OFFSET
1,1
COMMENTS
For k > 1, numbers k such that the digit 3 followed by k-2 occurrences of the digit 9 followed by the digits 37 is prime (see Example section).
a(31) > 3*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 39w37.
EXAMPLE
4 is in this sequence because 4*10^4 - 63 = 39937 is prime.
Initial terms and associated primes:
a(1) = 2, 337;
a(2) = 4, 39937;
a(3) = 5, 399937;
a(4) = 8, 399999937;
a(5) = 9, 3999999937; etc.
MATHEMATICA
Select[Range[2, 100000], PrimeQ[4*10^# - 63] &]
PROG
(PARI) is(n)=ispseudoprime(4*10^n - 63) \\ Charles R Greathouse IV, Jun 13 2017
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Dec 18 2016
EXTENSIONS
a(28)-a(30) from Robert Price, Oct 24 2018
STATUS
approved