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A111247 Numbers k such that (31*10^k - 121) / 9 is prime. 0

%I #21 Jan 17 2019 13:44:08

%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/primedifficulty.txt">Search for 34w31.</a>

%e 6 is there because 3444431 is prime.

%e 8 is there 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

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Last modified March 28 13:42 EDT 2024. Contains 371254 sequences. (Running on oeis4.)