For a natural number \(n\), let \(a_n\) be the number of congruence classes of triangles whose all three sides have integer length and its perimeter is \(n\). Obtain a formula for \(a_n\).

The best solution was submitted by 이도현 (수리과학과 2018학번, +4). Congratulations!

Here is the best solution of problem 2021-15.

Other solutions were submitted by 강한필 (전산학부 2016학번, +3), 고성훈 (수리과학과 2018학번, +3), 전해구 (기계공학과 졸업생, +3).