

A231605


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


2



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, 9, 9, 57, 371, 63, 51, 351, 17, 471, 89, 257, 39, 81, 109, 63, 3, 7, 973, 47, 39, 109, 737, 201, 429, 143, 7, 261, 9, 7, 129, 221, 69, 63, 167, 77, 493, 459, 587, 183, 129
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OFFSET

1,2


COMMENTS

When a(n)=1, we get a prime repunit and these indices+1 give A004023.  Michel Marcus, Nov 13 2013


LINKS

T. D. Noe, Table of n, a(n) for n = 1..1000


EXAMPLE

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


MATHEMATICA

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


CROSSREFS

Cf. A112733 (smallest prime of the form 1 followed by n copies of k).
Sequence in context: A124050 A107663 A298780 * A248307 A168159 A038297
Adjacent sequences: A231602 A231603 A231604 * A231606 A231607 A231608


KEYWORD

nonn,base


AUTHOR

T. D. Noe, Nov 13 2013


STATUS

approved



