(9*5^n+1)/2.

Terms

a(0) =5a(1) =23a(2) =113a(3) =563a(4) =2813a(5) =14063a(6) =70313a(7) =351563a(8) =1757813a(9) =8789063a(10) =43945313a(11) =219726563a(12) =1098632813a(13) =5493164063a(14) =27465820313a(15) =137329101563a(16) =686645507813