OFFSET
1,1
COMMENTS
For k > 1, numbers k such that the digit 2 followed by k-2 occurrences of the digit 7 followed by the digits 51 is prime (see Example section).
a(25) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 27w51.
EXAMPLE
4 is in this sequence because (25*10^4 - 241)/9 = 27751 is prime.
Initial terms and associated primes:
a(1) = 2, 251;
a(2) = 4, 27751;
a(3) = 5, 277751;
a(4) = 8, 277777751;
a(5) = 20, 277777777777777777751; etc.
MATHEMATICA
Select[Range[1, 100000], PrimeQ[(25*10^# - 241)/9] &]
PROG
(PARI) isok(k) = ispseudoprime((25*10^k-241)/9); \\ Altug Alkan, May 07 2018
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Jan 25 2017
EXTENSIONS
a(24) from Robert Price, May 07 2018
STATUS
approved