OFFSET
1,1
COMMENTS
For k > 1, numbers k such that the digit 3 followed by k-2 occurrences of the digit 8 followed by the digits 23 is prime (see Example section).
a(27) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 38w23.
EXAMPLE
5 is in this sequence because (35*10^5 - 593)/9 = 388823 is prime.
Initial terms and associated primes:
a(1) = 3, 3823;
a(2) = 5, 388823;
a(3) = 6, 3888823;
a(4) = 14; 388888888888823;
a(5) = 24, 3888888888888888888888823; etc.
MATHEMATICA
Select[Range[2, 100000], PrimeQ[(35*10^# - 593)/9] &]
PROG
(PARI) isok(n) = ispseudoprime((35*10^n - 593)/9) \\ Altug Alkan, Aug 15 2017
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Aug 15 2017
EXTENSIONS
a(24)-a(26) from Robert Price, Jul 18 2018
STATUS
approved