OFFSET
1,1
COMMENTS
Conjecture: All terms are prime.
a(9) > 10^5.
MATHEMATICA
Select[ Prime[ Range[1, 100000] ], PrimeQ[ (17^# + 4^#)/21 ]& ]
PROG
(PARI) is(n)=ispseudoprime((17^n+4^n)/21) \\ Charles R Greathouse IV, May 22 2017
CROSSREFS
KEYWORD
hard,nonn,more
AUTHOR
Robert Price, Aug 25 2013
STATUS
approved