a(n) = (-1)^(n-3)*binomial(n,3) - 1.

Terms

a(0) =0a(1) =-5a(2) =9a(3) =-21a(4) =34a(5) =-57a(6) =83a(7) =-121a(8) =164a(9) =-221a(10) =285a(11) =-365a(12) =454a(13) =-561a(14) =679a(15) =-817a(16) =968a(17) =-1141a(18) =1329a(19) =-1541a(20) =1770a(21) =-2025a(22) =2299a(23) =-2601a(24) =2924a(25) =-3277a(26) =3653a(27) =-4061a(28) =4494a(29) =-4961