a(n) = floor(n*(n-1)*(n-2)/16).

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =1a(5) =3a(6) =7a(7) =13a(8) =21a(9) =31a(10) =45a(11) =61a(12) =82a(13) =107a(14) =136a(15) =170a(16) =210a(17) =255a(18) =306a(19) =363a(20) =427a(21) =498a(22) =577a(23) =664a(24) =759a(25) =862a(26) =975a(27) =1096a(28) =1228a(29) =1370