(3*11^n+1)/2.

Terms

a(0) =2a(1) =17a(2) =182a(3) =1997a(4) =21962a(5) =241577a(6) =2657342a(7) =29230757a(8) =321538322a(9) =3536921537a(10) =38906136902a(11) =427967505917