a(n) = (1 - (-7)^n)/8.

Terms

a(0) =1a(1) =-6a(2) =43a(3) =-300a(4) =2101a(5) =-14706a(6) =102943a(7) =-720600a(8) =5044201a(9) =-35309406a(10) =247165843a(11) =-1730160900a(12) =12111126301a(13) =-84777884106a(14) =593445188743