OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digits 31 followed by k-1 occurrences of the digit 3 followed by the digit 7 is prime (see Example section).
a(27) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 313w7.
EXAMPLE
3 is in this sequence because (94*10^3 + 11)/3 = 31337 is prime.
Initial terms and associated primes:
a(1) = 1, 317;
a(2) = 2, 3137;
a(3) = 3, 31337;
a(4) = 5, 3133337;
a(5) = 10, 313333333337; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(94*10^# + 11)/3] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Feb 21 2017
EXTENSIONS
a(24)-a(26) from Robert Price, Apr 03 2020
STATUS
approved