OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 3 followed by k-2 occurrences of the digit 1 followed by the digits 97 is prime (see Example section).
a(26) > 10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 31w97.
EXAMPLE
5 is in this sequence because (28*10^5 + 773)/9 = 311197 is prime.
Initial terms and associated primes:
a(1) = 0, 89;
a(2) = 2, 397;
a(3) = 5, 311197;
a(4) = 6, 3111197;
a(5) = 11, 311111111197, etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(28*10^# + 773)/9] &]
PROG
(Magma) [n: n in [0..500] | IsPrime((28*10^n+773) div 9)]; // Vincenzo Librandi, Aug 01 2016
(PARI) is(n)=ispseudoprime((28*10^n+773)/9) \\ Charles R Greathouse IV, Jun 13 2017
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Robert Price, Jul 31 2016
STATUS
approved