a(n) is smallest prime > 2*a(n-1), a(1) = 13.

Terms

a(0) =13a(1) =29a(2) =59a(3) =127a(4) =257a(5) =521a(6) =1049a(7) =2099a(8) =4201a(9) =8419a(10) =16843a(11) =33703a(12) =67409a(13) =134837a(14) =269683a(15) =539389a(16) =1078787a(17) =2157587a(18) =4315183a(19) =8630387a(20) =17260781a(21) =34521589a(22) =69043189a(23) =138086407a(24) =276172823a(25) =552345671a(26) =1104691373a(27) =2209382761