OFFSET
1,3
COMMENTS
For k > 1, numbers k such that the digit 3 followed by k-2 occurrences of the digit 1 followed by the digits 51 is prime (see Example section).
a(23) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 31w51.
EXAMPLE
1 is in this sequence because (28*10^1 + 359)/9 = 71 is prime.
Initial terms and associated primes:
a(1) = 0, 43;
a(2) = 1, 71;
a(3) = 4, 31151;
a(4) = 6, 3111151;
a(5) = 9, 3111111151; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(28*10^# + 359)/9] &]
PROG
(PARI) isok(k) = isprime((28*10^k + 359)/9); \\ Michel Marcus, Nov 20 2017
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Nov 19 2017
STATUS
approved