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

Terms

a(0) =1a(1) =1a(2) =9a(3) =35a(4) =199a(5) =1005a(6) =5475a(7) =29469a(8) =161685a(9) =889759a(10) =4932641a(11) =27453471a(12) =153432241a(13) =860203135a(14) =4836370101a(15) =27257082723a(16) =153943314903a(17) =871064225325