a(n) = 7*Sum_{k=0..n} 6^k.

Terms

a(0) =7a(1) =49a(2) =301a(3) =1813a(4) =10885a(5) =65317a(6) =391909a(7) =2351461a(8) =14108773a(9) =84652645a(10) =507915877a(11) =3047495269a(12) =18284971621a(13) =109709829733a(14) =658258978405