a(n) = 7^n - 3*4^n + 2*3^n.

Terms

a(0) =1a(1) =19a(2) =205a(3) =1795a(4) =14221a(5) =106819a(6) =778765a(7) =5581315a(8) =39606541a(9) =279447619a(10) =1965098125a(11) =13792018435a(12) =96690872461a(13) =677427332419