Is there any closed expression for the following sum:
$$\sum_{i=0}^{l-k} \binom{n-l}{i} \binom{l-k}{i} \binom{l-i}{k}$$ where $ k<l < n/2$?
Is there any closed expression for the following sum:
$$\sum_{i=0}^{l-k} \binom{n-l}{i} \binom{l-k}{i} \binom{l-i}{k}$$ where $ k<l < n/2$?
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