G.f.: AGM( (1-x)/(1+x), (1+3*x)/(1-3*x) ).

Terms

a(0) =1a(1) =2a(2) =6a(3) =18a(4) =46a(5) =146a(6) =398a(7) =1234a(8) =3454a(9) =10610a(10) =30230a(11) =92290a(12) =265670a(13) =808210a(14) =2343182a(15) =7110994a(16) =20720798a(17) =62774578a(18) =183617910a(19) =555559554a(20) =1629845726a(21) =4926370114a(22) =14486707710a(23) =43752525762a(24) =128909169334a(25) =389075915954