a(n) = 2*a(n-2) + a(n-3) + a(n-4) for n>=4, a(n) = binomial(n,3) for n<4.

Terms

a(0) =0a(1) =0a(2) =0a(3) =1a(4) =0a(5) =2a(6) =1a(7) =5a(8) =4a(9) =13a(10) =14a(11) =35a(12) =45a(13) =97a(14) =139a(15) =274a(16) =420a(17) =784a(18) =1253a(19) =2262a(20) =3710a(21) =6561a(22) =10935a(23) =19094a(24) =32141a(25) =55684a(26) =94311a(27) =162603a(28) =276447a(29) =475201