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A294632
Numbers k such that 7*10^k - 99 is prime.
0
2, 8, 14, 30, 43, 61, 79, 84, 109, 121, 203, 211, 215, 224, 318, 392, 1061, 1304, 1535, 1559, 6989, 9201, 13485, 35462, 39408, 43136, 70680, 126600, 132884, 171535
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 01 is prime (see Example section).
a(31) > 2*10^5.
EXAMPLE
2 is in this sequence because 7*10^2 - 99 = 601 is prime.
Initial terms and associated primes:
a(1) = 2, 601;
a(2) = 8, 699999901;
a(3) = 14, 699999999999901;
a(4) = 30, 6999999999999999999999999999901;
a(5) = 43, 69999999999999999999999999999999999999999901; etc.
MATHEMATICA
Select[Range[2, 100000], PrimeQ[7*10^# - 99] &]
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Nov 05 2017
EXTENSIONS
a(28)-a(30) from Robert Price, Jun 29 2019
STATUS
approved