OFFSET
1,1
COMMENTS
For k > 1, numbers k such that the digit 3 followed by k-2 occurrences of the digit 2 followed by the digits 47 is prime (see Example section).
a(20) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 32w47.
EXAMPLE
2 is in this sequence because (29*10^2 + 223)/9 = 347 is prime.
Initial terms and associated primes:
a(1) = 2, 347;
a(2) = 5, 322247;
a(3) = 9, 3222222247;
a(4) = 11, 322222222247;
a(5) = 23, 322222222222222222222247; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(29*10^# + 223)/9] &]
PROG
(PARI) isok(k) = isprime((29*10^k + 223)/9); \\ Michel Marcus, Nov 24 2017
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Nov 24 2017
EXTENSIONS
a(19) from Robert Price, Sep 08 2018
STATUS
approved