a(n) = (1 - (-6)^n)/7.

Terms

a(0) =1a(1) =-5a(2) =31a(3) =-185a(4) =1111a(5) =-6665a(6) =39991a(7) =-239945a(8) =1439671a(9) =-8638025a(10) =51828151a(11) =-310968905a(12) =1865813431a(13) =-11194880585a(14) =67169283511a(15) =-403015701065