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URL: https://oeis.org/A288149

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A288149
Numbers k such that (68*10^k - 257)/9 is prime.
0
1, 2, 4, 8, 11, 20, 22, 32, 40, 62, 95, 104, 569, 710, 955, 1682, 2933, 4592, 10286, 16634, 19235, 20437, 31024, 48304, 79813, 128645, 148060
OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 7 followed by k-2 occurrences of the digit 5 followed by the digits 27 is prime (see Example section).
a(28) > 2*10^5.
EXAMPLE
4 is in this sequence because (68*10^4 - 257)/9 = 75527 is prime.
Initial terms and associated primes:
a(1) = 1, 47;
a(2) = 2, 727;
a(3) = 4, 75527;
a(4) = 8, 755555527;
a(5) = 11, 755555555527; etc.
MATHEMATICA
Select[Range[1, 100000], PrimeQ[(68*10^# - 257)/9] &]
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Jun 05 2017
EXTENSIONS
a(26)-a(27) from Robert Price, Jul 27 2019
STATUS
approved