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%I #11 Jan 17 2019 13:44:09
%S 1,3,4,6,7,52,53,103,131,199,294,426,780,1144,1876,2001,3507,5737,
%T 6657,12558,28303,31608,60643,74741,124648
%N Numbers k such that (7*10^k + 197)/3 is prime.
%C For k>1, numbers such that the digit 2 followed by k-2 occurrences of the digit 3 followed by the digits 99 is prime (see Example section).
%C a(26) > 2*10^5.
%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 23w99.</a>
%e 4 is in this sequence because (7*10^4 + 197)/3 = 23399 is prime.
%e Initial terms and primes associated:
%e a(1) = 1, 89;
%e a(2) = 3, 2399;
%e a(3) = 4, 23399;
%e a(4) = 6, 2333399;
%e a(5) = 7, 23333399; etc.
%p select(k -> isprime((7*10^k+197)/3), [$1..10000]); # _Robert Israel_, Jul 20 2017
%t Select[Range[0, 100000], PrimeQ[(7*10^# + 197)/3] &]
%Y Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
%K nonn,more,hard
%O 1,2
%A _Robert Price_, Jul 19 2017
%E a(25) from _Robert Price_, Jul 17 2018
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