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A289753 Numbers k such that 2*10^k - 43 is prime. 0
2, 6, 8, 13, 24, 38, 42, 47, 324, 338, 3957, 4243, 4623, 4692, 4824, 5741, 7611, 8403, 10499, 21386, 56083, 132493, 135083, 136057, 138162, 193981 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,1
COMMENTS
For k>1, numbers such that the digit 1 followed by k-2 occurrences of the digit 9 followed by the digits 57 is prime (see Example section).
a(27) > 2*10^5.
LINKS
Makoto Kamada, Search for 19w57.
EXAMPLE
6 is in this sequence because 2*10^6 - 43 = 1999957 is prime.
Initial terms and primes associated:
a(1) = 2, 157;
a(2) = 6, 1999957;
a(3) = 8, 199999957;
a(4) = 13, 19999999999957;
a(5) = 24, 1999999999999999999999957; etc.
MATHEMATICA
Select[Range[2, 100000], PrimeQ[2*10^# - 43] &]
CROSSREFS
Sequence in context: A294862 A087327 A266627 * A002511 A074383 A107505
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Jul 11 2017
EXTENSIONS
a(22)-a(26) from Robert Price, Apr 11 2018
STATUS
approved

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Last modified March 29 00:26 EDT 2024. Contains 371264 sequences. (Running on oeis4.)