a(n) = 7^n - a(n-1) for n>0, a(0)=0.

Terms

a(0) =0a(1) =7a(2) =42a(3) =301a(4) =2100a(5) =14707a(6) =102942a(7) =720601a(8) =5044200a(9) =35309407a(10) =247165842a(11) =1730160901a(12) =12111126300a(13) =84777884107a(14) =593445188742