G.f. A(x) satisfies 1 + 3*A(x) = Sum_{n=-oo..+oo} (x + A(x)^n)^n.

Terms

a(0) =1a(1) =1a(2) =2a(3) =5a(4) =18a(5) =55a(6) =187a(7) =659a(8) =2411a(9) =8888a(10) =33416a(11) =127319a(12) =490666a(13) =1907591a(14) =7477478a(15) =29515731a(16) =117220905a(17) =468044260a(18) =1877832933a(19) =7566468733a(20) =30606272800a(21) =124236563076a(22) =505910379074