Example:LetS={oo·a1,o:a2,…,ooa}, Determine the number r-permutations of s Solution: Let p be the number r- permutations of s, gr(x)=(1+x+x2/2!,,+xr/r!+…),then g(x)=(1+x+x2/2!+.+xTr!+.)k=(e)=ek ∑k 0 0▪ Example: Let S={·a1 ,·a2 ,…,·ak }， Determine the number r-permutations of S. ▪ Solution: Let pr be the number r￾permutations of S, ▪ gri(x)=(1+x+x2 /2!+…+xr /r!+…)，then ▪ g(x)=(1+x+x2 /2!+…+xr /r!+…)k=(ex ) k=ekx    =  = = = 0 0 ! ! ( ) r r r r r r x k r kx