Example: Use generating functions to determine the number of r-combinations of multiset s=oo.a1,oo a2,..., 00.ak. Solution: let b be the number of r- combinations of multiset s And let generating functions of (b be f(y (1+y+y2+..)k=?f(y) ∑C(k+r-1,r)y (1-y)▪ Example: Use generating functions to determine the number of r-combinations of multiset S={·a1 ,·a2 ,…, ·ak }. ▪ Solution: Let br be the number of r￾combinations of multiset S. And let generating functions of {br } be f(y)， ▪ (1+y+y2+…)k=? f(y)   = = + − − = 0 ( 1, ) (1 ) 1 ( ) r r k C k r r y y f y