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 A109080 Numbers n such that 5^n - 2 is prime. 18
 1, 2, 14, 26, 50, 126, 144, 260, 624, 1424, 10472, 19784, 24558, 797390 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS No more terms through 18000. No more terms through 50000. - Ryan Propper, Dec 10 2008 a(14) > 10^5. - Robert Price, Feb 03 2014 a(14) > 185000. - Tyler NeSmith, Dec 13 2020 Searched 185000 to 800000. - Paul Bourdelais, Mar 03 2021 LINKS F. Firoozbakht, M. F. Hasler, Variations on Euclid's formula for Perfect Numbers, JIS 13 (2010) #10.3.1 EXAMPLE 5^14 - 2 = 6103515623 is prime, hence 14 is in the sequence. MATHEMATICA Do[If[PrimeQ[5^n - 2], Print[n]], {n, 1, 18000}] PROG (PARI) for(n=1, 10^5, if(ispseudoprime(5^n-2), print1(n, ", "))) \\ Felix FrÃ¶hlich, Jun 05 2014 CROSSREFS Sequence in context: A194411 A017545 A280268 * A295083 A180877 A032461 Adjacent sequences:  A109077 A109078 A109079 * A109081 A109082 A109083 KEYWORD hard,nonn AUTHOR Ryan Propper, Jun 18 2005 EXTENSIONS Two more terms from Ryan Propper, Dec 10 2008 a(14) from Paul Bourdelais, Mar 03 2021 STATUS approved

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Last modified April 12 17:13 EDT 2021. Contains 342929 sequences. (Running on oeis4.)