a(0) = 6; thereafter a(n) = 4*a(n-1) + 1.

Terms

a(0) =6a(1) =25a(2) =101a(3) =405a(4) =1621a(5) =6485a(6) =25941a(7) =103765a(8) =415061a(9) =1660245a(10) =6640981a(11) =26563925a(12) =106255701a(13) =425022805a(14) =1700091221a(15) =6800364885a(16) =27201459541a(17) =108805838165a(18) =435223352661