Constant term in the expansion of (1 + x^2 + y^2 + 1/(x*y))^n.

Terms

a(0) =1a(1) =1a(2) =1a(3) =1a(4) =13a(5) =61a(6) =181a(7) =421a(8) =1261a(9) =5293a(10) =21421a(11) =73261a(12) =232321a(13) =789361a(14) =2954953a(15) =11127481a(16) =39961741a(17) =139908301a(18) =499315501a(19) =1835933293a(20) =6792310153a(21) =24827506873a(22) =90058277233a(23) =328509505633