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

Terms

a(0) =1a(1) =2a(2) =8a(3) =32a(4) =128a(5) =502a(6) =1904a(7) =6862a(8) =22784a(9) =64832a(10) =120008a(11) =-223606a(12) =-4311424a(13) =-33271366a(14) =-205802344a(15) =-1142307968a(16) =-5919738880a(17) =-29159028386a(18) =-137718099760a(19) =-626077804826