Injections count the of 1-1 functions f (nml one-to-one correspondence [n] [m] π=(f(1),f(2),.,f(n)》 n-permutation:E[m]of distinct elements (m)n=m(m-1)(m-n+1)= m! (m-n)! “m lower factorial n'Injections [n] [m] 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]