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

Terms

a(0) =2a(1) =26a(2) =242a(3) =2186a(4) =19682a(5) =177146a(6) =1594322a(7) =14348906a(8) =129140162a(9) =1162261466a(10) =10460353202a(11) =94143178826a(12) =847288609442