OFFSET
1,1
COMMENTS
For k > 1, numbers k such that the digit 1 followed by k-2 occurrences of the digit 8 followed by the digits 31 is prime (see Example section).
a(20) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 18w31.
EXAMPLE
2 is in this sequence because (17*10^2 - 521)/9 = 131 is prime.
Initial terms and associated primes:
a(1) = 2, 131;
a(2) = 3, 1831;
a(3) = 5, 188831;
a(4) = 8, 188888831;
a(5) = 9, 1888888831; etc.
MATHEMATICA
Select[Range[2, 100000], PrimeQ[(17*10^# - 521)/9] &]
PROG
(PARI) isok(k) = isprime((17*10^k - 521)/9); \\ Michel Marcus, Nov 14 2017
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Nov 13 2017
EXTENSIONS
a(19) from Robert Price, Apr 24 2018
STATUS
approved