G.f.: 1/sqrt(1-10*x^2 + x^4/(1-8*x^2)) + x/(1-9*x^2).

Terms

a(0) =1a(1) =1a(2) =5a(3) =9a(4) =37a(5) =81a(6) =301a(7) =729a(8) =2549a(9) =6561a(10) =22045a(11) =59049a(12) =193029a(13) =531441a(14) =1703469a(15) =4782969a(16) =15111573a(17) =43046721a(18) =134539837a(19) =387420489a(20) =1200901157a(21) =3486784401a(22) =10739313997a(23) =31381059609a(24) =96172251061a(25) =282429536481a(26) =862142190941