OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 6 followed by k-2 occurrences of the digit 2 followed by the digits 47 is prime (see Example section).
a(29) > 10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 62w47.
EXAMPLE
3 is in this sequence because (56*10^3 + 223)/9 = 6247 is prime.
Initial terms and associated primes:
a(1) = 0, 31;
a(2) = 2, 647;
a(3) = 3, 6247;
a(4) = 5, 622247;
a(5) = 8, 622222247, etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(56*10^# + 223)/9] &]
PROG
(Magma) [n: n in [0..500] | IsPrime((56*10^n+223) div 9)]; // Vincenzo Librandi, Aug 01 2016
(PARI) is(n)=ispseudoprime((56*10^n+223)/9) \\ Charles R Greathouse IV, Jun 13 2017
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Robert Price, Jul 31 2016
STATUS
approved