OFFSET
1,1
COMMENTS
For k > 1, numbers k such that the digit 2 followed by k-2 occurrences of the digit 5 followed by the digits 49 is prime (see Example section).
a(26) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 25w49.
EXAMPLE
6 is in this sequence because (23*10^6 - 59)/9 = is prime.
Initial terms and associated primes:
a(1) = 3, 2549;
a(2) = 6, 2555549;
a(3) = 10, 25555555549;
a(4) = 16, 25555555555555549;
a(5) = 22, 25555555555555555555549; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(23*10^# - 59)/9] &]
PROG
(PARI) isok(n) = isprime((23*10^n - 59)/9); \\ Indranil Ghosh, Mar 09 2017
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Mar 09 2017
EXTENSIONS
a(24)-a(25) from Robert Price, May 29 2018
STATUS
approved