G.f. Sum_{n>=0} a(n)*x^n = Sum_{n>=0} (1 + n*x + x^2)^n * x^n.

Terms

a(0) =1a(1) =1a(2) =2a(3) =6a(4) =16a(5) =51a(6) =172a(7) =626a(8) =2409a(9) =9791a(10) =41671a(11) =185224a(12) =855865a(13) =4100761a(14) =20314349a(15) =103827684a(16) =546388333a(17) =2955518901a(18) =16407286272a(19) =93350267922a(20) =543674327227