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 87 is prime (see Example section).
a(33) > 3*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 73w87.
EXAMPLE
4 is in this sequence because (22*10^4 + 161)/3 = 73387 is prime.
Initial terms and associated primes:
a(1) = 0, 61;
a(2) = 1, 127;
a(3) = 2, 787;
a(4) = 4, 73387;
a(5) = 5, 733387; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(22*10^# + 161)/3] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Feb 10 2017
EXTENSIONS
a(31)-a(32) from Robert Price, May 04 2019
STATUS
approved