a(-1)=-1; a(n) = 2*A000108(n) for n >= 0.

Terms

a(0) =-1a(1) =2a(2) =2a(3) =4a(4) =10a(5) =28a(6) =84a(7) =264a(8) =858a(9) =2860a(10) =9724a(11) =33592a(12) =117572a(13) =416024a(14) =1485800a(15) =5348880a(16) =19389690a(17) =70715340a(18) =259289580a(19) =955277400a(20) =3534526380a(21) =13128240840a(22) =48932534040a(23) =182965127280a(24) =686119227300