Olympiad problems

2021 Taiwan TST Round 3, N

Let $a_1$, $a_2$, $a_3$, $\ldots$ be a sequence of positive integers such that $a_1=2021$ and $$\sqrt{a_{n+1}-a_n}=\lfloor \sqrt{a_n} \rfloor. $$ Show that there are infinitely many odd numbers and infinitely many even numbers in this sequence. [i] Proposed by Li4, Tsung-Chen Chen, and Ming Hsiao.[/i]

2022 Taiwan TST Round 1, C

Let $\triangle P_1P_2P_3$ be an equilateral triangle. For each $n\ge 4$, [i]Mingmingsan[/i] can set $P_n$ as the circumcenter or orthocenter of $\triangle P_{n-3}P_{n-2}P_{n-1}$. Find all positive integer $n$ such that [i]Mingmingsan[/i] has a strategy to make $P_n$ equals to the circumcenter of $\triangle P_1P_2P_3$. [i]Proposed by Li4 and Untro368.[/i]

ming circumcircle geometry combinatorics coordinate method modular arithmetic number theory parity

2022 Taiwan TST Round 3, 4

Let $\mathcal{X}$ be the collection of all non-empty subsets (not necessarily finite) of the positive integer set $\mathbb{N}$. Determine all functions $f: \mathcal{X} \to \mathbb{R}^+$ satisfying the following properties: (i) For all $S$, $T \in \mathcal{X}$ with $S\subseteq T$, there holds $f(T) \le f(S)$. (ii) For all $S$, $T \in \mathcal{X}$, there hold \[f(S) + f(T) \le f(S + T),\quad f(S)f(T) = f(S\cdot T), \] where $S + T = \{s + t\mid s\in S, t\in T\}$ and $S \cdot T = \{s\cdot t\mid s\in S, t\in T\}$. [i]Proposed by Li4, Untro368, and Ming Hsiao.[/i]

ming function domain functional equation algebra combinatorics functional equations number theory

2022 Taiwan TST Round 2, A

Determine all functions $f: \mathbb{R}^+ \to \mathbb{R}^+$ satisfying \[f\bigl(x + y^2 f(y)\bigr) = f\bigl(1 + yf(x)\bigr)f(x)\] for any positive reals $x$, $y$, where $\mathbb{R}^+$ is the collection of all positive real numbers. [i]Proposed by Ming Hsiao.[/i]

ming functional equation algebra functional equations mathematical induction