a(n) = T(n,n-3), where T is the array in A026386.

Terms

a(0) =1a(1) =5a(2) =17a(3) =34a(4) =75a(5) =114a(6) =202a(7) =272a(8) =425a(9) =535a(10) =771a(11) =930a(12) =1267a(13) =1484a(14) =1940a(15) =2224a(16) =2817a(17) =3177a(18) =3925a(19) =4370a(20) =5291a(21) =5830a(22) =6942a(23) =7584a(24) =8905a(25) =9659a(26) =11207a(27) =12082a(28) =13875a(29) =14880