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A291868
Numbers k such that (47*10^k - 119)/9 is prime.
0
2, 3, 9, 15, 39, 45, 59, 137, 180, 224, 264, 476, 491, 504, 1905, 3144, 4784, 10893, 24483, 28602, 82706, 100283, 129789, 131886, 132125
OFFSET
1,1
COMMENTS
For k > 1, numbers k such that the digit 5 followed by k-2 occurrences of the digit 2 followed by the digits 09 is prime (see Example section).
a(26) > 2*10^5.
EXAMPLE
3 is in this sequence because (47*10^3 - 119)/9 = 5209 is prime.
Initial terms and associated primes:
a(1) = 2, 509;
a(2) = 3, 5209;
a(3) = 9, 5222222209;
a(4) = 15, 5222222222222209;
a(5) = 39, 5222222222222222222222222222222222222209; etc.
MATHEMATICA
Select[Range[1, 100000], PrimeQ[(47*10^# - 119)/9] &]
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Sep 04 2017
EXTENSIONS
a(22)-a(25) from Robert Price, May 30 2019
STATUS
approved