a(n) = floor( n*(n-1)*(n-2)*(n-3)/22 ).

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =1a(5) =5a(6) =16a(7) =38a(8) =76a(9) =137a(10) =229a(11) =360a(12) =540a(13) =780a(14) =1092a(15) =1489a(16) =1985a(17) =2596a(18) =3338a(19) =4228a(20) =5285a(21) =6529a(22) =7980a(23) =9660a(24) =11592a(25) =13800a(26) =16309a(27) =19145a(28) =22336a(29) =25910