G.f. A(x) = (1 + x*A(3x))^2.A135870G.f. A(x) = (1 + x*A(3x))^2.Terms a(0) =1a(1) =2a(2) =13a(3) =246a(4) =13554a(5) =2210436a(6) =1076561037External referencesoeis: A135870