a(n) = [x^n] (1 - 2*x + x^2 - 2*x^3 + x^4)/((1 - x)^2*(1 - 2*x)).

Terms

a(0) =1a(1) =2a(2) =4a(3) =6a(4) =9a(5) =14a(6) =23a(7) =40a(8) =73a(9) =138a(10) =267a(11) =524a(12) =1037a(13) =2062a(14) =4111a(15) =8208a(16) =16401a(17) =32786a(18) =65555a(19) =131092a(20) =262165a(21) =524310a(22) =1048599a(23) =2097176a(24) =4194329a(25) =8388634a(26) =16777243a(27) =33554460a(28) =67108893a(29) =134217758