(2*5^n + 2^n)/3.

Terms

a(0) =1a(1) =4a(2) =18a(3) =86a(4) =422a(5) =2094a(6) =10438a(7) =52126a(8) =260502a(9) =1302254a(10) =6510758a(11) =32552766a(12) =162761782a(13) =813804814a(14) =4069015878a(15) =20345063006a(16) =101725282262a(17) =508626345774