a(n)=(-1)^n(1 - (1/12)n(n + 1)(12 - n + n^2)).

Terms

a(0) =1a(1) =1a(2) =-6a(3) =17a(4) =-39a(5) =79a(6) =-146a(7) =251a(8) =-407a(9) =629a(10) =-934a(11) =1341a(12) =-1871a(13) =2547a(14) =-3394a(15) =4439a(16) =-5711a(17) =7241a(18) =-9062a(19) =11209a(20) =-13719a(21) =16631a(22) =-19986a(23) =23827a(24) =-28199a(25) =33149a(26) =-38726a(27) =44981a(28) =-51967a(29) =59739