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A281512 Numbers k such that 5*10^k - 81 is prime. 0
2, 3, 4, 13, 15, 22, 24, 28, 111, 148, 248, 294, 476, 1017, 1875, 2061, 2068, 2593, 4462, 6078, 14457, 14527, 23679, 33356, 36484, 50890, 51608, 113224 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,1
COMMENTS
For k>1, numbers such that the digit 4 followed by k-2 occurrences of the digit 9 followed by the digits 19 is prime (see Example section).
a(29) > 2*10^5.
LINKS
Makoto Kamada, Search for 49w19.
EXAMPLE
3 is in this sequence because 5*10^3 - 81 = 4919 is prime.
Initial terms and primes associated:
a(1) = 2, 419;
a(2) = 3, 4919;
a(3) = 4, 49919;
a(4) = 13, 49999999999919;
a(5) = 15, 4999999999999919; etc.
MATHEMATICA
Select[Range[1, 100000], PrimeQ[5*10^# - 81] &]
CROSSREFS
Sequence in context: A338478 A126129 A102947 * A295396 A236440 A357954
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Apr 13 2017
EXTENSIONS
a(28) from Robert Price, Jan 21 2019
STATUS
approved

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