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A282139
Numbers k such that (22*10^k - 19)/3 is prime.
0
1, 2, 4, 6, 14, 15, 24, 48, 78, 112, 122, 334, 504, 872, 914, 1230, 1506, 2442, 4095, 5022, 10615, 11460, 18303, 33675, 47701, 55592, 84159, 146661, 269517, 287085
OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 7 followed by k - 2 occurrences of the digit 3 followed by the digits 27 is prime (see Example section).
a(31) > 3*10^5.
EXAMPLE
4 is in this sequence because (22*10^4 - 19)/3 = 73327 is prime.
Initial terms and associated primes:
a(1) = 1, 67;
a(2) = 2, 727;
a(3) = 4, 73327;
a(4) = 6, 7333327;
a(5) = 14, 733333333333327; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(22 * 10^# - 19)/3] &]
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Feb 06 2017
EXTENSIONS
a(28) from Robert Price, Apr 27 2019
a(29)-a(30) from Robert Price, Oct 26 2023
STATUS
approved