First differences of 7^n (A000420).

Terms

a(0) =1a(1) =6a(2) =42a(3) =294a(4) =2058a(5) =14406a(6) =100842a(7) =705894a(8) =4941258a(9) =34588806a(10) =242121642a(11) =1694851494a(12) =11863960458a(13) =83047723206a(14) =581334062442