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 77 is prime (see Example section).
a(33) > 3*10^5. - Robert Price, Jul 10 2023
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 56w77.
EXAMPLE
5 is in this sequence because (17*10^5 + 31) / 3 = 566677 is prime.
Initial terms and associated primes:
a(1) = 1, 67;
a(2) = 2, 577;
a(3) = 5, 566677;
a(4) = 6, 5666677;
a(5) = 8, 5333333333351; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(17*10^# + 31) / 3] &]
PROG
(PARI) is(n)=ispseudoprime((17*10^n + 31)/3) \\ Charles R Greathouse IV, Jun 13 2017
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Jan 01 2017
EXTENSIONS
a(29)-a(31) from Robert Price, Jan 29 2019
a(32) from Robert Price, Jul 10 2023
STATUS
approved