26. Let S be the subset of the set of ordered pairs of integers defined recursively by Basis step: (0,0) E S. Recursive step: If (a, b) e S, then (a + 2, b+3) S and (a +3, b + 2) e S. a) List the elements of S produced by the first five ap- plications of the recursive definition. b) Use strong induction on the number of applications of the recursive step of the definition to show that 5|a +b when (a, b) e S. c) Use structural induction to show that 5|a + b when (a, b) ES