a(n) = floor((n+1)*(n-3)*(n-4)/12).

Terms

a(0) =1a(1) =1a(2) =0a(3) =0a(4) =0a(5) =1a(6) =3a(7) =8a(8) =15a(9) =25a(10) =38a(11) =56a(12) =78a(13) =105a(14) =137a(15) =176a(16) =221a(17) =273a(18) =332a(19) =400a(20) =476a(21) =561a(22) =655a(23) =760a(24) =875a(25) =1001a(26) =1138a(27) =1288a(28) =1450a(29) =1625