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%I #11 Jan 17 2019 13:57:13
%S 1,8,20,28,40,41,71,208,263,346,455,743,878,920,1451,2980,4678,7748,
%T 13301,106343,170932
%N Numbers k such that (46*10^k + 143)/9 is prime.
%C For k > 1, numbers such that the digit 5 followed by k-2 occurrences of the digit 1 followed by the digits 27 is prime (see Example section).
%C a(22) > 2*10^5.
%H Makoto Kamada, <a href="http://stdkmd.net/nrr">Factorization of near-repdigit-related numbers</a>.
%H Makoto Kamada, <a href="http://stdkmd.net/nrr/prime/primedifficulty.txt">Search for 51w27</a>
%e 8 is in this sequence because (46*10^8 + 143)/9 = 511111127 is prime.
%e Initial terms and primes associated:
%e a(1) = 1, 67;
%e a(2) = 8, 511111127;
%e a(3) = 20, 511111111111111111127;
%e a(4) = 28, 51111111111111111111111111127;
%e a(5) = 40, 51111111111111111111111111111111111111127; etc.
%t Select[Range[0, 100000], PrimeQ[(46*10^# + 143)/9] &]
%o (PARI) isok(k) = isprime((46*10^k + 143)/9); \\ _Michel Marcus_, Dec 01 2017
%Y Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
%K nonn,more,hard
%O 1,2
%A _Robert Price_, Nov 30 2017
%E a(20)-a(21) from _Robert Price_, Jan 17 2019
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