a(n) = largest prime q such that q | 2^p - 2 and p - 1 | q - 1, where p = prime(n).

Terms

a(0) =2a(1) =3a(2) =5a(3) =7a(4) =31a(5) =13a(6) =257a(7) =73a(8) =683a(9) =113a(10) =331a(11) =109a(12) =61681a(13) =5419a(14) =2796203a(15) =1613a(16) =3033169a(17) =1321a(18) =599479a(19) =122921a(20) =38737a(21) =22366891a(22) =8831418697a(23) =2931542417a(24) =22253377a(25) =268501a(26) =131071a(28) =279073a(29) =54410972897a(30) =77158673929