Expansion of g/(1 - x^3*g^3), where g = 1+x*g^2 is the g.f. of A000108.

Terms

a(0) =1a(1) =1a(2) =2a(3) =6a(4) =18a(5) =56a(6) =181a(7) =601a(8) =2037a(9) =7019a(10) =24515a(11) =86593a(12) =308799a(13) =1110249a(14) =4020162a(15) =14647422a(16) =53660766a(17) =197545356a(18) =730413708a(19) =2711279418a(20) =10099991244a(21) =37745850414a(22) =141480728682a(23) =531739303520