Number of not-necessarily-symmetric n X 2 crossword puzzle grids.

Terms

a(0) =1a(1) =5a(2) =15a(3) =39a(4) =97a(5) =237a(6) =575a(7) =1391a(8) =3361a(9) =8117a(10) =19599a(11) =47319a(12) =114241a(13) =275805a(14) =665855a(15) =1607519a(16) =3880897a(17) =9369317a(18) =22619535a(19) =54608391a(20) =131836321a(21) =318281037a(22) =768398399a(23) =1855077839a(24) =4478554081a(25) =10812186005a(26) =26102926095a(27) =63018038199a(28) =152139002497a(29) =367296043197