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 89 is prime (see Example section).
a(22) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 31w89.
EXAMPLE
2 is in this sequence because (28*10^2 + 701)/9 = 389 is prime.
Initial terms and associated primes:
a(1) = 1, 109;
a(2) = 2, 389;
a(3) = 4, 31189;
a(4) = 5, 311189;
a(5) = 14, 311111111111189; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(28*10^# + 701)/9] &]
PROG
(PARI) isok(k) = isprime((28*10^k + 701)/9); \\ Michel Marcus, Nov 22 2017
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Nov 21 2017
STATUS
approved