OFFSET
1,3
COMMENTS
For k > 1, numbers k such that the digits 77 followed by k-1 occurrences of the digit 9 followed by the digit 3 is prime (see Example section).
a(27) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 779w3.
EXAMPLE
2 is in this sequence because 78*10^2 - 7 = 7793 is prime.
Initial terms and associated primes:
a(1) = 0, 71;
a(2) = 1, 773;
a(3) = 2, 7793;
a(4) = 4, 779993;
a(5) = 11, 7799999999993; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[78*10^# - 7] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, May 21 2017
EXTENSIONS
a(25)-a(26) from Robert Price, Oct 08 2020
STATUS
approved