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

Terms

a(0) =1a(1) =2a(2) =5a(3) =14a(4) =39a(5) =111a(6) =314a(7) =894a(8) =2539a(9) =7224a(10) =20536a(11) =58413a(12) =166102a(13) =472410a(14) =1343448a(15) =3820748a(16) =10865805a(17) =30901790a(18) =87882171a(19) =249931270a(20) =710786078a(21) =2021427153a(22) =5748794540a(23) =16349171957a(24) =46495891170a(25) =132231060820a(26) =376055838670