a(n) = floor( n*(n-1)*(n-2)/20 ).

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =1a(5) =3a(6) =6a(7) =10a(8) =16a(9) =25a(10) =36a(11) =49a(12) =66a(13) =85a(14) =109a(15) =136a(16) =168a(17) =204a(18) =244a(19) =290a(20) =342a(21) =399a(22) =462a(23) =531a(24) =607a(25) =690a(26) =780a(27) =877a(28) =982a(29) =1096