a(n) = 5*6^n+1.

Terms

a(0) =6a(1) =31a(2) =181a(3) =1081a(4) =6481a(5) =38881a(6) =233281a(7) =1399681a(8) =8398081a(9) =50388481a(10) =302330881a(11) =1813985281a(12) =10883911681a(13) =65303470081a(14) =391820820481