Found problems: 32
2019-IMOC, C1
Given a natural number $n$, if the tuple $(x_1,x_2,\ldots,x_k)$ satisfies
$$2\mid x_1,x_2,\ldots,x_k$$
$$x_1+x_2+\ldots+x_k=n$$
then we say that it's an [i]even partition[/i]. We define [i]odd partition[/i] in a similar way. Determine all $n$ such that the number of even partitions is equal to the number of odd partitions.
2019 Junior Balkan Team Selection Tests - Romania, 4
Let $n$ be a positive integer. $2n+1$ tokens are in a row, each being black or white. A token is said to be [i]balanced[/i] if the number of white tokens on its left plus the number of black tokens on its right is $n$. Determine whether the number of [i]balanced[/i] tokens is even or odd.
2000 Saint Petersburg Mathematical Olympiad, 9.7
Define a complexity of a set $a_1,a_2,\dots,$ consisting of 0 and 1 to be the smallest positive integer $k$ such that for some positive integers $\epsilon_1,\epsilon_2,\dots, \epsilon_k$ each number of the sequence $a_n$, $n>k$, has the same parity as $\epsilon_1 a_{n-1}+\epsilon_2 a_{n-2}+\dots+\epsilon_k a_{n-k}$. Sequence $a_1,a_2,\dots,$ has a complexity of $1000$. What is the complexity of sequence $1-a_1,1-a_2,\dots,$.
[I]Proposed by A. Kirichenko[/i]
2024 Kyiv City MO Round 1, Problem 2
Is it possible to write the numbers from $1$ to $100$ in the cells of a of a $10 \times 10$ square so that:
1. Each cell contains exactly one number;
2. Each number is written exactly once;
3. For any two cells that are symmetrical with respect to any of the perpendicular bisectors of sides of the
original $10 \times 10$ square, the numbers in them must have the same parity.
The figure below shows examples of such pairs of cells, in which the numbers must have the same parity.
[img]https://i.ibb.co/b3P8t36/Kyiv-MO-2024-7-2.png[/img]
[i]Proposed by Mykhailo Shtandenko[/i]
2024 Nepal TST, P1
Let $a, b$ be positive integers. Prove that if $a^b + b^a \equiv 3 \pmod{4}$, then either $a+1$ or $b+1$ can't be written as the sum of two integer squares.
[i](Proposed by Orestis Lignos, Greece)[/i]
2000 Putnam, 1
Let $a_j$, $b_j$, $c_j$ be integers for $1 \le j \le N$. Assume for each $j$, at least one of $a_j$, $b_j$, $c_j$ is odd. Show that there exists integers $r, s, t$ such that $ra_j+sb_j+tc_j$ is odd for at least $\tfrac{4N}{7}$ values of $j$, $1 \le j \le N$.
2015 India IMO Training Camp, 3
Let $n > 1$ be a given integer. Prove that infinitely many terms of the sequence $(a_k )_{k\ge 1}$, defined by \[a_k=\left\lfloor\frac{n^k}{k}\right\rfloor,\] are odd. (For a real number $x$, $\lfloor x\rfloor$ denotes the largest integer not exceeding $x$.)
[i]Proposed by Hong Kong[/i]