a(n) = floor( n*(n-1)*(n-2)/11 ).

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =2a(5) =5a(6) =10a(7) =19a(8) =30a(9) =45a(10) =65a(11) =90a(12) =120a(13) =156a(14) =198a(15) =248a(16) =305a(17) =370a(18) =445a(19) =528a(20) =621a(21) =725a(22) =840a(23) =966a(24) =1104a(25) =1254a(26) =1418a(27) =1595a(28) =1786a(29) =1993