a(n) = (-1)^n*n*(n+1)*(2*n-5)/6.

Terms

a(0) =1a(1) =-1a(2) =-2a(3) =10a(4) =-25a(5) =49a(6) =-84a(7) =132a(8) =-195a(9) =275a(10) =-374a(11) =494a(12) =-637a(13) =805a(14) =-1000a(15) =1224a(16) =-1479a(17) =1767a(18) =-2090a(19) =2450a(20) =-2849a(21) =3289a(22) =-3772a(23) =4300a(24) =-4875a(25) =5499a(26) =-6174a(27) =6902a(28) =-7685a(29) =8525