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

Terms

a(0) =0a(1) =4a(2) =12a(3) =34a(4) =108a(5) =344a(6) =1104a(7) =3546a(8) =11396a(9) =36628a(10) =117732a(11) =378426a(12) =1216380a(13) =3909832a(14) =12567448a(15) =40395794a(16) =129844996a(17) =417363332a(18) =1341539196a(19) =4312135922a(20) =13860583628a(21) =44552347608a(22) =143205490528a(23) =460308235562