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A294572
Numbers k such that 8*10^k - 79 is prime.
0
5, 6, 13, 28, 64, 199, 233, 364, 457, 695, 1259, 1267, 1339, 1962, 2887, 3897, 6547, 7881, 11629, 13777, 15229, 17956, 24497, 48196
OFFSET
1,1
COMMENTS
For k > 1, numbers k such that the digit 7 followed by k-2 occurrences of the digit 9 followed by the digits 21 is prime (see Example section).
a(25) > 2*10^5.
EXAMPLE
5 is in this sequence because 8*10^5 - 79 = 799921 is prime.
Initial terms and associated primes:
a(1) = 5, 799921;
a(2) = 6, 7999921;
a(3) = 13, 79999999999921;
a(4) = 28, 79999999999999999999999999921; etc.
MATHEMATICA
Select[Range[1, 100000], PrimeQ[8*10^# - 79] &]
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Nov 02 2017
STATUS
approved