a(n) = floor(n*(n-1)*(n-2)*(n-3)/16).

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =1a(5) =7a(6) =22a(7) =52a(8) =105a(9) =189a(10) =315a(11) =495a(12) =742a(13) =1072a(14) =1501a(15) =2047a(16) =2730a(17) =3570a(18) =4590a(19) =5814a(20) =7267a(21) =8977a(22) =10972a(23) =13282a(24) =15939a(25) =18975a(26) =22425a(27) =26325a(28) =30712a(29) =35626