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

Terms

a(0) =1a(1) =1a(2) =0a(3) =-2a(4) =-5a(5) =-9a(6) =-14a(7) =-20a(8) =-27a(9) =-35a(10) =-44a(11) =-54a(12) =-65a(13) =-77a(14) =-90a(15) =-104a(16) =-119a(17) =-135a(18) =-152a(19) =-170a(20) =-189a(21) =-209a(22) =-230a(23) =-252a(24) =-275a(25) =-299a(26) =-324a(27) =-350a(28) =-377a(29) =-405