4. 4.2 Combinations of multisets If s is a multiset. a r-combination of s is an unordered selection of r of the objects of s Thus an r-combination of s is a submultiset of s. If s has n objects then there is only one n-combination of s, namely, s itself. If s contains objects of k different types, then there are k 1-combinations of s Example If S={2°a,l·b,3·e}, then the3- combinations of s are {2°a,1°b},{2a,1c},{1°a,l·b,l°c}, {l°a,2c},{lb,2c},仔3°c}▪ 4.4.2 Combinations of multisets ▪ If S is a multiset, a r-combination of S is an unordered selection of r of the objects of S. ▪ Thus an r-combination of S is a submultiset of S. If S has n objects，then there is only one n-combination of S, namely, S itself. ▪ If S contains objects of k different types, then there are k 1-combinations of S. ▪ Example If S={2•a,1•b,3•c}, then the 3-combinations of S are ▪ {2•a,1•b},{2•a,1•c}, {1•a,1•b,1•c}, {1•a,2•c},{1•b,2•c},{3•c}