OFFSET
1,2
COMMENTS
For k > 0, numbers k such that the digits 55 followed by k-1 occurrences of the digit 0 followed by the digit 7 is prime (see Example section).
a(24) > 3*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 550w7.
EXAMPLE
2 is in this sequence because 55*10^2 + 7 = 5507 is prime.
Initial terms and associated primes:
a(1) = 1, 557;
a(2) = 2, 5507;
a(3) = 4, 550007;
a(4) = 7, 550000007;
a(5) = 16, 550000000000000007; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[55*10^# + 7] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Oct 29 2017
STATUS
approved