Twice A024537.

Terms

a(0) =2a(1) =4a(2) =8a(3) =18a(4) =42a(5) =100a(6) =240a(7) =578a(8) =1394a(9) =3364a(10) =8120a(11) =19602a(12) =47322a(13) =114244a(14) =275808a(15) =665858a(16) =1607522a(17) =3880900a(18) =9369320a(19) =22619538a(20) =54608394a(21) =131836324a(22) =318281040a(23) =768398402a(24) =1855077842a(25) =4478554084a(26) =10812186008a(27) =26102926098a(28) =63018038202a(29) =152139002500