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A294379
Numbers k such that (62*10^k - 197)/9 is prime.
0
1, 5, 7, 14, 17, 40, 53, 95, 97, 113, 122, 125, 145, 374, 470, 485, 1685, 5324, 8035, 10745, 52448, 68059
OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 6 followed by k-2 occurrences of the digit 8 followed by the digits 67 is prime (see Example section).
a(23) > 2*10^5.
EXAMPLE
5 is in this sequence because (62*10^5 - 197)/9 = 6263 is prime.
Initial terms and associated primes:
a(1) = 1, 47;
a(2) = 5, 688867;
a(3) = 7, 68888867;
a(4) = 14, 688888888888867;
a(5) = 17, 688888888888888867; etc.
MATHEMATICA
Select[Range[1, 100000], PrimeQ[(62*10^# - 197)/9] &]
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Oct 29 2017
STATUS
approved