a(n) = floor( n*(n-1)*(n-2)*(n-3)/27 ).

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =0a(5) =4a(6) =13a(7) =31a(8) =62a(9) =112a(10) =186a(11) =293a(12) =440a(13) =635a(14) =889a(15) =1213a(16) =1617a(17) =2115a(18) =2720a(19) =3445a(20) =4306a(21) =5320a(22) =6502a(23) =7871a(24) =9445a(25) =11244a(26) =13288a(27) =15600a(28) =18200a(29) =21112