a(n) = T(n,n-3), where T is the array in A026374.

Terms

a(0) =1a(1) =6a(2) =17a(3) =45a(4) =75a(5) =144a(6) =202a(7) =330a(8) =425a(9) =630a(10) =771a(11) =1071a(12) =1267a(13) =1680a(14) =1940a(15) =2484a(16) =2817a(17) =3510a(18) =3925a(19) =4785a(20) =5291a(21) =6336a(22) =6942a(23) =8190a(24) =8905a(25) =10374a(26) =11207a(27) =12915a(28) =13875a(29) =15840