a(n) = (1 - (-8)^n)/9.

Terms

a(0) =1a(1) =-7a(2) =57a(3) =-455a(4) =3641a(5) =-29127a(6) =233017a(7) =-1864135a(8) =14913081a(9) =-119304647a(10) =954437177a(11) =-7635497415a(12) =61083979321a(13) =-488671834567