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A293277
Numbers k such that (77*10^k - 257)/9 is prime.
0
2, 3, 5, 8, 9, 23, 24, 32, 69, 96, 243, 381, 2237, 2426, 2427, 4041, 5195, 5516, 7053, 9690, 33933, 66237, 93249
OFFSET
1,1
COMMENTS
For k > 1, numbers k such that the digit 8 followed by k-2 occurrences of the digit 5 followed by the digits 27 is prime (see Example section).
a(24) > 2*10^5.
EXAMPLE
3 is in this sequence because (77*10^3 - 257)/9 = 8527 is prime.
Initial terms and associated primes:
a(1) = 2, 827;
a(2) = 3, 8527;
a(3) = 5, 855527;
a(4) = 8, 855555527;
a(5) = 9, 8555555527; etc.
MATHEMATICA
Select[Range[1, 100000], PrimeQ[(77*10^# - 257)/9] &]
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Oct 04 2017
STATUS
approved