a(n) = -2*n + 7^n - 5^n.

Terms

a(0) =0a(1) =0a(2) =20a(3) =212a(4) =1768a(5) =13672a(6) =102012a(7) =745404a(8) =5374160a(9) =38400464a(10) =272709604a(11) =1928498596a(12) =13597146552a(13) =95668307256a(14) =672119557196