Is there a combinatorial interpretation of the sum(adsbygoogle = window.adsbygoogle || []).push({});

[tex]\sum_{k=0}^{n}\binom{k}{r}\binom{n-k}{m-r}[/tex] ?

If it were a sum over r instead, it would be n C m. But I don't know about this one.

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# Combinatorial interpretation

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