G.f. A(x) satisfies A(x) = 1 / ((1 - x) * (1 - 2 * x * (1 + x + x^2) * A(x^3))).

Terms

a(0) =1a(1) =3a(2) =9a(3) =27a(4) =85a(5) =267a(6) =837a(7) =2631a(8) =8269a(9) =25983a(10) =81657a(11) =256623a(12) =806473a(13) =2534483a(14) =7965057a(15) =25031547a(16) =78665957a(17) =247221355a(18) =776935701a(19) =2441654415a(20) =7673320077a(21) =24114731415a(22) =75784701705a(23) =238166493207a(24) =748479273097