OFFSET
1,1
COMMENTS
For k > 1, numbers k such that the digit 1 followed by k-2 occurrences of the digit 3 followed by the digits 03 is prime (see Example section).
a(23) > 3*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 13w03.
EXAMPLE
3 is in this sequence because (4*10^3 - 91)/3 = 1303 is prime.
Initial terms and associated primes:
a(1) = 2, 103;
a(2) = 3, 1303;
a(3) = 5, 133303;
a(4) = 10, 13333333303;
a(5) = 13, 13333333333303; etc.
MATHEMATICA
Select[Range[2, 100000], PrimeQ[(4*10^# - 91)/3] &]
PROG
(PARI) isok(k) = ispseudoprime((4*10^k - 91)/3); \\ Altug Alkan, Mar 27 2018
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Jul 07 2017
EXTENSIONS
a(21) from Robert Price, Mar 27 2018
a(22) from Robert Price, Jul 02 2023
STATUS
approved