a(n) = phi(6^n).

Terms

a(0) =1a(1) =2a(2) =12a(3) =72a(4) =432a(5) =2592a(6) =15552a(7) =93312a(8) =559872a(9) =3359232a(10) =20155392a(11) =120932352a(12) =725594112a(13) =4353564672a(14) =26121388032a(15) =156728328192a(16) =940369969152