Let $\mathbb{R^+}$ denote the set of positive real numbers. Assume that $f:\mathbb{R^+} \to \mathbb{R^+}$ is a function satisfying the equations: $$ f(x^3)=f(x)^3 \quad \text{and} \quad f(2x)=f(x) $$for all $x \in \mathbb{R^+}$. Find all possible values of $f(\sqrt[2022]{2})$.
Problem
Source: Baltic Way 2022, Problem 1
Tags: function, algebra
CANBANKAN
12.11.2022 23:03
Let $g\colon \mathbb{R}\to\mathbb{R}$ such that $\log_2(f(x))=g(\log_2(x))$. Then $g(x)=g(x+1)$ and $g(3x)=3g(x)$.
I claim for any $x\in \mathbb{Q}^+$, $g(x)=0$. Indeed, let $x=3^a \frac{p}{q}$ where $p,q$ are integers with $\nu_3(q)=\nu_3(p)$. Then $g(x)=3^ag(\frac pq) = 3^a g(\frac pq + \frac{(3^m-1)p}{q}) = 3^{a+m} g(\frac pq)$ where $q\mid 3^m-1$, so $(3^{a+m}-3^a)g(\frac pq)=0$
Askenazy
07.12.2022 14:29
Thanks. What do you mean by nu function? You cannot choose m such that 3^m-1 is divisible by q
Marinchoo
07.12.2022 16:40
Let $g:\mathbb{R}^{+}\rightarrow \mathbb{R}^{+}$ be such that $g(x)=f(2^{x})$. Then $g(x)=g(x+1)$ and $g(3a)=g(a)^3$. Then if we pick $k=\frac{3^{\text{ord}_{674}(3)}-1}{674}\in\mathbb{N}$ we have that:
\[g\left(\frac{1}{2022}\right)^3=g\left(\frac{1}{674}\right)=g\left(\frac{674k+1}{674}\right)=g\left(\frac{3^{\text{ord}_{674}(3)}}{674}\right)=g\left(\frac{1}{674}\right)^{3^{\text{ord}_{674}(3)}}\]\[\Longrightarrow g\left(\frac{1}{674}\right)=1\Longrightarrow f(\sqrt[2022]{2})=g\left(\frac{1}{2022}\right)=1\]
CANBANKAN
07.12.2022 20:39
Askenazy wrote: Thanks. What do you mean by nu function? You cannot choose m such that 3^m-1 is divisible by q If $x$ is an integer $\nu_p(x)$ is the largest $k$ st $p^k|x$. We extend this to rationals with $\nu_p(x/y) = \nu_p(x)-\nu_p(y)$. Sorry I typoed; I meant $\nu_3(p)=\nu_3(q)$, then we can choose $m$ st $q/3^{\nu_3(q)}|3^m-1$.