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A280016
Numbers k such that 8*10^k - 99 is prime.
0
2, 3, 4, 9, 22, 50, 67, 82, 87, 89, 130, 134, 170, 179, 220, 832, 1425, 1648, 2356, 2434, 6556, 11391, 12610, 20278, 46509, 50253
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 01 is prime (see Example section).
a(27) > 3*10^5.
EXAMPLE
3 is in this sequence because 8*10^3-99 = 7901 is prime.
Initial terms and associated primes:
a(1) = 2, 701;
a(2) = 3, 7901;
a(3) = 4, 79901;
a(4) = 9, 7999999901;
a(5) = 22, 79999999999999999999901; etc.
MATHEMATICA
Select[Range[2, 100000], PrimeQ[8*10^#-99] &]
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Feb 21 2017
STATUS
approved