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A110053 Numbers n such that the string 111n is the decimal expansion of a prime number. 2

%I

%S 7,13,17,19,31,49,59,61,71,73,77,97,103,109,119,121,127,143,149,187,

%T 191,211,217,227,229,253,263,269,271,301,317,323,337,341,347,373,409,

%U 427,431,439,443,467,487,491,493,497,509,521,533,539,577,581,593,599

%N Numbers n such that the string 111n is the decimal expansion of a prime number.

%H Vincenzo Librandi, <a href="/A110053/b110053.txt">Table of n, a(n) for n = 1..2500</a>

%e 7 is in the sequence because 1117 is prime.

%e 73 is in the sequence because 11173 is prime.

%e 103 is in the sequence because 111103 is prime.

%p a:= proc(n) local k; for k from 1 +`if`(n=1, 0, a(n-1))

%p while not isprime(parse(""||111||k)) do od; a(n):=k

%p end:

%p seq(a(n), n=1..100); # _Alois P. Heinz_, Jul 30 2014

%t Select[Range[1000], PrimeQ[FromDigits[Join[{1, 1, 1}, IntegerDigits[ # ]]]] &] (* _Alonso Delarte_, Sep 06 2005 *)

%t Select[Range[600], PrimeQ[111 10^IntegerLength[#] + #] &] (* _Vincenzo Librandi_, Jul 30 2014 *)

%o (MAGMA) [ n: n in [1..600] | IsPrime(Seqint(Intseq(n) cat [1, 1, 1])) ];

%K nonn,base

%O 1,1

%A _Parthasarathy Nambi_, Sep 04 2005

%E More terms from _Alonso del Arte_, Sep 06 2005

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Last modified May 15 22:33 EDT 2021. Contains 343937 sequences. (Running on oeis4.)