OFFSET
1,2
COMMENTS
Numbers k such that the digits 15 followed by k occurrences of the digit 7 is prime (see Example section).
a(25) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 157w.
EXAMPLE
8 is in this sequence because (142*10^8 - 7)/9 = 1577777777 is prime.
Initial terms and associated primes:
a(1) = 1, 157;
a(2) = 8, 1577777777;
a(3) = 16, 157777777777777777;
a(4) = 23, 1577777777777777777777777;
a(5) = 32, 1577777777777777777777777777777777; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(142*10^# - 7)/9] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Feb 12 2017
EXTENSIONS
a(24) from Robert Price, Dec 31 2019
STATUS
approved