Monday, August 17, 2015

Prime binomial sum

I believe this is a problem from an International Mathematical Olympiad. [Don't know the year].

$p \ge 5$ is a prime number.

Show that

$$ \sum_{k=1}^{\lfloor 2p/3 \rfloor} \binom{p}{k}$$ is divisible by $p^2$.


[Solution]

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