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A294378
Numbers k such that (197*10^k - 11)/3 is prime.
0
1, 2, 5, 11, 24, 49, 50, 59, 67, 71, 91, 605, 726, 899, 1214, 3307, 3409, 3961, 11426, 30096, 70241, 80089, 108446
OFFSET
1,2
COMMENTS
For k > 0, numbers k such that the digits 65 followed by k-1 occurrences of the digit 6 followed by the digit 3 is prime (see Example section).
a(24) > 2*10^5.
EXAMPLE
2 is in this sequence because (197*10^2 - 11)/3 = 6563 is prime.
Initial terms and associated primes:
a(1) = 1, 653;
a(2) = 2, 6563;
a(3) = 5, 6566663;
a(4) = 11, 6566666666663;
a(5) = 24, 65666666666666666666666663; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(197*10^# - 11)/3] &]
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Oct 29 2017
EXTENSIONS
a(23) from Robert Price, Mar 22 2020
STATUS
approved