Magic Letters

Consider a string, s, of length n consisting of characters in the set { a, b, c }. We want to know the number of different (i, j, k) triples (where 0 ≤ i , j, k s[i] = "a", s[j] = "b" and s[k] = "c" (j + 1)2 = (i + 1)(k + 1) We consider two triples,(i, j, k) and (x, y, z), to be different if and only if i ≠ x or j ≠ y, or k ≠ z . Given n and s, find and print the number of different (i, j, k) tr