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

Terms

a(0) =16a(1) =40a(2) =82a(3) =166a(4) =322a(5) =634a(6) =1234a(7) =2434a(8) =4786a(9) =9490a(10) =18802a(11) =37426a(12) =74482a(13) =148594a(14) =296434a(15) =592114a(16) =1182706a(17) =2363890a(18) =4724722a(19) =9446386a(20) =18886642a(21) =37767154a(22) =75522034a(23) =151031794a(24) =302039026a(25) =604053490a(26) =1208057842a(27) =2416066546a(28) =4832034802a(29) =9663971314