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A293685
Numbers k such that (25*10^k - 67)/3 is prime.
0
1, 2, 3, 4, 39, 63, 64, 82, 129, 441, 685, 947, 1042, 1063, 1840, 2311, 8200, 9641, 15982, 17643, 45389, 54443, 100343, 102076, 124076
OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 8 followed by k-2 occurrences of the digit 3 followed by the digits 11 is prime (see Example section).
a(26) > 2*10^5.
EXAMPLE
2 is in this sequence because (25*10^2 - 67)/3 = 811 is prime.
Initial terms and associated primes:
a(1) = 1, 61;
a(2) = 2, 811;
a(3) = 3, 8311;
a(4) = 4, 83311;
a(5) = 39, 8333333333333333333333333333333333333311; etc.
MATHEMATICA
Select[Range[1, 100000], PrimeQ[(25*10^# - 67)/3] &]
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Nov 10 2017
EXTENSIONS
a(23)-a(25) from Robert Price, Jun 19 2019
STATUS
approved