a(n) = 4*a(n-1) - a(n-2) with a(0) = 2, a(1) = 4.

Terms

a(0) =2a(1) =4a(2) =14a(3) =52a(4) =194a(5) =724a(6) =2702a(7) =10084a(8) =37634a(9) =140452a(10) =524174a(11) =1956244a(12) =7300802a(13) =27246964a(14) =101687054a(15) =379501252a(16) =1416317954a(17) =5285770564a(18) =19726764302a(19) =73621286644a(20) =274758382274