%I #10 Nov 11 2024 14:14:40

%S 5,31,271,929,2789,4153

%N Numbers k such that (11^k - 9^k)/2 is prime.

%C All terms are prime.

%C The corresponding primes: 51001, 95780952266636767336259095696501, ...

%C a(7) > 50000. - _Michael S. Branicky_, Nov 11 2024

%t Select[Range[1, 10000], PrimeQ[(11^# - 9^#)/2] &]

%o (PARI) for(n=1, 10000, if(isprime((11^n - 9^n)/2), print1(n, ", ")))

%Y Cf. A005808, A210506, A128027, A216181, A128347, A273598, A273599, A273600, A062577.

%K nonn,more,hard

%O 1,1

%A _Tim Johannes Ohrtmann_, May 26 2016