1/4 the number of (n+1) X (n+1) binary arrays with all 2 X 2 subblock sums the same.

Terms

a(0) =4a(1) =8a(2) =14a(3) =26a(4) =46a(5) =86a(6) =158a(7) =302a(8) =574a(9) =1118a(10) =2174a(11) =4286a(12) =8446a(13) =16766a(14) =33278a(15) =66302a(16) =132094a(17) =263678a(18) =526334a(19) =1051646a(20) =2101246a(21) =4200446a(22) =8396798a(23) =16789502a(24) =33570814a(25) =67133438a(26) =134250494a(27) =268484606a(28) =536936446a(29) =1073840126