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

Terms

a(0) =1a(1) =2a(2) =6a(3) =12a(4) =28a(5) =56a(6) =120a(7) =240a(8) =496a(9) =992a(10) =2016a(11) =4032a(12) =8128a(13) =16256a(14) =32640a(15) =65280a(16) =130816a(17) =261632a(18) =523776a(19) =1047552a(20) =2096128a(21) =4192256a(22) =8386560a(23) =16773120a(24) =33550336a(25) =67100672a(26) =134209536a(27) =268419072a(28) =536854528a(29) =1073709056