IMC 2008 Day 2 P3 - Power of 2 divides binomial sum
Source: Problem 3
July 28, 2008
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Problem Statement
Let be a positive integer. Prove that 2^{n\minus{}1} divides
\sum_{0\leq k < n/2} \binom{n}{2k\plus{}1}5^k.
Source: Problem 3