AoPS Academy
[/quote]
, do not answer with "
is a solution" only. Either you post any kind of proof or at least something unexpected (like "
is the smallest solution). Someone that does not see that is a solution of the above without your post is completely wrong here, this is an IMO-level forum.
be a linear transformation on linear space 
satisfying:
but 
, and 
is the eigensubspace of eigenvalue 
. Prove that can be decomposed to 
-cyclic subspace's direct sum. over the field 
:
be an integer. In a configuration of an 
board, each of the 
cells contains an arrow, either pointing up, down, left, or right. Given a starting configuration, Turbo the snail starts in one of the cells of the board and travels from cell to cell. In each move, Turbo moves one square unit in the direction indicated by the arrow in her cell (possibly leaving the board). After each move, the arrows in all of the cells rotate 
counterclockwise. We call a cell good if, starting from that cell, Turbo visits each cell of the board exactly once, without leaving the board, and returns to her initial cell at the end. Determine, in terms of 
, the maximum number of good cells over all possible starting configurations.
, let 
be the excircle opposite to . Let 
and 
be the points where is tangent to 
, and 
, respectively. The circle 
intersects line 
at 
and 
. Let 
be the midpoint of 
. Prove that the circle 
is tangent to .
positive real numbers such that 
. Prove that :
such that for all 
,![\[
f(x + f(2y)) + f(x^2 - y) = f(f(x)) f(x + 1) + 2y - f(y).
\]](http://latex.artofproblemsolving.com/d/1/0/d10c01d7d416243614b24f04218d157e8b025ff2.png)
and 
functions are given with following properties:
is strict increasing and for each 
there holds 
or 
.
if holds and 
if holds.
and 
such that for all 
we have 
be an acute triangle. Points 
, and 
lie on a line in this order and satisfy 
. Let and 
be the midpoints of and 
, respectively. Suppose triangle 
is acute, and let 
be its orthocentre. Points and lie on lines 
and 
, respectively, such that 
and are concyclic and pairwise different, and 
and are concyclic and pairwise different. Prove that 
and are concyclic.
pairwise distinct complex numbers are written on a board. It turns out that there are exactly 760 ways to choose two numbers 
and 
from the board such that:![\[
a^2 + b^2 + 1 = 2ab
\]](http://latex.artofproblemsolving.com/8/f/c/8fc5df8dbc8e1647edfaa96dcfa8a0dfdac0c961.png)
Ways that differ by the order of selection are considered the same. Prove that there exist two numbers 
and 
from the board such that:![\[
c^2 + d^2 + 2025 = 2cd
\]](http://latex.artofproblemsolving.com/2/7/0/2706d72797c5e392b3f068a8174bd257e4d93fa1.png)
, 
are even?
is not the fractional part, but the sequence 
? Note that 
.
cannot be expressed in terms of other known constants (unless the recursion is carefully constructed so that the limit has a specific value, e.g. if the recursion appeared as a contest problem rather than a research problem).
: 
. This gives trivial bounds from below by monotonicity and a way to improve bounds from above (if you compute for some large values ).
is not the fractional part, but the sequence ? Note that . cannot be expressed in terms of other known constants (unless the recursion is carefully constructed so that the limit has a specific value, e.g. if the recursion appeared as a contest problem rather than a research problem).: . This gives trivial bounds from below by monotonicity and a way to improve bounds from above (if you compute for some large values ). is not the fractional part.
and computing for some small values of you can show something like
for large values of . If my numerical computations are correct, this should hold e.g. for all 
.
and show that 
.
thus it suffices to show that
which is equivalent to
which should be easy to show. If something does not work out, you can probably fix it by adapting the constants in the claimed estimate of .
![$A_n=\left[n\right]^3{,}f\left(n\right)=\min \left|S\right|{,}S\ satisfy\ S\subseteq \ A_nand\ \forall \ x\in \ A_n{,}\exists \ y{,}z\in \ S\ {,}x{,}y{,}zcollinear$](http://latex.artofproblemsolving.com/a/a/6/aa671ae30c20a584500d6362790124ad6cbb67be.png)
I can prove that the sequence 
is convergent by monotone bounded convergence theorem.
ist convergent/bounded above? My endeavours left me in doubt if this is really the case. Thank you.