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A109355 Numbers k such that the string 23572357k is prime. 1

%I

%S 7,19,31,37,43,57,61,103,169,213,229,243,273,337,339,349,357,387,421,

%T 441,453,463,469,483,531,541,559,577,583,597,621,631,637,643,681,687,

%U 729,759,789,819,877,901,927,931,937,1011,1033,1047,1051,1071,1077,1093,1101,1141,1143,1201

%N Numbers k such that the string 23572357k is prime.

%H Robert Israel, <a href="/A109355/b109355.txt">Table of n, a(n) for n = 1..10000</a>

%e k=7 is in the sequence because 23572357k = 235723577 is prime.

%e k=213 is in the sequence because 23572357k = 23572357213 is prime.

%p q:= 23572357:

%p select(n -> isprime(q*10^(1+ilog10(n))+n), [seq(i,i=1..10000,2)]); # _Robert Israel_, Jan 22 2018

%t Select[Range[1250], PrimeQ[23572357*10^IntegerLength[#] + #] &] (* _Michael De Vlieger_, Jan 22 2018 *)

%o (MAGMA) [ n: n in [1..1750] | IsPrime(Seqint(Intseq(n) cat [7, 5, 3, 2, 7, 5, 3, 2])) ]; // _Vincenzo Librandi_, Feb 02 2011

%K nonn,base

%O 1,1

%A _Parthasarathy Nambi_, Aug 22 2005

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Last modified September 18 22:34 EDT 2021. Contains 347548 sequences. (Running on oeis4.)