a(n) = 3*9^n + 1.

Terms

a(0) =4a(1) =28a(2) =244a(3) =2188a(4) =19684a(5) =177148a(6) =1594324a(7) =14348908a(8) =129140164a(9) =1162261468a(10) =10460353204a(11) =94143178828a(12) =847288609444