OFFSET
1,3
COMMENTS
For k > 1, numbers k such that the digit 9 followed by k-2 occurrences of the digit 3 followed by the digits 97 is prime (see Example section).
a(35) > 3*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 93w97.
EXAMPLE
3 is in this sequence because (28*10^3 + 191)/3 = 9397 is prime.
Initial terms and associated primes:
a(1) = 0, 73;
a(2) = 1, 157:
a(3) = 2, 997;
a(4) = 3, 9397;
a(5) = 5, 933397, etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(28*10^# + 191)/3] &]
PROG
(PARI) is(n)=ispseudoprime((28*10^n + 191)/3) \\ Charles R Greathouse IV, Jun 13 2017
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Robert Price, May 13 2016
EXTENSIONS
a(31)-a(33) from Robert Price, Feb 27 2020
a(34) from Robert Price, Jul 12 2023
STATUS
approved