a(n) = (7-4*7^n)/3.

Terms

a(0) =1a(1) =-7a(2) =-63a(3) =-455a(4) =-3199a(5) =-22407a(6) =-156863a(7) =-1098055a(8) =-7686399a(9) =-53804807a(10) =-376633663a(11) =-2636435655a(12) =-18455049599a(13) =-129185347207a(14) =-904297430463