a(n) = ((3^(n-1) + 1)^3 - 1)/3^n.

Terms

a(0) =7a(1) =37a(2) =271a(3) =2269a(4) =19927a(5) =177877a(6) =1596511a(7) =14355469a(8) =129159847a(9) =1162320517a(10) =10460530351a(11) =94143710269a(12) =847290203767