a(n) = 4*12^n - 1.

Terms

a(0) =3a(1) =47a(2) =575a(3) =6911a(4) =82943a(5) =995327a(6) =11943935a(7) =143327231a(8) =1719926783a(9) =20639121407a(10) =247669456895