PL-OM 1990 Problem 6

Prove that for all integers $n > 2$ , $3| \sum\limits_{i=0}^{[n/3]} (-1)^i C _n ^{3i}$

Dataset text AI-MO AoPS. 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.