%I #15 Jul 31 2019 09:06:38
%S 17,691,113749
%N Numbers k such that (41^k + 1)/42 is prime.
%C Larger terms correspond to probable primes.
%H P. Bourdelais, <a href="https://listserv.nodak.edu/cgi-bin/wa.exe?A2=NMBRTHRY;417ab0d6.0906">A Generalized Repunit Conjecture</a>
%t Do[p=Prime[n]; If[ PrimeQ[(41^p + 1)/42], Print[p]], {n, 1, 1000000}]
%o (PARI) is(n)=ispseudoprime((41^n+1)/42)
%K nonn,bref,hard,more
%O 1,1
%A _Paul Bourdelais_, Jul 29 2019