Number of symmetric n X 3 crossword puzzle grids.

Terms

a(0) =1a(1) =3a(2) =11a(3) =21a(4) =61a(5) =111a(6) =309a(7) =565a(8) =1563a(9) =2859a(10) =7907a(11) =14459a(12) =39991a(13) =73117a(14) =202227a(15) =369729a(16) =1022587a(17) =1869581a(18) =5170829a(19) =9453757a(20) =26146909a(21) =47804041a(22) =132214959a(23) =241726811a(24) =668560673a(25) =1222320359a(26) =3380656559a(27) =6180808253a(28) =17094691953a(29) =31253992007