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

Terms

a(0) =0a(1) =2a(2) =24a(3) =218a(4) =1776a(5) =13682a(6) =102024a(7) =745418a(8) =5374176a(9) =38400482a(10) =272709624a(11) =1928498618a(12) =13597146576a(13) =95668307282a(14) =672119557224