a(n) = floor( n*(n-1)*(n-2)/19 ).

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =1a(5) =3a(6) =6a(7) =11a(8) =17a(9) =26a(10) =37a(11) =52a(12) =69a(13) =90a(14) =114a(15) =143a(16) =176a(17) =214a(18) =257a(19) =306a(20) =360a(21) =420a(22) =486a(23) =559a(24) =639a(25) =726a(26) =821a(27) =923a(28) =1034a(29) =1153