OFFSET
1,3
COMMENTS
For k > 1, numbers k such that the digit 5 followed by k-2 occurrences of the digit 3 followed by the digits 51 is prime (see Example section).
a(28) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 53w51.
EXAMPLE
3 is in this sequence because (16*10^3 + 53) / 3 = 5351 is prime.
Initial terms and associated primes:
a(1) = 0, 23;
a(2) = 1, 71;
a(3) = 3, 5351;
a(4) = 8, 533333351;
a(5) = 12, 5333333333351; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(16*10^# + 53) / 3] &]
PROG
(PARI) is(n)=ispseudoprime((16*10^n + 53)/3) \\ Charles R Greathouse IV, Jun 13 2017
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Dec 29 2016
EXTENSIONS
a(27) from Robert Price, Mar 08 2019
STATUS
approved