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A364644
Numbers k such that floor(10^k/7) is prime.
1
7, 25, 355, 823
OFFSET
1,1
COMMENTS
Numbers k such that A090519(k) = 7.
All terms == 1 (mod 6).
Numbers k such that (10^k-3)/7 is prime.
a(5) > 20000 if it exists. - Hugo Pfoertner, Jul 31 2023
EXAMPLE
a(1) = 7 is a term because floor(10^7/7) = 1428571 is prime.
MAPLE
select(n -> isprime(floor(10^n/7)), [seq(i, i=1..1000, 6)]);
CROSSREFS
Cf. A090519.
Sequence in context: A197651 A198022 A231579 * A197721 A261421 A081298
KEYWORD
nonn,more,hard
AUTHOR
Robert Israel, Jul 31 2023
STATUS
approved