%I #24 May 31 2024 20:44:11
%S 2,6,8,21,66,84,86,89,93,104,170,249,273,413,824,1392,1589,1679,3125,
%T 3185,3482,4089,4772,5138,5738,54761,86364
%N Numbers k such that (31*10^k - 121) / 9 is prime.
%C Variant of repunit primes: each value is 3444...44431, with k-2 4's.
%C a(28) > 2*10^5. - _Robert Price_, Jun 07 2016
%H Makoto Kamada, <a href="https://stdkmd.net/nrr">Factorization of near-repdigit-related numbers</a>.
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/prime/prime_difficulty.txt">Search for 34w31</a>.
%e 6 is a term because 3444431 is prime.
%e 8 is a term because 344444431 is prime.
%t Select[Range[1200],PrimeQ[(31*10^# - 121)/9]&] (* _Vladimir Joseph Stephan Orlovsky_, Feb 01 2012 *)
%o (PARI) is(n)=ispseudoprime((31*10^n - 121)/9) \\ _Charles R Greathouse IV_, Jun 08 2016
%K nonn,more
%O 1,1
%A Joao da Silva (zxawyh66(AT)yahoo.com), Oct 31 2005
%E Edited by _Don Reble_, Nov 09 2005
%E a(16) - a(23) from _Vladimir Joseph Stephan Orlovsky_, Feb 01 2012
%E a(24) - a(27) from _Robert Price_, Jun 07 2016