MathDB
Mutually non-generable functions

Source: Vietnam TST 2023 P2

April 13, 2023
algebra

Problem Statement

Given three functions
P(x)=(x21)2023,Q(x)=(2x+1)14,R(x)=(2x+1+2x)34.P(x) = (x^2-1)^{2023}, Q(x) = (2x+1)^{14}, R(x) = \left(2x+1+\frac 2x \right)^{34}.
Initially, we pick a set SS containing two of these functions, and we perform some operations on it. Allowed operations include:
- We can take two functions p,qSp,q \in S and add one of p+q,pqp+q, p-q, or pqpq to SS. - We can take a function pSp \in S and add pkp^k to SS for kk is an arbitrary positive integer of our choice. - We can take a function pSp \in S and choose a real number tt, and add to SS one of the function p+t,pt,ptp+t, p-t, pt.
Show that no matter how we pick SS in the beginning, there is no way we can perform finitely many operations on SS that would eventually yield the third function not in SS.