OFFSET
1,3
COMMENTS
For k > 0, numbers k such that the digits 31 followed by k-1 occurrences of the digit 3 followed by the digit 1 is prime (see Example section).
a(27) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 313w1.
EXAMPLE
4 is in this sequence because (94*10^4 - 7) / 3 = 313331 is prime.
Initial terms and associated primes:
a(1) = 0, 29;
a(2) = 1, 311;
a(3) = 4, 313331;
a(4) = 5, 3133331;
a(5) = 9, 31333333331; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(94*10^# - 7) / 3] &]
PROG
(PARI) is(n)=ispseudoprime((94*10^n - 7)/3) \\ Charles R Greathouse IV, Jun 13 2017
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Nov 18 2016
EXTENSIONS
a(26) from Robert Price, Mar 10 2020
STATUS
approved