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A289535
Numbers k such that (151*10^k - 7)/9 is prime.
0
1, 4, 6, 12, 13, 15, 18, 30, 49, 72, 799, 832, 877, 1122, 1968, 2046, 8176, 11976, 16444, 20896, 32941, 67027, 94885, 154434
OFFSET
1,2
COMMENTS
For k > 0, numbers k such that the digits 16 followed by k occurrences of the digit 7 (see Example section).
a(25) > 2*10^5.
EXAMPLE
4 is in this sequence because (151*10^4 - 7)/9 = 167777 is prime.
Initial terms and associated primes:
a(1) = 1, 167;
a(2) = 4, 167777;
a(3) = 6, 16777777;
a(4) = 12, 16777777777777;
a(5) = 13, 167777777777777; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(151*10^# - 7)/9] &]
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Jul 07 2017
EXTENSIONS
a(24) from Robert Price, Jan 08 2020
STATUS
approved