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A287017
Positive numbers k such that (10^(k+2)*109 + 89)/9 is prime.
0
5, 13, 41, 107, 3049, 52727
OFFSET
1,1
COMMENTS
Or '12'||...'1'...||'21' in decimal form is prime ('1' concatenated k times to which the prefix '12' and the suffix '21' are concatenated once).
a(1)...a(6) themselves are primes. a(7) > 70000 (if it exists).
EXAMPLE
5 is a term as 121111121 is prime (as a string, it consists of '1' concatenated 5 times to which the prefix '12' and the suffix '21' are concatenated once).
MATHEMATICA
ParallelMap[ If[ PrimeQ[ (10^(#+2)*109+89)/9], #, Nothing]&, Range[3500]]
PROG
(PARI) is(n)=ispseudoprime((10^(n+2)*109+89)/9) \\ Charles R Greathouse IV, Jun 13 2017
CROSSREFS
Sequence in context: A034735 A305464 A200150 * A229747 A182300 A080925
KEYWORD
nonn,hard,more,base
AUTHOR
Mikk Heidemaa, May 19 2017
STATUS
approved