Number of inequivalent tilings of a 4 X n rectangle using integer-sided square tiles.

Terms

a(0) =1a(1) =1a(2) =4a(3) =6a(4) =13a(5) =39a(6) =115a(7) =295a(8) =861a(9) =2403a(10) =7048a(11) =20377a(12) =60008a(13) =175978a(14) =519589a(15) =1532455a(16) =4531277a(17) =13395656a(18) =39639758a(19) =117301153a(20) =347248981a(21) =1028011708a(22) =3043852214a(23) =9012879842a(24) =26689014028a(25) =79033362580a(26) =234045889421a(27) =693101137571