

A082387


Numbers k such that (5^k + 2^k)/7 is prime.


25



3, 17, 19, 47, 101, 1709, 2539, 5591, 6037, 8011, 19373, 26489, 27427
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OFFSET

1,1


COMMENTS

Next term > 35000.  Giovanni Resta, Apr 06 2006
For (2^k + 5^k)/7 to be a prime, k must be a prime too.  Alexander Adamchuk, Feb 06 2007
No other terms less than 100000.  Robert Price, Apr 17 2012
Some of the results were computed using the PrimeFormGW (PFGW) primalitytesting program.  Hugo Pfoertner, Nov 14 2019


LINKS

Table of n, a(n) for n=1..13.


EXAMPLE

a(1)=3 because (5^3 + 2^3)/7 = (125 + 8)/7 = 19 is prime.


MATHEMATICA

Do[f=(2^n+5^n)/7; If[PrimeQ[f], Print[{n, f}]], {n, 1, 35000}] (* Alexander Adamchuk, Feb 06 2007 *)
Select[Range[0, 10000], PrimeQ[(5^# + 2^#)/7] &] (* G. C. Greubel, Nov 02 2018 *)


PROG

(PARI) is(n)=ispseudoprime((5^n + 2^n)/7) \\ Charles R Greathouse IV, Apr 28 2015
(MAGMA) [n : n in [1..10000]  IsPrime((5^n+2^n) div 7)]; // G. C. Greubel, Nov 02 2018


CROSSREFS

Cf. A057469 ((2^k + 3^k)/5 is prime).
Sequence in context: A103088 A226925 A259772 * A032923 A018750 A177208
Adjacent sequences: A082384 A082385 A082386 * A082388 A082389 A082390


KEYWORD

more,nonn


AUTHOR

Hugo Pfoertner, May 22 2003


EXTENSIONS

More terms from Hugo Pfoertner, Jun 23 2003
More terms from Giovanni Resta, Apr 06 2006


STATUS

approved



