$f(m,n)=\binom{m}{m}\binom{n}{1}+\binom{m}{m1}\binom{n}{2}+\cdots+\binom{m}{2}\binom{n}{m1}+\binom{m}{1}\binom{n}{m}=?$
Can we simplify the expression above?
$f(m,n)=\binom{m}{m}\binom{n}{1}+\binom{m}{m1}\binom{n}{2}+\cdots+\binom{m}{2}\binom{n}{m1}+\binom{m}{1}\binom{n}{m}=?$ Can we simplify the expression above? 


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