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A270831 Numbers k such that (7*10^k + 71)/3 is prime. 498

%I

%S 1,2,3,4,5,7,23,29,37,39,40,89,115,189,227,253,301,449,533,607,969,

%T 1036,1207,1407,1701,3493,7147,11342,21638,22327,25575,25648,34079,

%U 39974,47719,49913,74729,100737,103531,168067

%N Numbers k such that (7*10^k + 71)/3 is prime.

%C For n>1, numbers such that the digit 2 followed by n-2 occurrences of the digit 3 followed by the digits 57 is prime (see Example section).

%C a(41) > 2*10^5.

%H Makoto Kamada, <a href="https://stdkmd.net/nrr/prime/primedifficulty.txt">Search for 23w57.</a>

%e 3 is in this sequence because (7*10^3 + 71)/3 = 2357 is prime.

%e Initial terms and primes associated:

%e a(1) = 1, 47;

%e a(2) = 2, 257;

%e a(3) = 3, 2357;

%e a(4) = 4, 23357;

%e a(5) = 5, 233357, etc.

%t Select[Range[0, 100000], PrimeQ[(7*10^# + 71)/3] &]

%o (PARI) lista(nn) = {for(n=1, nn, if(ispseudoprime((7*10^n + 71)/3), print1(n, ", "))); } \\ _Altug Alkan_, Mar 23 2016

%Y Cf. A056654, A268448, A269303, A270339, A270613.

%K nonn

%O 1,2

%A _Robert Price_, Mar 23 2016

%E a(38)-a(40) from _Robert Price_, May 21 2018

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Last modified August 8 06:27 EDT 2020. Contains 336290 sequences. (Running on oeis4.)