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Appears to be the numbers k such that the sequence 5^n mod k has period length k-1. All terms are congruent to 2 or 3 mod 5. - Gary Detlefs, May 21 2014
Vincenzo Librandi, Table of n, a(n) for n = 1..1000
Index entries for primes by primitive root
pr=5; Select[Prime[Range], MultiplicativeOrder[pr, # ] == #-1 &]
Sequence in context: A045333 A040141 A235627 * A113425 A245590 A110480
Adjacent sequences: A019332 A019333 A019334 * A019336 A019337 A019338
David W. Wilson