OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 6 followed by k-2 occurrences of the digit 3 followed by the digits 13 is prime (see Example section).
a(26) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 63w13.
EXAMPLE
4 is in this sequence because (19*10^4 - 61)/3 = 63313 is prime.
Initial terms and associated primes:
a(1) = 1, 43;
a(2) = 2, 613;
a(3) = 4, 63313;
a(4) = 10, 63333333313;
a(5) = 17, 633333333333333313; etc.
MATHEMATICA
Select[Range[1, 100000], PrimeQ[(19*10^# - 61)/3] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Apr 29 2017
EXTENSIONS
a(25) from Robert Price, May 04 2019
STATUS
approved