a(n) = n*7^n - 1.

Terms

a(0) =6a(1) =97a(2) =1028a(3) =9603a(4) =84034a(5) =705893a(6) =5764800a(7) =46118407a(8) =363182462a(9) =2824752489a(10) =21750594172a(11) =166095446411