1/16 the number of (n+1) X 6 0..3 arrays with all 2 X 2 subblocks having the same four values.

Terms

a(0) =169a(1) =181a(2) =202a(3) =244a(4) =322a(5) =478a(6) =778a(7) =1378a(8) =2554a(9) =4906a(10) =9562a(11) =18874a(12) =37402a(13) =74458a(14) =148378a(15) =296218a(16) =591514a(17) =1182106a(18) =2362522a(19) =4723354a(20) =9443482a(21) =18883738a(22) =37761178a(23) =75516058a(24) =151019674a(25) =302026906a(26) =604029082a(27) =1208033434a(28) =2416017562a(29) =4831985818