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A291608
Numbers k such that 56*10^k - 3 is prime.
0
0, 1, 3, 6, 10, 18, 21, 34, 44, 105, 154, 163, 189, 382, 421, 2473, 2664, 4590, 9804, 19950, 29992, 38457, 40249, 124656, 185139, 195032
OFFSET
1,3
COMMENTS
For k > 0, numbers k such that the digits 55 followed by k-1 occurrences of the digit 9 followed by the digit 7 is prime (see Example section).
a(27) > 2*10^5.
EXAMPLE
3 is in this sequence because 56*10^3 - 3 = 55997 is prime.
Initial terms and associated primes:
a(1) = 0, 53;
a(2) = 1, 557;
a(3) = 3, 55997;
a(4) = 6, 55999997;
a(5) = 10, 559999999997; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[56*10^# - 3] &]
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Aug 27 2017
EXTENSIONS
a(24)-a(26) from Robert Price, Mar 17 2020
STATUS
approved