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URL: https://oeis.org/A113916

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A113916
Numbers k such that (p^k-1)/(p-1) is prime where p is the k-th prime.
0
3, 7, 19, 31, 307, 421
OFFSET
1,1
COMMENTS
Note that k must itself be a prime.
a(7) > 20000 if it exists. - Michael S. Branicky, Dec 29 2025
MATHEMATICA
Do[ p = Prime[n]; If[PrimeQ[(p^n - 1)/(p - 1)], Print[n]], {n, 1800}] (* Robert G. Wilson v, Jan 30 2006 *)
CROSSREFS
Sequence in context: A102271 A145039 A112633 * A227502 A268065 A049490
KEYWORD
nonn,hard,more
AUTHOR
Joao da Silva (zxawyh66(AT)yahoo.com), Jan 30 2006
STATUS
approved