

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
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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

Table of n, a(n) for n=1..14.
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^n2), 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



