a(n) = 4*a(n-1) - 4 for n>0, a(0)=3.

Terms

a(0) =3a(1) =8a(2) =28a(3) =108a(4) =428a(5) =1708a(6) =6828a(7) =27308a(8) =109228a(9) =436908a(10) =1747628a(11) =6990508a(12) =27962028a(13) =111848108a(14) =447392428a(15) =1789569708a(16) =7158278828a(17) =28633115308a(18) =114532461228a(19) =458129844908