OFFSET
1,1
COMMENTS
For k > 1, numbers k such that the digit 1 followed by k-2 occurrences of the digit 9 followed by the digits 73 is prime (see Example section).
a(27) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 19w73.
EXAMPLE
3 is in this sequence because 2*10^3 - 27 = 1973 is prime.
Initial terms and associated primes:
a(1) = 2, 173;
a(2) = 3, 1973;
a(3) = 4, 19973;
a(4) = 9, 1999999973;
a(5) = 26, 199999999999999999999999973; etc.
MATHEMATICA
Select[Range[2, 100000], PrimeQ[2*10^# - 27] &]
PROG
(PARI) isok(k) = isprime(2*10^k - 27); \\ Michel Marcus, Feb 19 2017
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Feb 18 2017
EXTENSIONS
a(25)-a(26) from Robert Price, May 05 2018
STATUS
approved