OFFSET
1,1
COMMENTS
For k > 1, numbers k such that the digit 3 followed by k-2 occurrences of the digit 6 followed by the digits 31 is prime (see Example section).
a(26) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 36w31.
EXAMPLE
3 is in this sequence because (11*10^3 - 107) / 3 = 3631 is prime.
Initial terms and associated primes:
a(1) = 2, 331;
a(2) = 3, 3631;
a(3) = 5, 366631;
a(4) = 7, 36666631;
a(5) = 8, 366666631; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(11*10^# - 107) / 3] &]
PROG
(PARI) is(n)=ispseudoprime((11*10^n - 107)/3) \\ Charles R Greathouse IV, Jun 13 2017
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Nov 23 2016
EXTENSIONS
First comment and second link corrected by Robert Price, May 23 2018
a(25) from Robert Price, Sep 30 2018
STATUS
approved