a(n) = 4*3^n - 2*2^n - 1.

Terms

a(0) =1a(1) =7a(2) =27a(3) =91a(4) =291a(5) =907a(6) =2787a(7) =8491a(8) =25731a(9) =77707a(10) =234147a(11) =704491a(12) =2117571a(13) =6360907a(14) =19099107a(15) =57330091a(16) =172055811a(17) =516298507a(18) =1549157667a(19) =4647997291a(20) =13945040451a(21) =41837218507a(22) =125515849827a(23) =376555938091