OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 5 followed by k-2 occurrences of the digit 6 followed by the digits 41 is prime (see Example section).
a(32) > 3*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 56w41.
EXAMPLE
3 is in this sequence because (17*10^3-77)/3 = 5641 is prime.
Initial terms and associated primes:
a(1) = 1, 31;
a(2) = 2, 541;
a(3) = 3, 5641;
a(4) = 6, 5666641;
a(5) = 9, 5666666641, etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(17*10^# - 77)/3] &]
PROG
(PARI) is(n)=ispseudoprime((17*10^n - 77)/3) \\ Charles R Greathouse IV, Jun 13 2017
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Robert Price, May 20 2016
EXTENSIONS
a(31) from Robert Price, Aug 21 2019
STATUS
approved