a(n) = 3*7^n.

Terms

a(0) =3a(1) =21a(2) =147a(3) =1029a(4) =7203a(5) =50421a(6) =352947a(7) =2470629a(8) =17294403a(9) =121060821a(10) =847425747a(11) =5931980229a(12) =41523861603a(13) =290667031221