OFFSET
1,2
COMMENTS
For k > 1, numbers that begin with the digit 3 followed by k-2 occurrences of the digit 9 followed by the digits 79 are prime (see Example section).
a(36) > 3*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 39w79.
EXAMPLE
4 is in this sequence because 4*10^4 - 21 = 39979, which is prime.
Initial terms and associated primes:
a(1) = 1, 19;
a(2) = 2, 379;
a(3) = 4, 39979;
a(4) = 5, 399979;
a(5) = 7, 39999979, etc.
MATHEMATICA
Select[Range[1, 100000], PrimeQ[4*10^# - 21] &]
PROG
(PARI) is(n)=ispseudoprime(4*10^n - 21) \\ Charles R Greathouse IV, Jun 13 2017
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Robert Price, Apr 17 2016
EXTENSIONS
a(32)-a(34) from Robert Price, Sep 10 2018
a(35) from Robert Price, Jun 01 2023
STATUS
approved