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A231605 The least number k such that the decimal number formed from 1 followed by n copies of k is prime. 2

%I

%S 1,9,7,37,9,13,9,47,7,13,11,17,7,87,3,63,147,1,67,137,21,1,23,19,163,

%T 9,9,57,371,63,51,351,17,471,89,257,39,81,109,63,3,7,973,47,39,109,

%U 737,201,429,143,7,261,9,7,129,221,69,63,167,77,493,459,587,183,129

%N The least number k such that the decimal number formed from 1 followed by n copies of k is prime.

%C When a(n)=1, we get a prime repunit and these indices+1 give A004023. - _Michel Marcus_, Nov 13 2013

%H T. D. Noe, <a href="/A231605/b231605.txt">Table of n, a(n) for n = 1..1000</a>

%e For n = 2, the numbers to consider are 111, 122, 133, 144, 155, 166, 177, 188, 199,.... The first prime is 199.

%t Table[k = 1; While[p = FromDigits[Join[{1}, Flatten[Table[IntegerDigits[k], {n}]]]]; ! PrimeQ[p], k = k + 2]; k, {n, 65}]

%Y Cf. A112733 (smallest prime of the form 1 followed by n copies of k).

%K nonn,base

%O 1,2

%A _T. D. Noe_, Nov 13 2013

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Last modified June 17 17:03 EDT 2019. Contains 324196 sequences. (Running on oeis4.)