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A293026
Numbers k such that 7*10^k - 83 is prime.
0
2, 3, 11, 15, 23, 26, 46, 68, 76, 111, 146, 147, 444, 843, 896, 1390, 2242, 3131, 4251, 4252, 6299, 58815, 93692
OFFSET
1,1
COMMENTS
For k > 1, numbers k such that the digit 6 followed by k-2 occurrences of the digit 9 followed by the digits 17 is prime (see Example section).
a(24) > 2*10^5.
EXAMPLE
3 is in this sequence because 7*10^3 - 83 = 6917 is prime.
Initial terms and associated primes:
a(1) = 2, 617;
a(2) = 3, 6917;
a(3) = 11, 699999999917;
a(4) = 15, 6999999999999917;
a(5) = 23, 699999999999999999999917; etc.
MATHEMATICA
Select[Range[1, 100000], PrimeQ[7*10^# - 83] &]
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Sep 28 2017
STATUS
approved