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

Terms

a(0) =1a(1) =9a(2) =18a(3) =25a(4) =27a(5) =21a(6) =4a(7) =-27a(8) =-75a(9) =-143a(10) =-234a(11) =-351a(12) =-497a(13) =-675a(14) =-888a(15) =-1139a(16) =-1431a(17) =-1767a(18) =-2150a(19) =-2583a(20) =-3069a(21) =-3611a(22) =-4212a(23) =-4875a(24) =-5603a(25) =-6399a(26) =-7266a(27) =-8207a(28) =-9225a(29) =-10323