Let $a, b, c$ be positive integers whose greatest common divisor is $1$. Determine whether there always exists a positive integer $n$ such that, for every positive integer $k$, the number $2^n$ is not a divisor of $a^k+b^k+c^k$.
Source: 2009 Peru Iberoamerican TST problem 5
Tags: number theory
Let $a, b, c$ be positive integers whose greatest common divisor is $1$. Determine whether there always exists a positive integer $n$ such that, for every positive integer $k$, the number $2^n$ is not a divisor of $a^k+b^k+c^k$.