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A280969
Numbers k such that 7*10^k - 17 is prime.
0
1, 2, 3, 11, 16, 33, 35, 37, 39, 42, 64, 94, 123, 136, 308, 309, 310, 357, 965, 1109, 2178, 8916, 9471, 18330, 62127, 252209
OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 6 followed by k-2 occurrences of the digit 9 followed by the digits 83 is prime (see Example section).
a(27) > 3*10^5.
EXAMPLE
3 is in this sequence because 7*10^3 - 17 = 6983 is prime.
Initial terms and associated primes:
a(1) = 1, 53;
a(2) = 2, 683;
a(3) = 3, 6983;
a(4) = 11, 699999999983;
a(5) = 16, 69999999999999983; etc.
MATHEMATICA
Select[Range[1, 100000], PrimeQ[7*10^# - 17] &]
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Jan 11 2017
EXTENSIONS
Comment corrected by Harvey P. Dale, Apr 10 2019
a(26) from Robert Price, May 31 2023
STATUS
approved