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

Terms

a(0) =1a(1) =1a(2) =1a(3) =3a(4) =7a(5) =11a(6) =21a(7) =53a(8) =113a(9) =211a(10) =451a(11) =1049a(12) =2223a(13) =4517a(14) =9881a(15) =22203a(16) =47531a(17) =100531a(18) =220933a(19) =489737a(20) =1059137a(21) =2284401a(22) =5025959a(23) =11088703a(24) =24161133a(25) =52644061a(26) =115913011a(27) =255469863a(28) =559494883a(29) =1226060651