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A272271 Numbers k such that 7*10^k - 23 is prime. 0
1, 2, 3, 23, 29, 34, 35, 38, 52, 57, 61, 82, 186, 209, 251, 366, 394, 426, 786, 979, 1382, 2037, 4557, 8995, 12774, 19170, 21828, 23259, 32003, 41831, 44999, 56785, 76483, 97987, 110468 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 6 followed by k-2 occurrences of the digit 9 followed by the digits 77 is prime (see Example section).
a(36) > 3*10^5.
LINKS
Makoto Kamada, Search for 69w77.
EXAMPLE
3 is in this sequence because 7*10^3 - 23 = 6977 is prime.
Initial terms and associated primes:
a(1) = 1, 47;
a(2) = 2, 677;
a(3) = 3, 6977;
a(4) = 23, 699999999999999999999977;
a(5) = 29, 699999999999999999999999999977, etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[7*10^# - 23] &]
PROG
(PARI) is(n)=ispseudoprime(7*10^n - 23) \\ Charles R Greathouse IV, Jun 13 2017
CROSSREFS
Sequence in context: A280894 A042585 A143279 * A068887 A364142 A260128
KEYWORD
nonn,more
AUTHOR
Robert Price, Apr 24 2016
EXTENSIONS
a(35) from Robert Price, Jul 27 2019
STATUS
approved

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Last modified June 20 03:21 EDT 2024. Contains 373512 sequences. (Running on oeis4.)