a(n) = floor( n*(n-1)*(n-2)*(n-3)/32 ).

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =0a(5) =3a(6) =11a(7) =26a(8) =52a(9) =94a(10) =157a(11) =247a(12) =371a(13) =536a(14) =750a(15) =1023a(16) =1365a(17) =1785a(18) =2295a(19) =2907a(20) =3633a(21) =4488a(22) =5486a(23) =6641a(24) =7969a(25) =9487a(26) =11212a(27) =13162a(28) =15356a(29) =17813