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A293910
Numbers k such that (26*10^k - 173)/3 is prime.
0
1, 2, 3, 8, 11, 13, 17, 28, 46, 67, 136, 536, 619, 746, 2420, 2672, 8228, 10861, 13424, 16047, 18075, 37720, 56371, 75055
OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 8 followed by k-2 occurrences of the digit 6 followed by the digits 09 is prime (see Example section).
a(25) > 2*10^5.
EXAMPLE
2 is in this sequence because (26*10^2 - 173)/3 = 809 is prime.
Initial terms and associated primes:
a(1) = 1, 29;
a(2) = 2, 809;
a(3) = 3, 8609;
a(4) = 8, 866666609;
a(5) = 11, 866666666609; etc.
MATHEMATICA
Select[Range[1, 100000], PrimeQ[(26*10^# - 173)/3] &]
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Oct 19 2017
STATUS
approved