G.f.: -sqrt(1 - 4*x)*(2*x - 1)/(3*x - 1).

Terms

a(0) =-1a(1) =1a(2) =1a(3) =3a(4) =11a(5) =41a(6) =151a(7) =549a(8) =1977a(9) =7075a(10) =25229a(11) =89831a(12) =319881a(13) =1140523a(14) =4075321a(15) =14603243a(16) =52501659a(17) =189440937a(18) =686181711a(19) =2495243373a(20) =9109701699a(21) =33388293177a(22) =122840931891a(23) =453622854873