G.f. A(x) satisfies 1 + 4*A(x) = Sum_{n=-oo..+oo} (2*x + A(x)^n)^n.

Terms

a(0) =1a(1) =2a(2) =5a(3) =13a(4) =46a(5) =170a(6) =666a(7) =2648a(8) =10944a(9) =46296a(10) =199828a(11) =873616a(12) =3863994a(13) =17255100a(14) =77710773a(15) =352486026a(16) =1608824138a(17) =7383210030a(18) =34048297506a(19) =157700809314a(20) =733283992543