G.f.: ((1+2*x)*sqrt(1-6*x^2+x^4)-1+5*x^2-2*x^3)/(2*x*(1-6*x^2)).

Terms

a(0) =1a(1) =1a(2) =2a(3) =4a(4) =8a(5) =18a(6) =36a(7) =86a(8) =172a(9) =426a(10) =852a(11) =2162a(12) =4324a(13) =11166a(14) =22332a(15) =58438a(16) =116876a(17) =309042a(18) =618084a(19) =1648154a(20) =3296308a(21) =8851206a(22) =17702412a(23) =47813790a(24) =95627580a(25) =259585002a(26) =519170004a(27) =1415431266a(28) =2830862532a(29) =7747200558