This website contains problems from math contests. Problems and corresponding tags were obtained from the Art of Problem Solving website.

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Found problems: 313

2017 Thailand Mathematical Olympiad, 9
Tags: functional equation , functional , algebra
Determine all functions $f$ on the set of positive rational numbers such that $f(xf(x) + f(y)) = f(x)^2 + y$ for all positive rational numbers $x, y$.

I Soros Olympiad 1994-95 (Rus + Ukr), 11.5
Tags: algebra , functional , functional equation
Is there a function $f(x)$ defined for all $x$ and such that for some $a$ and all $x$ holds the equality $$f(x) + f(2x^2 - 1) = 2x + a?$$

1991 Poland - Second Round, 4
Tags: algebra , functional equation , functional
Find all monotone functions $ f: \mathbb{R} \to \mathbb{R} $ satisfying the equation $$ f(4x)-f(3x) = 2x \ \ \text{ for } \ \ x \in \mathbb{R}.$$

1982 Austrian-Polish Competition, 6
Tags: functional , functional equation , algebra
An integer $a$ is given. Find all real-valued functions $f (x)$ defined on integers $x \ge a$, satisfying the equation $f (x+y) = f (x) f (y)$ for all $x,y \ge a$ with $x + y \ge a$.

2009 Belarus Team Selection Test, 3
Tags: algebra , functional , functional equation
a) Does there exist a function $f: N \to N$ such that $f(f(n))=f(n+1) - f(n)$ for all $n \in N$? b) Does there exist a function $f: N \to N$ such that $f(f(n))=f(n+2) - f(n)$ for all $n \in N$? I. Voronovich

2021 Austrian MO National Competition, 4
Tags: algebra , functional equation , functional
Let $a$ be a real number. Determine all functions $f: R \to R$ with $f (f (x) + y) = f (x^2 - y) + af (x) y$ for all $x, y \in R$. (Walther Janous)

MathLinks Contest 2nd, 4.1
Tags: algebra , functional
The real polynomial $f \in R[X]$ has an odd degree and it is given that $f$ is co-prime with $g(x) = x^2 - x - 1$ and $$f(x^2 - 1) = f(x)f(-x), \forall x \in R.$$ Prove that $f$ has at least two complex non-real roots.

1973 Swedish Mathematical Competition, 6
Tags: functional , functional inequality , algebra
$f(x)$ is a real valued function defined for $x \geq 0$ such that $f(0) = 0$, $f(x+1)=f(x)+\sqrt{x}$ for all $x$, and \[ f(x) < \frac{1}{2}f\left(x - \frac{1}{2}\right)+\frac{1}{2}f\left(x + \frac{1}{2}\right) \quad \text{for all} \quad x \geq \frac{1}{2} \] Show that $f\left(\frac{1}{2}\right)$ is uniquely determined.

I Soros Olympiad 1994-95 (Rus + Ukr), 10.1
Tags: algebra , number theory , functional
The function $f: Z \to Z$ satisfies the following conditions: 1) $f(f(n))=n$ for all integers $n$ 2) $f(f(n+2)+2) = n$ for all integers $n$ 3) $f(0)=1$. Find the value of $f(1995)$ and $f(-1994)$.

2010 Indonesia TST, 1
Tags: algebra , functional , functional equation
Find all functions $ f : R \to R$ that satisfies $$xf(y) - yf(x)= f\left(\frac{y}{x}\right)$$ for all $x, y \in R$.

2017 QEDMO 15th, 11
Tags: algebra , functional , functional equation
Let $G$ be a finite group and $f: G \to G$ a map, such that $f (xy) = f (x) f (y)$ for all $x, y \in G$ and $f (x) = x^{-1}$ for more than $\frac34$ of all $x \in G$ is fulfilled. Show that $f (x) =x^{-1}$ even holds for all $x \in G$ holds.

2005 Thailand Mathematical Olympiad, 14
Tags: algebra , functional equation , functional
A function $f : Z \to Z$ is given so that $f(m + n) = f(m) + f(n) + 2mn - 2548$ for all positive integers $m, n$. Given that $f(2548) = -2548$, find the value of $f(2)$.

2018 Costa Rica - Final Round, 4
Tags: functional , functional inequality , algebra
Determine if there exists a function f: $N^*\to N^*$ that satisfies that for all $n \in N^*$, $$10^{f (n)} <10n + 1 <10^{f (n) +1}.$$ Justify your answer. Note: $N^*$ denotes the set of positive integers.

1979 Kurschak Competition, 2
Tags: algebra , functional , functional inequality , function
$f$ is a real-valued function defined on the reals such that $f(x) \le x$ and $f(x + y) \le f(x) + f(y)$ for all $x, y$. Prove that $f(x) = x$ for all $x$.

1990 Chile National Olympiad, 4
Tags: algebra , functional
The function $g$, with domain and real numbers, fulfills the following: $\bullet$ $g (x) \le x$, for all real $x$ $\bullet$ $g (x + y) \le g (x) + g (y)$ for all real $x,y$ Find $g (1990)$.

2004 Switzerland Team Selection Test, 11
Tags: functional equation , functional
Find all injective functions $f : R \to R$ such that for all real $x \ne y$ , $f\left(\frac{x+y}{x-y}\right) = \frac{f(x)+ f(y)}{f(x)- f(y)}$

OIFMAT II 2012, 2
Tags: number theory , functional , functional equation in n
Find all functions $ f: N \rightarrow N $ such that: $\bullet$ $ f (m) = 1 \iff m = 1 $; $\bullet$ If $ d = \gcd (m, n) $, then $ f (mn) = \frac {f (m) f (n)} {f (d)} $; and $\bullet$ $ \forall m \in N $, we have $ f ^ {2012} (m) = m $. Clarification: $f^n (a) = f (f^{n-1} (a))$

2007 Estonia Team Selection Test, 5
Tags: functional , continuous , functional equation , algebra
Find all continuous functions $f: R \to R$ such that for all reals $x$ and $y$, $f(x+f(y)) = y+f(x+1)$.

2017 Thailand Mathematical Olympiad, 3
Tags: functional equation , functional , algebra
Determine all functions $f : R \to R$ satisfying $f(f(x) - y) \le xf(x) + f(y)$ for all real numbers $x, y$.

2011 QEDMO 10th, 1
Tags: algebra , functional equation , functional
Find all functions $f: R\to R$ with the property that $xf (y) + yf (x) = (x + y) f (xy)$ for all $x, y \in R$.

1996 Czech And Slovak Olympiad IIIA, 5
Tags: functional , functional equation , algebra
For which integers $k$ does there exist a function $f : N \to Z$ such that $f(1995) =1996$ and $f(xy) = f(x)+ f(y)+k f(gcd(x,y))$ for all $x,y \in N$?

2013 Costa Rica - Final Round, F1
Tags: algebra , functional equation , functional
Find all functions $f: R\to R$ such that for all real numbers $x, y$ is satisfied that $$f (x + y) = (f (x))^{ 2013} + f (y).$$

2009 Thailand Mathematical Olympiad, 5
Tags: algebra , functional equation , functional
Determine all functions $f : R\to R$ satisfying: $$f(xy + 2x + 2y - 1) = f(x)f(y) + f(y) + x -2$$ for all real numbers $x, y$.

2019 Saudi Arabia Pre-TST + Training Tests, 4.2
Tags: functional , functional equation , algebra
Find all functions $f : R^2 \to R$ that for all real numbers $x, y, z$ satisfies to the equation $f(f(x,z), f(z, y))= f(x, y) + z$

2022 Auckland Mathematical Olympiad, 9
Tags: number theory , functional equation , functional , algebra
Does there exist a function $f(n)$, which maps the set of natural numbers into itself and such that for each natural number $n > 1$ the following equation is satisfi ed $$f(n) = f(f(n - 1)) + f(f(n + 1))?$$