Problem 4: Divisibility

by henderson, Feb 12, 2016, 6:21 PM

$$\bf\color{red}Problem \ 4 \  $$Prove that for all integers $n\geq m$ \[\frac{\gcd(m,n)}{n}\cdot\binom{n}{m}\]is an integer.
$$\bf\color{red}Solution \  $$There exist integers $a,b$ such that $gcd(m,n)=am+bn.$
So, we get \[ \frac{gcd(m,n)}{n} \binom{n}{m}=a\frac{m}{n}\binom{n}{m} +b\binom{n}{m}=a\binom{n-1}{m-1} +b\binom{n}{m}.\]
This post has been edited 3 times. Last edited by henderson, Sep 12, 2016, 2:43 PM

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"Do not worry too much about your difficulties in mathematics, I can assure you that mine are still greater." - Albert Einstein

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