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A281641
Numbers k such that (5*10^k - 107)/3 is prime.
0
2, 4, 5, 7, 8, 10, 46, 60, 204, 324, 348, 1268, 1525, 2152, 2250, 3398, 4073, 4939, 6835, 12502, 26012, 28339, 50861, 53855, 84469, 86011
OFFSET
1,1
COMMENTS
For k > 1, numbers k such that the digit 1 followed by k-2 occurrences of the digit 6 followed by the digits 31 is prime (see Example section).
a(27) > 2*10^5.
EXAMPLE
4 is in this sequence because (5*10^4 - 107)/3 = 16631 is prime.
Initial terms and associated primes:
a(1) = 2, 131;
a(2) = 4, 16631;
a(3) = 5, 166631;
a(4) = 7, 16666631;
a(5) = 8, 166666631; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(5*10^# - 107)/3] &]
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Jan 25 2017
STATUS
approved