a(n) = floor(n*(n-1)*(n-2)*(n-3)/21).

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =1a(5) =5a(6) =17a(7) =40a(8) =80a(9) =144a(10) =240a(11) =377a(12) =565a(13) =817a(14) =1144a(15) =1560a(16) =2080a(17) =2720a(18) =3497a(19) =4429a(20) =5537a(21) =6840a(22) =8360a(23) =10120a(24) =12144a(25) =14457a(26) =17085a(27) =20057a(28) =23400a(29) =27144