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) primality-testing program. - Hugo Pfoertner, Nov 14 2019
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
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