3*10^n-2*9^n.

Terms

a(0) =1a(1) =12a(2) =138a(3) =1542a(4) =16878a(5) =181902a(6) =1937118a(7) =20434062a(8) =213906558a(9) =2225159022a(10) =23026431198a(11) =237237880782