a(n) = 2^(n-2)*(2^(n+4)-(-1)^n+13).

Terms

a(0) =7a(1) =23a(2) =76a(3) =284a(4) =1072a(5) =4208a(6) =16576a(7) =65984a(8) =262912a(9) =1050368a(10) =4197376a(11) =16784384a(12) =67121152a(13) =268464128a(14) =1073790976a(15) =4295081984a(16) =17180065792a(17) =68719935488a(18) =274878693376