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

Terms

a(0) =0a(1) =1a(2) =2a(3) =6a(4) =18a(5) =52a(6) =148a(7) =420a(8) =1192a(9) =3384a(10) =9608a(11) =27280a(12) =77456a(13) =219920a(14) =624416a(15) =1772896a(16) =5033760a(17) =14292288a(18) =40579904a(19) =115217984a(20) =327136896a(21) =928835456a(22) =2637230208a(23) =7487852800a(24) =21260161280a(25) =60363694336a(26) =171389837824a(27) =486624896512