Number of solutions to k_1 + 2*k_2 + ... + n*k_n = 0, where k_i are from {-1,0,1}, i=1..n.

Terms

a(0) =1a(1) =1a(2) =1a(3) =3a(4) =7a(5) =15a(6) =35a(7) =87a(8) =217a(9) =547a(10) =1417a(11) =3735a(12) =9911a(13) =26513a(14) =71581a(15) =194681a(16) =532481a(17) =1464029a(18) =4045117a(19) =11225159a(20) =31268577a(21) =87404465a(22) =245101771a(23) =689323849a(24) =1943817227a(25) =5494808425a(26) =15568077235a(27) =44200775239a(28) =125739619467