G.f.: 1 / AGM(1-5*x+x^2, 1+3*x+x^2).

Terms

a(0) =1a(1) =1a(2) =4a(3) =11a(4) =47a(5) =172a(6) =725a(7) =2945a(8) =12592a(9) =53607a(10) =233115a(11) =1017428a(12) =4488097a(13) =19893325a(14) =88746008a(15) =397610355a(16) =1789394067a(17) =8081593288a(18) =36622787565a(19) =166442457597a(20) =758467464848