>=Henry Gould > 2f(n) + f(n-1) = (3n+1)C(n) + (-1)^n This formula doesn't seem right; I don't get the same numbers(?) Is there still a typo? Of course with any similar expression we can solve for f(n) in terms of f(n-1), and then we can substitute (-1)^k a(k) for f(k) and get the recurrence for the original a(n).