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

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =1a(5) =2a(6) =5a(7) =9a(8) =15a(9) =22a(10) =32a(11) =45a(12) =60a(13) =78a(14) =99a(15) =124a(16) =152a(17) =185a(18) =222a(19) =264a(20) =310a(21) =362a(22) =420a(23) =483a(24) =552a(25) =627a(26) =709a(27) =797a(28) =893a(29) =996