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A258778 Least base b >= 2 such that prime(n) is an absolute prime in base b. 2
3, 2, 3, 2, 5, 3, 5, 5, 4, 4, 2, 7, 7, 6, 7, 4, 8, 8, 9, 6, 8, 9, 11, 7, 7, 9, 11, 11, 13, 10, 2, 10, 12, 11, 13, 17, 12, 11, 12, 9, 16, 9, 6, 13, 15, 10, 6, 11, 19, 12, 19, 13, 11, 16, 7, 17, 19, 19, 12, 7, 16, 19, 7, 10, 13, 19, 22, 7, 19, 19, 18, 18, 21, 10 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

a(n) < prime(n) for n > 1. This is true since prime(n) in base prime(n)-1 is written as 11 which is an absolute prime.

Conjecture: a(n) < prime(n)-1 for n > 2.

LINKS

Chai Wah Wu, Table of n, a(n) for n = 1..10000

Wikipedia, Permutable prime

EXAMPLE

a(78) = 13. prime(78) = 397 in base 10 and 397_10 = 247_13. Rearranging the digits in base 13, we get 274_13 = 433_10, 427_13 = 709_10, 472_13 = 769_10, 724_13 = 1213_10, 742_13 = 1237_10, all of which are prime.

CROSSREFS

Cf. A003459, A258706, A258802.

Sequence in context: A165258 A238393 A092962 * A112924 A267148 A230406

Adjacent sequences:  A258775 A258776 A258777 * A258779 A258780 A258781

KEYWORD

nonn

AUTHOR

Chai Wah Wu, Jun 11 2015

STATUS

approved

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Last modified March 25 20:10 EDT 2019. Contains 321477 sequences. (Running on oeis4.)