OFFSET
1,1
COMMENTS
For k > 1, numbers k such that the digit 1 followed by k-2 occurrences of the digit 5 followed by the digits 31 is prime (see Example section).
a(28) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 15w31.
EXAMPLE
3 is in this sequence because (14*10^3 - 221)/9 = 1531 is prime.
Initial terms and associated primes:
a(1) = 2, 131;
a(2) = 3, 1531;
a(3) = 15, 1555555555555531;
a(4) = 17, 155555555555555531;
a(5) = 21, 1555555555555555555531; etc.
MATHEMATICA
Select[Range[2, 100000], PrimeQ[(14*10^# - 221)/9] &]
PROG
(PARI) isok(k) = ispseudoprime((14*10^k - 221)/9); \\ Altug Alkan, Apr 12 2018
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Feb 13 2017
EXTENSIONS
a(24)-a(27) from Robert Price, Apr 12 2018
STATUS
approved