KöMaL 2013 A588

Let $a$ be a positive integer. Prove that all prime divisors of 7 $a^2$ ( $a$ +1)-1 are of the form 7 $k \pm$ 1.

Official text KöMaL. Quoted with source.

Solutions

No solution is on olymmath yet. Official and human solutions come first when they arrive. AI solutions come after them, with a label.