a(n) = [x^n] 1/(1 + x + x^2)^n.

Terms

a(0) =1a(1) =-1a(2) =1a(3) =2a(4) =-15a(5) =49a(6) =-98a(7) =48a(8) =561a(9) =-2860a(10) =8151a(11) =-12948a(12) =-9282a(13) =149226a(14) =-594320a(15) =1428952a(16) =-1448655a(17) =-5538975a(18) =37450900a(19) =-122995950a(20) =239589735a(21) =-37528755a(22) =-1886983020a(23) =8939152560a(24) =-24579514050a(25) =35197176924a(26) =51580335366a(27) =-541312482256