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\frac{n^5}{120} - \frac{n^3}{24} + \frac{n}{30} is integer

Source: Polish MO second round 1957 p1

August 29, 2024
number theoryInteger

Problem Statement

Prove that if n n is an integer, then n5120n324+n30 \frac{n^5}{120} - \frac{n^3}{24} + \frac{n}{30} is also an integer.