1/4 the number of (n+1) X 6 binary arrays with all 2 X 2 subblock sums the same.

Terms

a(0) =25a(1) =27a(2) =30a(3) =36a(4) =46a(5) =66a(6) =102a(7) =174a(8) =310a(9) =582a(10) =1110a(11) =2166a(12) =4246a(13) =8406a(14) =16662a(15) =33174a(16) =66070a(17) =131862a(18) =263190a(19) =525846a(20) =1050646a(21) =2100246a(22) =4198422a(23) =8394774a(24) =16785430a(25) =33566742a(26) =67125270a(27) =134242326a(28) =268468246a(29) =536920086