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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
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 (list; graph; refs; listen; history; text; internal format)
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

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Last modified May 19 06:48 EDT 2019. Contains 323386 sequences. (Running on oeis4.)