(8*7^n+1)/3.

Terms

a(0) =3a(1) =19a(2) =131a(3) =915a(4) =6403a(5) =44819a(6) =313731a(7) =2196115a(8) =15372803a(9) =107609619a(10) =753267331a(11) =5272871315a(12) =36910099203a(13) =258370694419