a(n) = 3*12^n-1.

Terms

a(0) =2a(1) =35a(2) =431a(3) =5183a(4) =62207a(5) =746495a(6) =8957951a(7) =107495423a(8) =1289945087a(9) =15479341055a(10) =185752092671