. Segments
$BB', DD'$, and $PQ$ share a perpendicular bisector which passes through $O$, so
$OP=OQ$.
\paragraph{Second solution via isosceles triangles (from contestants)}
Let $T = \ol{BQ} \cap \ol{DP}$.
\begin{center}
\begin{asy}
size(200);
path omega = circle((0, 0), 1);
pair A = dir(70);
pair B = dir(180);
pair C = dir(200);
pair D = dir(340);
pair O = (0, 0);
real x = 0.2;
real y = 0.3;
pair X = (1-x) * A + x * B;
pair Y = (1-y) * C + y * D;
pair P = 2 * foot(circumcenter(A, D, X), X, Y) - X;
pair Q = 2 * foot(circumcenter(B, C, Y), X, Y) - Y;
pair T = extension(B, Q, D, P);
dot("$A$", A, A);
dot("$B$", B, dir(135));
dot("$C$", C, C);
dot("$D$", D, 2*dir(5));
dot("$O$", O, dir(60));
dot("$X$", X, dir(A+B));
dot("$Y$", Y, dir(C+D));
dot("$P$", P, dir(30));
dot("$Q$", Q, dir(-60));
dot("$T$", T, dir(240));
draw(omega);
draw(A -- B -- C -- D -- cycle);
draw(circumcircle(A, D, X));
draw(circumcircle(B, C, Y));
draw(X -- Y);
draw(P -- O -- Q, dotted);
draw(B -- T -- P, dashed);
pair O1 = circumcenter(B, O, D);
real r = abs(O1 - O);
draw(arc(O1, r, 45, 120), dashed);
\end{asy}
\end{center}
Note that $PQT$ is isosceles because
\[ \dang PQT = \dang YQB = \dang BCD = \dang BAD = \dang XPD = \dang TPQ. \]
Then $(BODT)$ is cyclic because
\[\dang BOD = 2 \dang BCD = \dang PQT + \dang TPQ = \dang BTD.\]
Since $BO=OD$, $\ol{TO}$ is an angle bisector of $\dang BTD$. Since $\triangle PQT$ is isosceles, $\ol{TO} \perp \ol{PQ}$, so $OP = OQ$.
\paragraph{Third solution using a parallelogram (from contestants)}
Let $(BCY)$ meet $\ol{AB}$ again at $W$ and let $(ADX)$ meet $\ol{CD}$ again at $Z$. Additionally, let $O_1$ be the center of $(ADX)$ and $O_2$ be the center of $(BCY)$.
\begin{center}
\begin{asy}
size(200);
path omega = circle((0, 0), 1);
pair A = dir(70);
pair B = dir(180);
pair C = dir(200);
pair D = dir(340);
pair O = (0, 0);
real x = 0.2;
real y = 0.3;
pair X = (1-x) * A + x * B;
pair Y = (1-y) * C + y * D;
pair P = 2 * foot(circumcenter(A, D, X), X, Y) - X;
pair Q = 2 * foot(circumcenter(B, C, Y), X, Y) - Y;
pair T = extension(B, Q, D, P);
draw(omega);
draw(A -- B ^^ C -- D);
draw(circumcircle(A, D, X));
draw(circumcircle(B, C, Y));
draw(X -- Y);
pair W = 2 * foot(circumcenter(B, C, Y), A, B) - B;
pair Z = 2 * foot(circumcenter(A, D, X), C, D) - D;
pair O1 = circumcenter(A, D, X);
pair O2 = circumcenter(B, C, Y);
pair Op = circumcenter(X, Y, Z);
draw(W -- Z);
draw(circumcircle(X, Y, Z), dashed);
dot("$A$", A, A);
dot("$B$", B, B);
dot("$C$", C, C);
dot("$D$", D, D);
dot("$X$", X, dir(A+B));
dot("$Y$", Y, dir(C+D));
dot("$P$", P, dir(-45));
dot("$Q$", Q, dir(0));
dot("$W$", W, dir(A+B));
dot("$Z$", Z, dir(C+D));
dot("$O$", O, dir(300));
dot("$O_1$", O1, dir(O1));
dot("$O_2$", O2, dir(O2));
dot("$O'$", Op, dir(300));
draw(O -- O1 -- Op -- O2 -- cycle, dotted);
\end{asy}
\end{center}
Note that $(WXYZ)$ is cyclic since
\[\dang XWY + \dang YZX = \dang YWB + \dang XZD = \dang YCB + \dang XAD = 0^\circ,\]
so let $O'$ be the center of $(WXYZ)$. Since $\ol{AD} \parallel \ol{WY}$ and $\ol{BC} \parallel \ol{XZ}$ by Reim's theorem, $OO_1O'O_2$ is a parallelogram.
To finish the problem, note that projecting $O_1$, $O_2$, and $O'$ onto
$\ol{XY}$ gives the midpoints of $\ol{PX}$, $\ol{QY}$, and
$\ol{XY}$. Since $OO_1O'O_2$ is a parallelogram, projecting $O$ onto
$\ol{XY}$ must give the midpoint of $\ol{PQ}$, so $OP=OQ$.
\paragraph{Fourth solution using congruent circles (from contestants)}
Let the angle bisector of $\dang BOD$ meet $\ol{XY}$ at $K$.
\begin{center}
\begin{asy}
size(200);
path omega = circle((0, 0), 1);
pair A = dir(70);
pair B = dir(180);
pair C = dir(200);
pair D = dir(340);
pair O = (0, 0);
real x = 0.2;
real y = 0.3;
pair X = (1-x) * A + x * B;
pair Y = (1-y) * C + y * D;
pair P = 2 * foot(circumcenter(A, D, X), X, Y) - X;
pair Q = 2 * foot(circumcenter(B, C, Y), X, Y) - Y;
pair T = extension(B, Q, D, P);
dot("$A$", A, A);
dot("$B$", B, B);
dot("$C$", C, C);
dot("$D$", D, D);
dot("$O$", O, 2*dir(15));
dot("$X$", X, 1/3*dir(A+B));
dot("$Y$", Y, dir(C+D));
dot("$P$", P, dir(0));
dot("$Q$", Q, dir(-30));
pair K = extension(O, foot(O, B, D), X, Y);
dot("$K$", K, 2*dir(K));
draw(X -- K -- O);
draw(B -- O -- D);
draw(omega);
draw(A -- B -- C -- D -- cycle);
draw(circumcircle(A, D, X));
draw(circumcircle(B, C, Y));
draw(X -- Y);
draw(circumcircle(O, P, D), dotted);
draw(circumcircle(O, Q, B), dotted);
\end{asy}
\end{center}
Then $(BQOK)$ is cyclic because $\dang KOD = \dang BAD = \dang KPD$, and $(DOPK)$ is cyclic similarly.
By symmetry over $KO$, these circles have the same radius $r$, so
\[ OP = 2r \sin \angle OKP = 2r \sin \angle OKQ = OQ \]
by the Law of Sines.
\paragraph{Fifth solution by ratio calculation (from contestants)}
Let $\ol{XY}$ meet $(ABCD)$ at $X'$ and $Y'$.
\begin{center}
\begin{asy}
size(200);
path omega = circle((0, 0), 1);
pair A = dir(70);
pair B = dir(180);
pair C = dir(200);
pair D = dir(340);
pair O = (0, 0);
real x = 0.2;
real y = 0.3;
pair X = (1-x) * A + x * B;
pair Y = (1-y) * C + y * D;
pair P = 2 * foot(circumcenter(A, D, X), X, Y) - X;
pair Q = 2 * foot(circumcenter(B, C, Y), X, Y) - Y;
pair T = extension(B, Q, D, P);
pair Xp = intersectionpoints(circumcircle(A,B,C), Y + (Y-X)*10 -- X + (X-Y)*10)[0];
pair Yp = intersectionpoints(circumcircle(A,B,C), Y + (Y-X)*10 -- X + (X-Y)*10)[1];
fill(D -- P -- Xp -- cycle, lightblue+opacity(0.5));
fill(B -- Yp -- D -- cycle, lightblue+opacity(0.5));
// fill(B -- D -- X -- cycle, lightgreen+opacity(0.5));
//fill(B -- Xp -- D -- cycle, lightgreen+opacity(0.5));
dot("$A$", A, A);
dot("$B$", B, B);
dot("$C$", C, C);
dot("$D$", D, D);
dot("$O$", O, -dir(A+B+C+D));
dot("$X$", X, dir(A+B));
dot("$Y$", Y, dir(-45));
dot("$P$", P, dir(0));
dot("$Q$", Q, dir(-30));
draw(omega);
draw(A -- B -- C -- D -- cycle);
draw(circumcircle(A, D, X));
draw(circumcircle(B, C, Y));
draw(Xp -- Yp);
dot("$X'$", Xp, dir(Xp));
dot("$Y'$", Yp, dir(Yp));
\end{asy}
\end{center}
Since $\dang Y'BD = \dang PX'D$ and $\dang BY'D = \dang BAD = \dang X'PD$,
\[ \triangle BY'D \sim \triangle XP'D \implies PX' = BY' \cdot \frac{DX'}{BD}.\]
Similarly,
\[ \triangle BX'D \sim \triangle BQY' \implies QY' = DX' \cdot \frac{BY'}{BD}.\]
Thus $PX' = QY'$, which gives $OP=OQ$.
\paragraph{Sixth solution using radical axis (from author)}
Without loss of generality, assume $\ol{AD} \nparallel \ol{BC}$, as this case holds by continuity. Let $(BCY)$ meet $\ol{AB}$ again at $W$, let $(ADX)$ meet $\ol{CD}$ again at $Z$, and let $\ol{WZ}$ meet $(ADX)$ and $(BCY)$ again at $R$ and $S$.
\begin{center}
\begin{asy}
size(200);
path omega = circle((0, 0), 1);
pair A = dir(70);
pair B = dir(180);
pair C = dir(200);
pair D = dir(340);
pair O = (0, 0);
real x = 0.2;
real y = 0.3;
pair X = (1-x) * A + x * B;
pair Y = (1-y) * C + y * D;
pair P = 2 * foot(circumcenter(A, D, X), X, Y) - X;
pair Q = 2 * foot(circumcenter(B, C, Y), X, Y) - Y;
pair T = extension(B, Q, D, P);
draw(omega);
draw(A -- B ^^ C -- D);
draw(circumcircle(A, D, X));
draw(circumcircle(B, C, Y));
draw(X -- Y);
pair W = 2 * foot(circumcenter(B, C, Y), A, B) - B;
pair Z = 2 * foot(circumcenter(A, D, X), C, D) - D;
pair R = 2 * foot(circumcenter(A, D, X), W, Z) - Z;
pair S = 2 * foot(circumcenter(B, C, Y), W, Z) - W;
draw(W -- Z);
draw(circumcircle(P, Q, R), dashed);
draw(circumcircle(X, Y, Z), dashed);
dot("$A$", A, A);
dot("$B$", B, B);
dot("$C$", C, C);
dot("$D$", D, D);
dot("$X$", X, dir(A+B));
dot("$Y$", Y, dir(C+D));
dot("$P$", P, dir(-45));
dot("$Q$", Q, dir(0));
dot("$W$", W, dir(A+B));
dot("$Z$", Z, dir(C+D));
dot("$R$", R, dir(90));
dot("$S$", S, dir(210));
\end{asy}
\end{center}
Note that $(WXYZ)$ is cyclic since
\[\dang XWY + \dang YZX = \dang YWB + \dang XZD = \dang YCB + \dang XAD = 0^\circ\]
and $(PQRS)$ is cyclic since
\[\dang PQS = \dang YQS = \dang YWS = \dang PXZ = \dang PRZ = \dang SRP.\]
Additionally, $\ol{AD} \parallel \ol{PR}$ since
\[\dang DAX + \dang AXP + \dang XPR = \dang YWX + \dang WXY + \dang XYW = 0^\circ,\]
and $\ol{BC} \parallel \ol{SQ}$ similarly.
Lastly, $(ABCD)$ and $(PQRS)$ are concentric; if not, using the radical axis theorem twice shows that their radical axis must be parallel to both $\ol{AD}$ and $\ol{BC}$, contradiction.
\paragraph{Seventh solution using Cayley-Bacharach (author)}
Define points $W, Z, R, S$ as in the previous solution.
\begin{center}
\begin{asy}[width = 0.5\textwidth]
path omega = circle((0, 0), 1);
pair A = dir(70);
pair B = dir(180);
pair C = dir(200);
pair D = dir(340);
pair O = (0, 0);
real x = 0.2;
real y = 0.3;
pair X = (1-x) * A + x * B;
pair Y = (1-y) * C + y * D;
pair P = 2 * foot(circumcenter(A, D, X), X, Y) - X;
pair Q = 2 * foot(circumcenter(B, C, Y), X, Y) - Y;
draw(omega, green);
draw(A -- B ^^ C -- D, red);
draw(circumcircle(A, D, X), blue);
draw(circumcircle(B, C, Y), blue);
draw(X -- Y, green);
pair W = 2 * foot(circumcenter(B, C, Y), A, B) - B;
pair Z = 2 * foot(circumcenter(A, D, X), C, D) - D;
pair R = 2 * foot(circumcenter(A, D, X), W, Z) - Z;
pair S = 2 * foot(circumcenter(B, C, Y), W, Z) - W;
draw(W -- Z, green);
draw(circumcircle(P, Q, R), red+dashed);
dot("$A$", A, A);
dot("$B$", B, B);
dot("$C$", C, C);
dot("$D$", D, D);
dot("$X$", X, dir(A+B));
dot("$Y$", Y, dir(C+D));
dot("$P$", P, dir(-45));
dot("$Q$", Q, dir(0));
dot("$W$", W, dir(A+B));
dot("$Z$", Z, dir(C+D));
dot("$R$", R, dir(90));
dot("$S$", S, dir(210));
\end{asy}
\end{center}
The quartics $(ADXZ) \cup (BCWY)$ and $\ol{XY} \cup \ol{WZ} \cup (ABCD)$ meet at the 16 points
\[A, B, C, D, W, X, Y, Z, P, Q, R, S, I, I, J, J,\]
where $I$ and $J$ are the circular points at infinity. Since $\ol{AB} \cup \ol{CD} \cup (PQR)$ contains the 13 points
\[A,B,C,D,P,Q,R,W,X,Y,Z,I,J,\]
it must contain $S$, $I$, and $J$ as well, by quartic Cayley-Bacharach.
Thus, $(PQRS)$ is cyclic and intersects $(ABCD)$ at $I$, $I$, $J$, and $J$, implying that the two circles are concentric, as desired.
\begin{remark*}
[Author comments]
Holden says he came up with this problem via the Cayley-Bacharach solution,
by trying to get two quartics to intersect.
\end{remark*}
\pagebreak
\subsection{TSTST 2021/2, proposed by Merlijn Staps}
\textsl{Available online at \url{https://aops.com/community/p23586635}.}
\begin{mdframed}[style=mdpurplebox,frametitle={Problem statement}]
Let $a_1 < a_2 < a_3 < a_4 < \cdots$ be an
infinite sequence of real numbers in the interval $(0,1)$.
Show that there exists a number that occurs
exactly once in the sequence
\[ \frac{a_1}{1}, \; \frac{a_2}{2}, \;
\frac{a_3}{3}, \; \frac{a_4}{4}, \; \ldots. \]
\end{mdframed}
We present three solutions.
\paragraph{Solution 1 (Merlijn Staps)}
We argue by contradiction, so suppose that for each $\lambda$ for which the set
$S_\lambda = \{k : a_k/k = \lambda\}$ is non-empty, it contains at least two
elements. Note that $S_\lambda$ is always a finite set because $a_k =
k\lambda$ implies $k < 1/\lambda$.
Write $m_\lambda$ and $M_\lambda$ for the smallest and largest element of
$S_\lambda$, respectively, and define $T_\lambda = \{m_\lambda,
m_\lambda+1,\dotsc,M_\lambda\}$ as the smallest set of consecutive positive
integers that contains $S_\lambda$. Then all $T_\lambda$ are sets of at least
two consecutive positive integers, and moreover the $T_\lambda$ cover
$\NN$. Additionally, each positive integer is covered finitely many times
because there are only finitely many possible values of $m_{\lambda}$ smaller
than any fixed integer.
Recall that if three intervals have a point in common then one
of them is contained in the union of the other two. Thus, if any positive
integer is covered more than twice by the sets $T_{\lambda}$, we may throw out
one set while maintaining the property that the $T_{\lambda}$
cover $\NN$. By using the fact that each positive integer is covered
finitely many times, we can apply this process so that each positive integer is
eventually covered at most twice.
%By repeatedly throwing away some of the $T_\lambda$ we may assume
%that each positive integer is contained in at most two sets $T_\lambda$, while
%still maintaining the property that the $T_\lambda$ cover $\NN$; this
%follows from the fact that if three intervals have a point in common then one
%of them is contained in the union of the other two.
Let $\Lambda$ denote the set
of the $\lambda$-values for which $T_\lambda$ remains in our collection of sets;
then $\bigcup_{\lambda \in \Lambda} T_\lambda = \NN$ and each positive
integer is contained in at most two sets $T_\lambda$.
We now obtain
\[
\sum_{\lambda \in \Lambda} \sum_{k \in T_\lambda} (a_{k+1}-a_k) \le 2 \sum_{k \ge 1} (a_{k+1} - a_k) \le 2.
\]
On the other hand, because $a_{m_\lambda} = \lambda m_\lambda$ and $a_{M_\lambda} = \lambda M_\lambda$, we have
\begin{align*}
2\sum_{k \in T_\lambda} (a_{k+1} - a_k) &\ge2 \sum_{m_\lambda \le k < M_\lambda}
(a_{k+1} - a_k) = 2(a_{M_\lambda}-a_{m_\lambda}) = 2(M_\lambda-m_\lambda)\lambda
\\
& = 2(M_\lambda - m_\lambda) \cdot
\frac{a_{m_\lambda}}{m_\lambda} \ge
(M_\lambda - m_\lambda + 1) \cdot
\frac{a_1}{m_\lambda} \ge a_1 \cdot
\sum_{k \in T_\lambda} \frac1k.
\end{align*}
Combining this with our first estimate, and using the fact that the $T_\lambda$ cover $\NN$, we obtain
\[
4 \ge 2 \sum_{\lambda \in \Lambda} \sum_{k \in T_\lambda} (a_{k+1}-a_k) \ge a_1 \sum_{\lambda \in \Lambda} \sum_{k \in T_\lambda} \frac1k \ge a_1 \sum_{k \ge 1} \frac1k,
\]
contradicting the fact that the harmonic series diverges.
\paragraph{Solution 2 (Sanjana Das)}
Assume for the sake of contradiction that no number appears exactly once in the
sequence. For every $i < j$ with $a_i/i = a_j/j$, draw an edge
between $i$ and $j$, so every $i$ has an edge (and being connected by an edge is
a transitive property). Call $i$ \emph{good} if it has an edge with some $j > i$.
First, each $i$ has finite degree -- otherwise \[\frac{a_{x_1}}{x_1} = \frac{a_{x_2}}{x_2} = \cdots\] for an infinite increasing sequence of positive integers $x_i$, but then the $a_{x_i}$ are unbounded.
Now we use the following process to build a sequence of indices whose $a_i$ we can lower-bound:
\begin{itemize}
\item Start at $x_1 = 1$, which is good.
\item If we're currently at good index $x_i$, then let $s_i$ be the largest positive integer such that $x_i$ has an edge to $x_i + s_i$. (This exists because the degrees are finite.)
\item Let $t_i$ be the smallest positive integer for which $x_i + s_i + t_i$ is good, and let this be $x_{i + 1}$. This exists because if all numbers $k \leq x \leq 2k$ are bad, they must each connect to some number less than $k$ (if two connect to each other, the smaller one is good), but then two connect to the same number, and therefore to each other -- this is the idea we will use later to bound the $t_i$ as well.
\end{itemize}
Then $x_i = 1 + s_1 + t_1 + \cdots + s_{i - 1} + t_{i - 1}$, and we have
\[a_{x_{i + 1}} > a_{x_i + s_i} = \frac{x_i + s_i}{x_i} a_{x_i} = \frac{1 + (s_1
+ \cdots + s_{i - 1} + s_i) + (t_1 + \cdots + t_{i - 1})}{1 + (s_1 + \cdots +
s_{i - 1}) + (t_1 + \cdots + t_{i - 1})}a_{x_i}.\] This means
\[c_n := \frac{a_{x_n}}{a_1} > \prod_{i = 1}^{n-1} \frac{1 + (s_1 + \cdots +
s_{i-1} + s_{i}) + (t_1 + \cdots + t_{i-1})}{1 + (s_1 + \cdots + s_{i-1}) + (t_1
+ \cdots + t_{i-1})}.\]
\begin{lemma*}
$t_1 + \cdots + t_n \leq s_1 + \cdots + s_n$ for each $n$.
\end{lemma*}
\begin{proof}
Consider $1 \leq i \leq n$. Note that for every $i$, the $t_i - 1$ integers strictly between $x_i + s_i$ and $x_i + s_i + t_i$ are all bad, so each such index $x$ must have an edge to some $y < x$.
First we claim that if $x \in (x_i + s_i, x_i + s_i + t_i)$, then $x$ cannot
have an edge to $x_j$ for any $j \leq i$. This is because $x > x_i + s_i
\geq x_j + s_j$, contradicting the fact that $x_j + s_j$ is the largest
neighbor of $x_j$.
This also means $x$ doesn't have an edge to $x_j + s_j$ for any $j \leq i$, since if it did, it would have an edge to $x_j$.
Second, no two bad values of $x$ can have an edge, since then the smaller
one is good. This also means no two bad $x$ can have an edge to the same $y$.
Then each of the $\sum (t_i - 1)$ values in the intervals $(x_i+s_i,
x_i+s_i+t_i)$ for $1 \leq i \leq n$ must have an edge to an unique $y$ in one
of the intervals $(x_i, x_i + s_i)$ (not necessarily with the same $i$). Therefore
\[\sum (t_i - 1) \leq \sum (s_i - 1)\implies \sum t_i \leq \sum s_i.\qedhere\]
\end{proof}
Now note that if $a > b$, then $\frac{a + x}{b + x} = 1 + \frac{a - b}{b + x}$
is decreasing in $x$. This means
\[c_n > \prod_{i = 1}^{n-1} \frac{1 + 2s_1 + \cdots + 2s_{i-1} + s_{i}}{1 + 2s_1
+ \cdots + 2s_{i-1}} > \prod_{i = 1}^{n-1} \frac{1 + 2s_1 + \cdots +
2s_{i-1} + 2s_{i}}{1 + 2s_1 + \cdots + 2s_{i-1} + s_{i}},\]
By multiplying both products, we have a telescoping product, which results in
\[c_n^2 \geq 1 + 2s_1 + \cdots + 2s_n + 2s_{n + 1}.\]
The right hand side is unbounded since the $s_i$ are positive integers, while
$c_n = a_{x_n}/a_1 < 1/a_1$ is bounded, contradiction.
\paragraph{Solution 3 (Gopal Goel)}
Suppose for sake of contradiction that the problem is false. Call an index $i$ a
\emph{pin} if
\[\frac{a_j}{j} = \frac{a_i}{i} \implies j\ge i.\]
\begin{lemma*}
There exists $k$ such that if we have $\tfrac{a_i}{i}=\tfrac{a_j}{j}$ with
$j > i \ge k$, then $j\le 1.1i$.
\end{lemma*}
\begin{proof}
Note that for any $i$, there are only finitely many $j$ with
$\frac{a_j}{j}=\frac{a_i}{i}$, otherwise $a_j=\frac{ja_i}{i}$ is unbounded.
Thus it suffices to find $k$ for which $j\leq 1.1i$ when $j > i\geq k$.
Suppose no such $k$ exists. Then, take a pair $j_1>i_1$ such that
$\tfrac{a_{j_1}}{j_1} = \tfrac{a_{i_1}}{i_1}$ and $j_1>1.1i_1$, or
$a_{j_1}>1.1a_{i_1}$. Now, since $k=j_1$ can't work, there exists a pair
$j_2>i_2\ge i_1$ such that $\tfrac{a_{j_2}}{j_2} = \tfrac{a_{i_2}}{i_2}$ and
$j_2>1.1i_2$, or $a_{j_2}>1.1a_{i_2}$. Continuing in this fashion, we see that
\[a_{j_\ell}>1.1a_{i_\ell}> 1.1a_{j_{\ell-1}},\]
so we have that $a_{j_\ell}>1.1^\ell a_{i_1}$. Taking $\ell>\log_{1.1}(1/a_1)$ gives the desired contradiction.
\end{proof}
\begin{lemma*}
For $N>k^2$, there are at most $0.8N$ pins in $[\sqrt{N},N)$.
\end{lemma*}
\begin{proof}
By the first lemma, we see that the number of pins in $[\sqrt{N},\tfrac{N}{1.1})$ is at most the number of non-pins in $[\sqrt{N},N)$. Therefore, if the number of pins in $[\sqrt{N},N)$ is $p$, then we have
\[p-N\left(1-\frac{1}{1.1}\right)\le N-p,\]
so $p\le 0.8N$, as desired.
\end{proof}
We say that $i$ is the pin of $j$ if it is the smallest index such that
$\tfrac{a_i}{i}=\tfrac{a_j}{j}$. The pin of $j$ is always a pin.
Given an index $i$, let $f(i)$ denote the largest index less than $i$ that is
not a pin (we leave the function undefined when no such index exists, as we are
only interested in the behavior for large $i$). Then $f$ is weakly increasing
and unbounded by the first lemma. Let $N_0$ be a positive integer such that
$f(\sqrt{N_0}) > k$.
Take any $N>N_0$ such that $N$ is not a pin. Let $b_0=N$, and $b_1$ be the pin
of $b_0$. Recursively define $b_{2i}=f(b_{2i-1})$, and $b_{2i+1}$ to be the pin of $b_{2i}$.
Let $\ell$ be the largest odd index such that $b_\ell\ge\sqrt{N}$. We first show
that $b_\ell\le 100\sqrt{N}$. Since $N > N_0$, we have $b_{\ell+1} > k$. By the
choice of $\ell$ we have $b_{\ell+2}<\sqrt{N}$, so \[b_{\ell+1}<1.1b_{\ell+2}<
1.1\sqrt{N}\] by the first lemma. We see
that all the indices from $b_{\ell+1}+1$ to $b_\ell$ must be pins, so we have
at least $b_\ell-1.1\sqrt{N}$ pins in $[\sqrt{N},b_\ell)$. Combined with the
second lemma, this shows that $b_\ell\le 100\sqrt{N}$.
Now, we have that $a_{b_{2i}}=\tfrac{b_{2i}}{b_{2i+1}}a_{b_{2i+1}}$ and
$a_{b_{2i+1}}> a_{b_{2i+2}}$, so combining gives us
\[\frac{a_{b_0}}{a_{b_\ell}}> \frac{b_0}{b_1}\frac{b_2}{b_3}\cdots\frac{b_{\ell-1}}{b_\ell}.\]
Note that there are at least
\[(b_1-b_2)+(b_3-b_4)+\cdots+(b_{\ell-2}-b_{\ell-1})\]
pins in $[\sqrt{N},N)$, so by the second lemma, that sum is at most $0.8N$. Thus,
\begin{align*}
(b_0-b_1)+(b_2-b_3)+\cdots+(b_{\ell-1}-b_\ell)
&=b_0-[(b_1-b_2)+\dotsb + (b_{\ell-2}-b_{\ell-1})]-b_\ell
\\ &\ge 0.2N-100\sqrt{N}.
\end{align*}
Then
\begin{align*}
\frac{b_0}{b_1}\frac{b_2}{b_3}\cdots
\frac{b_{\ell-1}}{b_\ell} &\ge 1+\frac{b_0-b_1}{b_1}+\cdots+\frac{b_{\ell-1}-b_\ell}{b_\ell} \\
&> 1 +\frac{b_0-b_1}{b_0} + \dotsb +
\frac{b_{\ell-1}-b_{\ell}}{b_0} \\
&\ge 1+\frac{0.2N-100\sqrt{N}}{N},
\end{align*}
which is at least $1.01$ if $N_0$ is large enough. Thus, we see that
\[a_N>1.01 a_{b_{\ell}} \geq 1.01 a_{\lfloor\sqrt{N}\rfloor}\]
if $N>N_0$ is not a pin. Since there are arbitrarily large non-pins, this implies that the sequence $(a_n)$ is unbounded, which is the desired contradiction.
\pagebreak
\subsection{TSTST 2021/3, proposed by Merlijn Staps}
\textsl{Available online at \url{https://aops.com/community/p23586679}.}
\begin{mdframed}[style=mdpurplebox,frametitle={Problem statement}]
Find all positive integers $k > 1$ for which there exists a positive integer
$n$ such that $\binom{n}{k}$ is divisible by $n$, and $\binom{n}{m}$ is not
divisible by $n$ for $2\leq m < k$.
\end{mdframed}
Such an $n$ exists for any $k$.
First, suppose $k$ is prime. We choose $n=(k-1)!$. For
$m t_p$, so $\nu_p(n-i) = \nu_p(i)$;
\item If $p \mid k$ and $i \neq k_p$, then we have $\nu_p(i), \nu_p(n-i) \le t_p$ and $\nu_p(n) \ge t_p$, so again $\nu_p(n-i) = \nu_p(i)$;
\item If $p \mid k$ and $i = k_p$, then we have $\nu_p(n-i) = \nu_p(i) + \nu_p(k)$ by \eqref{eq:cong2}.
\end{itemize}
We conclude that $\nu_p(n-i) = \nu_p(i)$ always holds, except when $i = k_p$, when we have $\nu_p(n-i) = \nu_p(i) + \nu_p(k)$ (this formula holds irrespective of whether $p \mid k$ or $p \nmid k$).
We can now show that $\binom{n}{k}$ is divisible by $n$, which amounts to
showing that $k!$ divides $(n-1)(n-2) \dotsm (n-k+1)$. Indeed, for each prime $p \le k$ we have
\begin{align*}
\nu_p\left( (n-1)(n-2) \ldots (n-k+1) \right) &= \nu_p(n-k_p) + \sum_{i \nu_p(k)$, then it follows that
\begin{align*}
\nu_p\left( (n-1)(n-2) \ldots (n-m+1) \right)
&= \nu_p(k) + \sum_{i=1}^{m-1} \nu_p(i) \\
&< \nu_p(m) + \sum_{i=1}^{m-1}
\nu_p(i) \\
&= \nu_p(m!),
\end{align*}
so $m!$ cannot divide $(n-1)(n-2) \ldots (n-m+1)$. On the other hand, suppose
that $\nu_p(m) \le \nu_p(k)$ for all $p \mid k$, which would mean that $m \mid
k$ and hence $m \le \frac{k}{2}$. Consider a prime $p$ dividing $m$. We have
$k_p \ge \frac{k}{2}$, because otherwise $2k_p$ could have been used instead of
$k_p$. It follows that $m \le \frac{k}{2} \le k_p$. Therefore, we obtain
\begin{align*}
\nu_p\left( (n-1)(n-2) \ldots (n-m+1) \right) &= \sum_{i=1}^{m-1} \nu_p(n-i) \\
&= \sum_{i=1}^{m-1} \nu_p(i) \\
&= \nu_p((m-1)!) < \nu_p(m!),
\end{align*}
showing that $(n-1)(n-2) \cdots (n-m+1)$ is not divisible by $m!$.
This shows that $\binom{n}{m}$ is not divisible by $n$ for $m a/2$, then $(\dagger)$ forces $r^2+s^2 \le 2b$,
giving the last case.
\end{proof}
Hence, there exists some particular pair $(r,s)$
for which there are infinitely many solutions
$(m,n)$. Simplifying the system gives
\begin{gather*}
an = 2rm + r^2-b \\
2sn = am + b-s^2
\end{gather*}
Since the system is linear,
for there to be infinitely many solutions $(m, n)$
the system must be dependent.
This gives \[\frac{a}{2s}=\frac{2r}{a}=\frac{r^2-b}{b-s^2}\]
so $a = 2\sqrt{rs}$ and $b = \frac{s^2\sqrt{r}+r^2\sqrt{s}}{\sqrt{r}+\sqrt{s}}$.
Since $rs$ must be square, we can reparametrize as $r=kx^2$, $s=ky^2$, and $\gcd(x, y)=1$.
This gives
\begin{align*}
a &= 2kxy \\
b &= k^2xy(x^2-xy+y^2).
\end{align*}
Thus, $a \mid 2b$, as desired.
\pagebreak
\subsection{TSTST 2021/5, proposed by Vincent Huang}
\textsl{Available online at \url{https://aops.com/community/p23864182}.}
\begin{mdframed}[style=mdpurplebox,frametitle={Problem statement}]
Let $T$ be a tree on $n$ vertices with exactly $k$ leaves.
Suppose that there exists a subset of
at least $\frac{n+k-1}{2}$ vertices of $T$,
no two of which are adjacent.
Show that the longest path in $T$ contains
an even number of edges.
\end{mdframed}
The longest path in $T$ must go between two leaves. The solutions presented here
will solve the problem by showing that in the unique $2$-coloring of $T$, all
leaves are the same color.
\paragraph{Solution 1 (Ankan Bhattacharya, Jeffery Li)}
\begin{lemma*}
If $S$ is an independent set of $T$, then
\[\sum_{v\in S}\deg(v)\leq n-1.\]
Equality holds if and only if $S$ is one of the two components of the unique
$2$-coloring of $T$.
\end{lemma*}
\begin{proof}
Each edge of $T$ is incident to at most one vertex of $S$, so we obtain the
inequality by counting how many vertices of $S$ each edge is incident to. For
equality to hold, each edge is incident to exactly one vertex of $S$, which
implies the $2$-coloring.
\end{proof}
We are given that there exists an independent set of at least $\frac{n+k-1}{2}$
vertices. By greedily choosing vertices of smallest degree, the sum of the
degrees of these vertices is at least
\[k+2\cdot\frac{n-k-1}{2}=n-1.\]
Thus equality holds everywhere, which implies that the independent set contains
every leaf and is one of the components of the $2$-coloring.
\paragraph{Solution 2 (Andrew Gu)}
\begin{lemma*}
The vertices of $T$ can be partitioned into $k-1$ paths (i.e. the induced
subgraph on each set of vertices is a path) such that all edges of $T$
which are not part of a path are incident to an endpoint of a path.
\end{lemma*}
\begin{proof}
Repeatedly trim the tree by taking a leaf and removing the longest path
containing that leaf such that the remaining graph is still a tree.
\end{proof}
Now given a path of $a$ vertices, at most $\frac{a+1}{2}$ of those vertices can
be in an independent set of $T$. By the lemma, $T$ can be partitioned into $k-1$
paths of $a_1, \dotsc, a_{k-1}$ vertices, so the maximum size of an independent
set of $T$ is
\[\sum \frac{a_i+1}{2}=\frac{n+k-1}{2}.\]
For equality to hold, each path in the partition must have an odd number of
vertices, and has a unique $2$-coloring in red and blue where the endpoints are
red. The unique independent set of $T$ of size $\frac{n+k-1}{2}$ is then the set of red
vertices. By the lemma, the edges of $T$ which are not part of a path connect an
endpoint of a path (which is colored red) to another vertex (which must be blue,
because the red vertices are independent). Thus the coloring of the paths
extends to the unique $2$-coloring of $T$. The leaves of $T$ are endpoints of
paths, so they are all red.
\pagebreak
\subsection{TSTST 2021/6, proposed by Nikolai Beluhov}
\textsl{Available online at \url{https://aops.com/community/p23864189}.}
\begin{mdframed}[style=mdpurplebox,frametitle={Problem statement}]
Triangles $ABC$ and $DEF$ share circumcircle $\Omega$ and incircle $\omega$
so that points $A$, $F$, $B$, $D$, $C$, and $E$ occur in this order along $\Omega$.
Let $\Delta_A$ be the triangle formed by lines $AB$, $AC$, and $EF$,
and define triangles $\Delta_B$, $\Delta_C, \dotsc, \Delta_F$ similarly.
Furthermore, let $\Omega_A$ and $\omega_A$ be the circumcircle and incircle
of triangle $\Delta_A$, respectively, and define circles
$\Omega_B$, $\omega_B, \dotsc, \Omega_F$, $\omega_F$ similarly.
\begin{enumerate}[(a)]
\item Prove that the two common external tangents to circles $\Omega_A$ and $\Omega_D$
and the two common external tangents to circles $\omega_A$ and $\omega_D$
are either concurrent or pairwise parallel.
\item Suppose that these four lines meet at point $T_A$,
and define points $T_B$ and $T_C$ similarly.
Prove that points $T_A$, $T_B$, and $T_C$ are collinear.
\end{enumerate}
\end{mdframed}
\begin{center}
\begin{asy}
import geometry;
linemargin = 0;
real R = 120;
pair O = (0, 0);
pair A = rotate(35, O)*(R, 0);
pair B = rotate(210, O)*(R, 0);
pair C = rotate(330, O)*(R, 0);
circle omega = incircle(A, B, C);
circle k = circle((point) O, R);
pair D = rotate(255, O)*(R, 0);
line[] ts = tangents(omega, (point) D);
pair E = 2*(projection(ts[0])*O) - D;
pair F = 2*(projection(ts[1])*O) - D;
pair K = extension(A, B, E, F);
pair L = extension(A, B, F, D);
pair M = extension(B, C, F, D);
pair N = extension(B, C, D, E);
pair P = extension(C, A, D, E);
pair Q = extension(C, A, E, F);
triangle dA = triangle(Q, A, K);
triangle dB = triangle(L, B, M);
triangle dC = triangle(N, C, P);
triangle dD = triangle(M, D, N);
triangle dE = triangle(P, E, Q);
triangle dF = triangle(K, F, L);
circle oA = incircle(dA);
circle oB = incircle(dB);
circle oC = incircle(dC);
circle oD = incircle(dD);
circle oE = incircle(dE);
circle oF = incircle(dF);
circle kA = circumcircle(dA);
circle kB = circumcircle(dB);
circle kC = circumcircle(dC);
circle kD = circumcircle(dD);
circle kE = circumcircle(dE);
circle kF = circumcircle(dF);
point similitude(circle a, circle b) {
return extension(a.C, b.C, a.C + (0, a.r), b.C + (0, b.r));
}
point TA = similitude(oA, oD);
point TB = similitude(oB, oE);
point TC = similitude(oC, oF);
draw(A--B--C..cycle);
draw(D--E--F..cycle);
draw(omega);
draw(k);
pen otp = red+opacity(0.5);
draw(TA--projection(tangents(oD, TA)[0])*oD.C, otp);
draw(TA--projection(tangents(oD, TA)[1])*oD.C, otp);
draw(TB--projection(tangents(oB, TB)[0])*oB.C, otp);
draw(TB--projection(tangents(oB, TB)[1])*oB.C, otp);
draw(TC--projection(tangents(oF, TC)[0])*oF.C, otp);
draw(TC--projection(tangents(oF, TC)[1])*oF.C, otp);
//draw(tangents(oA, TA), otp);
//draw(tangents(oB, TB), otp);
//draw(tangents(oC, TC), otp);
pen ktp = lightolive+opacity(0.5);
draw(TA--projection(tangents(kD, TA)[0])*kD.C, ktp);
draw(TA--projection(tangents(kD, TA)[1])*kD.C, ktp);
draw(TB--projection(tangents(kB, TB)[0])*kB.C, ktp);
draw(TB--projection(tangents(kB, TB)[1])*kB.C, ktp);
draw(TC--projection(tangents(kF, TC)[0])*kF.C, ktp);
draw(TC--projection(tangents(kF, TC)[1])*kF.C, ktp);
//draw(tangents(kA, TA), ktp);
//draw(tangents(kB, TB), ktp);
//draw(tangents(kC, TC), ktp);
pen op = blue;
draw(oA, op);
draw(oB, op);
draw(oC, op);
draw(oD, op);
draw(oE, op);
draw(oF, op);
pen kp = heavygreen;
draw(kA, kp);
draw(kB, kp);
draw(kC, kp);
draw(kD, kp);
draw(kE, kp);
draw(kF, kp);
draw(line(TA, TB), 1.0 + heavymagenta);
dot(A);
dot(B);
dot(C);
dot(D);
dot(E);
dot(F);
dot(TA);
dot(TB);
dot(TC);
label("$A$", A, dir(A));
label("$B$", B, dir(B));
label("$C$", C, dir(C));
label("$D$", D, dir(D));
label("$E$", E, dir(E));
label("$F$", F, dir(F));
label("$T_A$", TA, dir((1, 0)));
label("$T_B$", TB, dir((1, 0)));
label("$T_C$", TC, dir((1, 0)));
\end{asy}
\end{center}
Let $I$ and $r$ be the center and radius of $\omega$, and let $O$ and $R$ be the
center and radius of $\Omega$. Let $O_A$ and $I_A$ be the circumcenter and
incenter of triangle $\Delta_A$, and define $O_B$, $I_B, \dotsc, I_F$
similarly. Let $\omega$ touch $EF$ at $A_1$, and define $B_1$, $C_1, \dotsc,
F_1$ similarly.
\paragraph{Part (a)} All solutions to part (a) will prove the stronger claim
that
\[(\Omega_A\cup \omega_A)\sim (\Omega_D\cup \omega_D).\]
The four lines will concur at the homothetic center of these figures (possibly
at infinity).
\subparagraph{Solution 1 (author)} Let the second tangent to $\omega$ parallel to $EF$ meet lines $AB$ and $AC$ at $P$ and $Q$, respectively, and let the second tangent to $\omega$ parallel to $BC$ meet lines $DE$ and $DF$ at $R$ and $S$, respectively. Furthermore, let $\omega$ touch $PQ$ and $RS$ at $U$ and $V$, respectively.
Let $h$ be inversion with respect to $\omega$. Then $h$ maps $A$, $B$, and $C$ onto the midpoints of the sides of triangle $D_1E_1F_1$. So $h$ maps $k$ onto the Euler circle of triangle $D_1E_1F_1$.
Similarly, $h$ maps $k$ onto the Euler circle of triangle $A_1B_1C_1$.
Therefore, triangles $A_1B_1C_1$ and $D_1E_1F_1$ share a common nine-point
circle $\gamma$. Let $K$ be its center; its radius equals $\frac{1}{2}r$.
Let $H$ be the reflection of $I$ in $K$. Then $H$ is the common orthocenter of triangles $A_1B_1C_1$ and $D_1E_1F_1$.
Let $\gamma_U$ of center $K_U$ and radius $\frac{1}{2}r$ be the Euler circle of
triangle $UE_1F_1$, and let $\gamma_V$ of center $K_V$ and radius $\frac{1}{2}r$ be
the Euler circle of triangle $VB_1C_1$.
Let $H_U$ be the orthocenter of triangle $UE_1F_1$. Since quadrilateral
$D_1E_1F_1U$ is cyclic, vectors $\overrightarrow{HH_U}$ and
$\overrightarrow{D_1U}$ are equal. Consequently,
$\overrightarrow{KK_U}=\frac{1}{2}\overrightarrow{D_1U}$. Similarly,
$\overrightarrow{KK_V}=\frac{1}{2}\overrightarrow{A_1V}$.
Since both of $A_1U$ and $D_1V$ are diameters in $\omega$, vectors
$\overrightarrow{D_1U}$ and $\overrightarrow{A_1V}$ are equal. Therefore, $K_U$
and $K_V$ coincide, and so do $\gamma_U$ and $\gamma_V$.
As above, $h$ maps $\gamma_U$ onto the circumcircle of triangle $APQ$ and
$\gamma_V$ onto the circumcircle of triangle $DRS$. Therefore, triangles $APQ$
and $DRS$ are inscribed inside the same circle $\Omega_{AD}$.
Since $EF$ and $PQ$ are parallel, triangles $\Delta_A$ and $APQ$ are homothetic,
and so are figures $\Omega_A \cup \omega_A$ and $\Omega_{AD}\cup \omega$.
Consequently, we have
\[(\Omega_A\cup \omega_A)\sim (\Omega_{AD}\cup \omega)\sim (\Omega_D\cup
\omega_D),\]
which solves part (a).
%If figures $\omega_A \cup k_A$ and $\omega_D \cup k_D$ are noncongruent, then the two common external tangents to $\omega_A$ and $\omega_D$ and the two common external tangents to $k_A$ and $k_D$ meet at their homothetic center. Otherwise, the two common external tangents to $\omega_A$ and $\omega_D$ and the two common external tangents to $k_A$ and $k_D$ are pairwise parallel.
\subparagraph{Solution 2 (Michael Ren)} As in the previous solution, let the
second tangent to $\omega$ parallel to $EF$ meet $AB$ and $AC$ at $P$ and $Q$,
respectively. Let $(APQ)$ meet $\Omega$ again at $D'$, so that $D'$ is the
Miquel point of $\{AB, AC, BC, PQ\}$. Since the quadrilateral formed by these
lines has incircle $\omega$, it is classical that $D'I$ bisects $\angle PD'C$
and $BD'Q$ (e.g. by DDIT).
Let $\ell$ be the tangent to $\Omega$ at $D'$ and $D'I$ meet $\Omega$ again at
$M$. We have
\[
\measuredangle(\ell, D'B) = \measuredangle D'CB = \measuredangle D'QP =
\measuredangle(D'Q, EF).
\]
Therefore $D'I$ also bisects the angle between $\ell$ and the line parallel to
$EF$ through $D'$. This means that $M$ is one of the arc midpoints of $EF$.
Additionally, $D'$ lies on arc $BC$ not containing $A$, so $D'=D$.
Similarly, letting the second tangent to $\omega$ parallel to $BC$ meet $DE$ and
$DF$ again at $R$ and $S$, we have $ADRS$ cyclic.
\begin{lemma*}
There exists a circle $\Omega_{AD}$ tangent to $\Omega_A$ and $\Omega_D$ at
$A$ and $D$, respectively.
\end{lemma*}
\begin{proof}
(This step is due to Ankan Bhattacharya.) It is equivalent to have
$\measuredangle OAO_A = \measuredangle O_DDO$. Taking isogonals with respect
to the shared angle of $\triangle ABC$ and $\Delta_A$, we see that
\[\measuredangle OAO_A = \measuredangle(\perp EF, \perp BC) = \measuredangle
(EF, BC).\]
(Here, $\perp EF$ means the direction perpendicular to $EF$.) By symmetry,
this is equal to $\measuredangle O_DDO$.
\end{proof}
The circle $(ADPQ)$ passes through $A$ and $D$, and is tangent to $\Omega_A$ by
homothety. Therefore it coincides with $\Omega_{AD}$, as does $(ADRS)$. Like the
previous solution, we finish with
\[(\Omega_A\cup \omega_A)\sim (\Omega_{AD}\cup \omega)\sim (\Omega_D\cup
\omega_D).\]
\subparagraph{Solution 3 (Andrew Gu)}
Construct triangles homothetic to $\Delta_A$ and $\Delta_D$ (with positive ratio) which
have the same circumcircle; it suffices to show that these copies have the same
incircle as well. Let $O$ be the center of this common circumcircle, which we
take to be the origin, and $M_{XY}$ denote the point on the circle such that the
tangent at that point is parallel to line $XY$ (from the two possible choices,
we make the choice that corresponds to the arc midpoint on $\Omega$, e.g. $M_{AB}$
should correspond to the arc midpoint on the internal angle bisector of $ACB$).
The left diagram below shows the original triangles $ABC$ and $DEF$, while the
right diagram shows the triangles homothetic to $\Delta_A$ and $\Delta_D$.
\begin{center}
\begin{asy}
size(6cm);
pair O = origin;
pair Mbc = dir(-90);
pair Mca = dir(40);
pair Mab = dir(160);
pair A = -Mca*Mab/Mbc;
pair B = -Mab*Mbc/Mca;
pair C = -Mbc*Mca/Mab;
pair I = Mbc+Mca+Mab;
pair Mef = dir(75);
pair D = 2*foot(O, I, Mef)-Mef;
pair X = midpoint(D--I);
pair Y = X+5*dir(90)*(X-D);
pair Mfd = intersectionpoints(X--Y, unitcircle)[0];
pair Mde = 2*foot(O, X, Mfd)-Mfd;
pair E = -Mde*Mef/Mfd;
pair F = -Mef*Mfd/Mde;
draw(unitcircle);
draw(A--B--C--cycle, red);
draw(D--E--F--cycle, blue);
draw(incircle(A, B, C));
string[] names = {"$A$", "$B$", "$C$", "$M_{BC}$", "$M_{CA}$", "$M_{AB}$", "$I$", "$D$", "$E$", "$F$", "$M_{FD}$", "$M_{DE}$", "$M_{EF}$"};
pair[] pts = {A, B, C, Mbc, Mca, Mab, I, D, E, F, Mfd, Mde, Mef};
pair[] labels = {A, B, C, Mbc, Mca, Mab, I, D, E, F, Mfd, Mde, Mef};
for(int i=0; i
0$. There exists at least one mine-avoiding path, which must pass through either
$(0, 1)$ or $(1, 0)$. We consider two cases:
\textbf{Case 1: there exist mine-avoiding paths starting at both $(1, 0)$ and
$(0, 1)$.}
By the inductive hypothesis, there are at least $2^{n-1-|M|}$ mine-avoiding
paths starting from each of $(1, 0)$ and $(0, 1)$. Then the total number of
mine-avoiding paths is at least $2^{n-1-|M|}+2^{n-1-|M|}=2^{n-|M|}$.
\textbf{Case 2: only one of $(1, 0)$ and $(0, 1)$ is on a mine-avoiding path.}
Without loss of generality, suppose no mine-avoiding path starts at $(0, 1)$.
Then some element of $M$ must be of the form $(0, k)$ for $1\leq k\leq n$ in
order to block the path along the $y$-axis. This element can be ignored for any
mine-avoiding path starting at $(1, 0)$. By the inductive hypothesis, there are
at least $2^{n-1-(|M|-1)}=2^{n-|M|}$ mine-avoiding paths.
This completes the induction step, which solves the problem.
\paragraph{Solution 2}
\begin{lemma*}
If $|M| d$, the pattern changes to
\[S_n = \sum_{j = 1}^d (-1)^{j + 1}c_jS_{n - j}.\]
\begin{lemma*}
All of the $c_i$ are integers except for $c_d$. Furthermore, $c_d$ is $1/p$
times an integer.
\end{lemma*}
\begin{proof}
The $q$th cyclotomic polynomial is
\[\Phi_q(x)=1+x^{p^{r-1}}+x^{2p^{r-1}}+\dotsb+x^{(p-1)p^{r-1}}.\]
The polynomial
\[Q(x) = 1+(1+x)^{p^{r-1}}+(1+x)^{2p^{r-1}}+\dotsb+(1+x)^{(p-1)p^{r-1}}\]
has roots $\omega-1$ for $\omega \in S_q$, so it is equal to $p(-x)^dP(-1/x)$ by
comparing constant coefficients. Comparing the remaining coefficients, we find
that $c_n$ is $1/p$ times the $x^{n}$ coefficient of $Q$.
Since $(x + y)^p \equiv x^p + y^p \pmod{p}$, we conclude that, modulo $p$,
\begin{align*}
Q(x) &\equiv 1 + \big(1 + x^{p^{r - 1}}\big) + \big(1 + x^{p^{r - 1}}\big)^2 + \dotsb + \big(1 + x^{p^{r -
1}}\big)^{p - 1} \\
&\equiv \Big[\big(1 + x^{p^{r - 1}}\big)^p - 1\Big]/x^{p^{r - 1}}.
\end{align*}
Since $\binom{p}{j}$ is a multiple of $p$ when $0 < j < p$, it follows that all
coefficients of $Q(x)$ are multiples of $p$ save for the leading one. Therefore,
$c_n$ is an integer when $n < d$, while $c_d$ is $1/p$ times an integer.
\end{proof}
By the recurrences above, $S_n$ is an integer for $n < d$. When $r = 1$, we get
that $dc_d$ is not an integer, so $S_d$ is not an integer, either. Thus the
answer for $r=1$ is $n=p-1$.
Suppose now that $r \ge 2$. Then $dc_d$ does become an integer, so $S_d$ is an integer as well.
%Using our recurrence for the $S_n$ with $n > d$, we see that the smallest $n$
%with $S_n$ non-integer is $d$ larger than the smallest $n$ with $S_n$ not a multiple of $p$.
\begin{lemma*}
For all $n$ with $1 \le n \le d$, we have $\nu_p(nc_n) \ge r - 2$.
Furthermore, the smallest $n$ such that $\nu_p(nc_n) = r - 2$ is $d - p^{r - 1} +
1$.
\end{lemma*}
\begin{proof}
The value of $nc_n$ is $1/p$ times the coefficient of $x^{n-1}$ in the
derivative $Q'(x)$. This derivative is
\[p^{r - 1}(1 + x)^{p^{r - 1} - 1}\left[\sum_{k = 1}^{p - 1} k(1 + x)^{(k - 1)p^{r -
1}}\right].\]
What we want to prove reduces to showing that all coefficients of the
polynomial in the square brackets are multiples of $p$ except for the leading one.
Using the same trick $(x + y)^p \equiv x^p + y^p \pmod{p}$ as before and also
writing $w$ for $x^{p^{r - 1}}$, modulo $p$ the polynomial in the square brackets
becomes
\[1 + 2(1 + w) + 3(1 + w)^2 + \dotsb + (p - 1)(1 + w)^{p - 2}.\]
This is the derivative of
\[1 + (1 + w) + (1 + w)^2 + \dotsb + (1 + w)^{p - 1} = [(1 + w)^p - 1]/w\]
and so, since $\binom{p}{j}$ is a multiple of $p$ when $0 < j < p$, we are
done.
\end{proof}
Finally, we finish the problem with the following claim.
\begin{claim*}
Let $m=d-p^{r-1}$. Then for all $k\geq 0$ and $1\leq j\leq d$, we have
\begin{align*}
\nu_p(S_{kd+m+1}) &= r-2-k \\
\nu_p(S_{kd+m+j}) &\geq r-2-k.
\end{align*}
\end{claim*}
\begin{proof}
First, $S_1, S_2, \dotsc, S_m$ are all divisible by $p^{r - 1}$ by Newton's
identities and the second lemma. Then $\nu_p(S_{m + 1}) = r - 2$ because
\[\nu_p((m+1)c_{m+1})=r-2,\]
and all other terms in the recurrence relation are divisible by $p^{r-1}$. We
can similarly check that $\nu_p(S_{n})\geq r-2$ for $m+1\leq n \leq d$.
Newton's identities combined with the first lemma now imply
the following for $n > d$:
\begin{itemize}
\item If $\nu_p(S_{n-j})\geq \ell$ for all $1\leq j\leq d$ and
$\nu_p(S_{n-d})\geq \ell+1$, then $\nu_p(S_{n})\geq \ell$.
\item If $\nu_p(S_{n-j})\geq \ell$ for all $1\leq j\leq d$ and
$\nu_p(S_{n-d})=\ell$, then $\nu_p(S_{n})=\ell-1$.
\end{itemize}
Together, these prove the claim by induction.
\end{proof}
By the claim, the smallest $n$ for which $\nu_p(S_n) < 0$ (equivalent to $S_n$
not being an integer, by the recurrences) is
\[n=(r-1)d+m+1=((p-1)r-1)p^{r-1}+1.\]
\begin{remark*}
The original proposal was the following more general version:
\begin{quote}
Let $n$ be an integer with prime power factorization $q_1\cdots q_m$.
Let $S_n$ denote the set of primitive $n$th roots of unity. Find all tuples of nonnegative integers $(z_1,\dots,z_m)$ such that
\[
\sum_{\omega\in S_n} \frac{f(\omega)}{(1-\omega^{n/q_1})^{z_1}\cdots (1-\omega^{n/q_m})^{z_m}} \in \ZZ
\]
for all polynomials $f\in \ZZ[x]$.
\end{quote}
The maximal $z_i$ are exponents in the prime ideal factorization of the \href{https://en.wikipedia.org/wiki/Different_ideal}{different ideal} of the cyclotomic extension $\QQ(\zeta_n)/\QQ$.
\end{remark*}
\begin{remark*}
Let $F = (x^p - 1)/(x-1)$ be the minimal polynomial of $\zeta_p = e^{2\pi i/p}$ over $\QQ$.
A calculation of Euler shows that
\[
(\ZZ[\zeta_p])^*
:= \{\alpha = g(\zeta_p)\in \QQ[\zeta_p]: \sum_{\omega \in S_p}
f(\omega)g(\omega)\in \ZZ\; \forall f\in \ZZ[x]\}
= \frac{1}{F'(\zeta_p)}\cdot \ZZ[\zeta_p],
\]
where
\[
F'(\zeta_p) = \frac{p\zeta_p^{p-1} - [1+\zeta_p+\cdots+\zeta_p^{p-1}]}{1-\zeta_p} = p(1-\zeta_p)^{-1}\zeta_p^{p-1}
\]
is $(1-\zeta_p)^{[p-1]-1} = (1-\zeta_p)^{p-2}$ times a unit of
$\ZZ[\zeta_p]$. Here, $(\ZZ[\zeta_p])^*$ is the dual lattice of
$\ZZ[\zeta_p]$.
\end{remark*}
%\begin{remark*}
%Let $\alpha$ be algebraic of degree $d$ over $\QQ$, and let $f\in \QQ[x]$ be the minimal polynomial of $\alpha$ (of degree $d$).
%Hidden in the previous remark is Euler's calculation: the trace of $\alpha^i/f'(\alpha)$ (sum over conjugates) is $0$ for $i=0,1,\dots,d-2$ and $1$ for $i=d-1$, proven either using partial fractions, Lagrange interpolation, or other symmetric sum techniques (e.g. Newton sums).
%All of this can be phrased more directly (e.g. in terms of symmetric sums of $S_n$), so one could likely use standard elementary methods to directly compute the sums in the problem statement.
%\end{remark*}
\begin{remark*}
Let $K = \QQ(\omega)$, so $(p)$ factors as $(1-\omega)^{p-1}$ in the ring of integers $\mathcal{O}_K$ (which, for cyclotomic fields, can be shown to be $\ZZ[\omega]$).
%total ramification
In particular, the \emph{ramification index} $e$ of $(1-\omega)$ over $p$ is the exponent, $p-1$.
Since $e = p-1$ is not divisible by $p$, we have so-called \emph{tame ramification}.
%So look at the exponent of $1-\omega$ in the inverse different, get $[p-1]-1 = p-2$.
Now by the
\href{https://en.wikipedia.org/wiki/Different_ideal\#Ramification}{ramification
theory} of Dedekind's different ideal, the exponent $z_1$ that works when $n=p$ is $e-1 = p-2$.
Higher prime powers are more interesting because of wild ramification: $p$ divides $\phi(p^r) = p^{r-1}(p-1)$ if and only if $r>1$.
(This is a similar phenomena to how Hensel's lemma for $x^2 - c$ is more interesting mod powers of 2 than mod odd prime powers.)
\end{remark*}
\begin{remark*}
Let $F = (x^q - 1)/(x^{q/p}-1)$ be the minimal polynomial of $\zeta_q = e^{2\pi i/q}$ over $\QQ$.
The aforementioned calculation of Euler shows that
\[
(\ZZ[\zeta_q])^*
:= \{\alpha = g(\zeta_q)\in \QQ[\zeta_q]: \sum_{\omega \in S_q}
f(\omega)g(\omega)\in \ZZ \; \forall f\in \ZZ[x]\}
= \frac{1}{F'(\zeta_q)}\cdot \ZZ[\zeta_q],
\]
where the chain rule implies (using the computation from the prime case)
\[
F'(\zeta_q) = [p(1-\zeta_p)^{-1}\zeta_p^{p-1}]\cdot \frac{q}{p}\zeta_q^{(q/p) - 1} = q(1-\zeta_p)^{-1}\zeta_q^{-1}.
\]
is $(1-\zeta_q)^{r\phi(q) - q/p} = (1-\zeta_q)^{z_q}$ times a unit of $\ZZ[\zeta_q]$.
\end{remark*}
\pagebreak
\pagebreak
\end{document}