Number of tatami tilings of a 3 X n grid (with monomers allowed).

Terms

a(0) =1a(1) =3a(2) =13a(3) =22a(4) =44a(5) =90a(6) =196a(7) =406a(8) =852a(9) =1778a(10) =3740a(11) =7822a(12) =16404a(13) =34346a(14) =72004a(15) =150822a(16) =316076a(17) =662186a(18) =1387596a(19) =2907262a(20) =6091780a(21) =12763778a(22) =26744268a(23) =56036566a(24) =117413804a(25) =246015450a(26) =515476036a(27) =1080072022a(28) =2263070868a(29) =4741795442