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A109080 Numbers n such that 5^n - 2 is prime. 19
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 Fröhlich_, Jun 05 2014
CROSSREFS
Sequence in context: A194411 A017545 A280268 * A295083 A180877 A032461
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 June 17 13:58 EDT 2024. Contains 373445 sequences. (Running on oeis4.)