The expressions a \plus{} b \plus{} c, ab \plus{} ac \plus{} bc, and abc are called the elementary symmetric expressions on the three letters a,b,c; symmetric because if we interchange any two letters, say a and c, the expressions remain algebraically the same. The common degree of its terms is called the order of the expression. Let Sk(n) denote the elementary expression on k different letters of order n; for example S_4(3) \equal{} abc \plus{} abd \plus{} acd \plus{} bcd. There are four terms in S4(3). How many terms are there in S9891(1989)? (Assume that we have 9891 different letters.) combinatorics unsolvedcombinatorics