Do there exist infinitely many positive integers not expressible in the form \[(a+b)+\log_2(b+c)-2^{c+a},\]where $a,b,c$ are positive integers?
Problem
Source: Russian TST 2021, Day 4 P1
Tags: number theory, representation
starchan
21.03.2023 20:28
solved with mxlcv and Kanav
The answer is yes. We show that not all large enough odd integers are expressible in this way. Suppose this were not the case and all odd $n \geqslant M$ are expressible in this way. Pick some $n > \max(10^{100}, M)$ and suppose $a, b, c$ are such that $n$ equals the expression. Obviously, we must have $b+c = 2^k$, for some positive integer $k$. We split into three cases:
1). $k < c+a$
In this case, $n = a+2^k-c+k-2^{c+a} \leqslant 2a-2^{a+c-1}$ which is negative for $a > 2$, which implies that the expression, is clearly always bounded, by in fact $20$, but here we assume $10^{100}$ for completeness. Thus, this case is not possible.
2). $k = c+a$
In this case, the expression is $2a$, contradicting the fact that $n$ is odd. This case is also not possible.
3). $k > c+a$
Suppose $k = c+a+\ell$ for some $\ell > 0$. The expression becomes $2a + 2^{a+c+\ell} - 2^{a+c} + \ell$. Let us count the number of values this expression can take which are at most $C$ for some large $C$. Because $C \geqslant 2^{a+c+\ell-1}$, we obtain that $a+c+\ell = \mathcal O(\log C)$. Because $a, c, \ell$ are all $\mathcal O(\log C)$ the number of values this expression takes which are at most $C$ is $\mathcal O(\log^3 C) = o(C)$. In particular, this contradicts the fact that we have a positive density of such expressible positive integers.