OFFSET
1,3
COMMENTS
For k > 1, numbers k such that the digit 7 followed by k-2 occurrences of the digit 3 followed by the digits 57 is prime (see Example section).
a(26) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 73w57.
EXAMPLE
2 is in this sequence because (22*10^2 + 71)/3 = 757 is prime.
Initial terms and associated primes:
a(1) = 0, 31;
a(2) = 1, 97;
a(3) = 2, 757;
a(4) = 6, 7333357;
a(5) = 20, 733333333333333333357; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(22*10^# + 71)/3] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, May 09 2017
STATUS
approved