(3^(4*n - 1) - 1)/2.
13, 1093, 88573, 7174453, 581130733, 47071589413, 3812798742493, 308836698141973, 25015772549499853, 2026277576509488133, 164128483697268538813, 13294407179478751643893, 1076846981537778883155373, 87224605504560089535585253, 7065193045869367252382405533
1
Bisection of A096053. These numbers appear to be the infinite part of the base-9 case of S000376.
T. D. Noe, Plot of 100 terms
T. D. Noe, Table of 100 terms
(Mma) Table[(3^(4*n - 1) - 1)/2, {n, 15}]
nonn
T. D. Noe, Nov 23 2014