a(n) = 9^n + 10^n.

Terms

a(0) =2a(1) =19a(2) =181a(3) =1729a(4) =16561a(5) =159049a(6) =1531441a(7) =14782969a(8) =143046721a(9) =1387420489a(10) =13486784401a(11) =131381059609