identity
Source: Ireland 1993
June 29, 2009
combinatorics unsolvedcombinatorics
Problem Statement
If are integers, prove the identity:
\displaystyle\sum_{d\equal{}1}^{\infty}\binom {n\minus{}r\plus{}1}{d} \binom {r\minus{}1} {d\minus{}1}\equal{}\binom {n}{r}.