a(n)={-1/(4x4x3x2x1)}(n-1)(n-2)(n-3)(n-4)
+{2/(7x3x2x1x1)}(n-1)(n-2)(n-3)(n-5)
+{-1/(3x2x1x1x2)}(n-1)(n-2)(n-4)(n-5)
+{2/(5x1x1x2x3)}(n-1)(n-3)(n-4)(n-5)
+{-1/(2x1x2x3x4)}(n-2)(n-3)(n-4)(n-5)
+b(n)(n-1)(n-2)(n-3)(n-4)(n-5)