Injections count the of 1-1 functions f:[ml马[ml one-to-one correspondence [n] [m] π=(f(1),f(2),..,f(n) n-permutation::π∈[m]of distinct elements m! (m)m=m(m-1)·(m-n+1)= (m-n)！ “m lower factorial n'”Injections count the # of 1-1 functions one-to-one correspondence ￾ ￾ [m] n of distinct elements ￾ = (f(1), f(2),...,f(n)) n-permutation: = m! (m ￾ n)! (m)n = m(m ￾ 1)···(m ￾ n + 1) “m lower factorial n” f : [n] 1-1 ￾￾ [m] [n] [m]