login
A294681
Numbers k such that (871*10^k - 7)/9 is prime.
0
1, 2, 7, 26, 28, 31, 38, 47, 59, 226, 233, 656, 752, 1073, 1808, 3646, 4252, 5246, 24686, 37952, 59599
OFFSET
1,2
COMMENTS
Numbers k such that the digits 96 followed by k occurrences of the digit 7 is prime (see Example section).
a(22) > 2*10^5.
EXAMPLE
2 is in this sequence because (871*10^2 - 7)/9 = 9677 is prime.
Initial terms and associated primes:
a(1) = 1, 967;
a(2) = 2, 9677;
a(3) = 7, 967777777;
a(4) = 26, 9677777777777777777777777777;
a(5) = 28, 967777777777777777777777777777; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(871*10^# - 7)/9] &]
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Nov 06 2017
STATUS
approved