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A286916 Positive numbers n such that (10^(n+2)*331-133)/9 is prime. 2
7, 13, 23, 409, 1147, 2597, 4561 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Or '36'||...'7'...||'63' in decimal form is prime ('7' concatenated n times to which the prefix '36' and the suffix '63' are concatenated once).

a(1)...a(4) and a(7) themselves are primes, a(8)>50000 (if it exists).

LINKS

Table of n, a(n) for n=1..7.

EXAMPLE

a(1)=7 as 36777777763 is prime (as string, it consists '7' concatenated 7 times to which the prefix '36' and the suffix '63' are concatenated once).

MATHEMATICA

ParallelMap[ If[ PrimeQ[ (10^(#+2)*331-133)/9], #, Nothing]&, Range[5000]]

PROG

(PARI) is(n)=ispseudoprime((10^(n+2)*331-133)/9) \\ Charles R Greathouse IV, Jun 13 2017

CROSSREFS

Sequence in context: A316118 A265766 A228727 * A032690 A188557 A090992

Adjacent sequences:  A286913 A286914 A286915 * A286917 A286918 A286919

KEYWORD

nonn,base,hard,more

AUTHOR

Mikk Heidemaa, May 17 2017

STATUS

approved

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Last modified September 15 08:13 EDT 2019. Contains 327062 sequences. (Running on oeis4.)