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A293278 Numbers k such that 9*10^k - 19 is prime. 0
1, 2, 5, 6, 7, 10, 11, 22, 24, 138, 140, 152, 698, 1180, 1270, 2970, 3932, 4972, 5982, 6287, 8936, 16014, 25186, 59862, 130998 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,2
COMMENTS
For k > 1, numbers such that the digit 8 followed by k-2 occurrences of the digit 9 followed by the digits 81 is prime (see Example section).
a(26) > 2*10^5.
LINKS
Makoto Kamada, Search for 89w81.
EXAMPLE
5 is in this sequence because 9*10^5 - 19 = 899981 is prime.
Initial terms and primes associated:
a(1) = 1, 71;
a(2) = 2, 881;
a(3) = 5, 899981;
a(4) = 6, 8999981;
a(5) = 7, 89999981; etc.
MATHEMATICA
Select[Range[1, 100000], PrimeQ[9*10^# - 19] &]
CROSSREFS
Sequence in context: A014528 A359375 A286751 * A087791 A334880 A285032
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Oct 04 2017
EXTENSIONS
a(25) from Robert Price, Jul 27 2019
STATUS
approved

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Last modified February 24 10:36 EST 2024. Contains 370294 sequences. (Running on oeis4.)