a(n) = 7^n - 5^n + 1^n.

Terms

a(0) =1a(1) =3a(2) =25a(3) =219a(4) =1777a(5) =13683a(6) =102025a(7) =745419a(8) =5374177a(9) =38400483a(10) =272709625a(11) =1928498619a(12) =13597146577a(13) =95668307283a(14) =672119557225