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A287687
Numbers k such that 9*10^k - 29 is prime.
0
1, 3, 5, 6, 15, 18, 21, 79, 291, 325, 359, 397, 455, 539, 735, 1164, 1653, 1795, 1829, 2622, 3798, 4013, 4795, 8823, 9349, 9366, 97247, 111201
OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 8 followed by k-2 occurrences of the digit 9 followed by the digits 71 is prime (see Example section).
a(29) > 2*10^5.
EXAMPLE
3 is in this sequence because 9*10^3 - 29 = 8971 is prime.
Initial terms and associated primes:
a(1) = 1, 61;
a(2) = 3, 8971;
a(3) = 5, 899971;
a(4) = 6, 8999971;
a(5) = 15, 8999999999999971; etc.
MATHEMATICA
Select[Range[1, 100000], PrimeQ[9*10^# - 29] &]
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, May 29 2017
EXTENSIONS
a(28) from Robert Price, Sep 25 2019
STATUS
approved