G.f.: Product_{k>=1} (1+x^k)^(3*k+1).

Terms

a(0) =1a(1) =4a(2) =13a(3) =42a(4) =117a(5) =310a(6) =785a(7) =1896a(8) =4433a(9) =10062a(10) =22248a(11) =48080a(12) =101821a(13) =211682a(14) =432795a(15) =871520a(16) =1730491a(17) =3391894a(18) =6568996a(19) =12580316a(20) =23841774a(21) =44742634a(22) =83193865a(23) =153347110a(24) =280336704a(25) =508499474a(26) =915540681a(27) =1636805438a(28) =2906642396a(29) =5128530946