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

Terms

a(0) =0a(1) =0a(2) =1a(3) =3a(4) =9a(5) =26a(6) =73a(7) =202a(8) =553a(9) =1504a(10) =4073a(11) =11003a(12) =29689a(13) =80094a(14) =216201a(15) =584295a(16) =1581729a(17) =4290648a(18) =11666337a(19) =31802925a(20) =86935089a(21) =238325838a(22) =655282305a(23) =1807106112a(24) =4998491721a(25) =13867056936a(26) =38583280297a(27) =107660033505a(28) =301241301633a(29) =845158142178