a(n) = floor( n*(n-1)*(n-2)/25 ).

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =0a(5) =2a(6) =4a(7) =8a(8) =13a(9) =20a(10) =28a(11) =39a(12) =52a(13) =68a(14) =87a(15) =109a(16) =134a(17) =163a(18) =195a(19) =232a(20) =273a(21) =319a(22) =369a(23) =425a(24) =485a(25) =552a(26) =624a(27) =702a(28) =786a(29) =876