OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 1 followed by k-2 occurrences of the digit 5 followed by the digits 81 is prime (see Example section).
a(22) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 15w81.
EXAMPLE
2 is in this sequence because (59*10^2 - 77)/9 = 181 is prime.
Initial terms and associated primes:
a(1) = 1, 41;
a(2) = 2, 181;
a(3) = 4, 15581;
a(4) = 5, 155581;
a(5) = 8, 155555581; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(59*10^# - 77)/9] &]
PROG
(PARI) isok(k) = isprime((14*10^k + 229)/9); \\ Michel Marcus, Nov 12 2017
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Nov 11 2017
EXTENSIONS
a(21) from Robert Price, Mar 14 2018
STATUS
approved