G.f.: (1+x^2)/((1-3x)(1-x-x^2)).

Terms

a(0) =1a(1) =4a(2) =15a(3) =49a(4) =154a(5) =473a(6) =1437a(7) =4340a(8) =13067a(9) =39277a(10) =117954a(11) =354061a(12) =1062505a(13) =3188036a(14) =9564951a(15) =28696217a(16) =86090858a(17) =258276145a(18) =774834213a(19) =2324511988a(20) =6973551091a(21) =20920677749a(22) =62762072850a(23) =188286282629a(24) =564858951569