Question

solve question 2 this from Topics in Combinatorial.

Recall that an array A: Z2 C can be thought of as an "infinite-by-infinite matrix, going in both directions." The product of two arrays A and B has entries r,c kez analogous to standard matrix multiplication. This product is well-defined if for any r, c, there are only finitely many k where Ar,xBrc 0. Recall that A: Z2 C is a Riordan array if there are Laurent series f 0, g where Ar x](g(x) f(x)) r,c 2. An array A: Z2-+ C is upper triangular if whenever r > c, we have Ar 0. Show that the product of two upper-triangular arrays is r.c well-defined and an upper-triangular array.

Answer 1

coment please!!!.

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