OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 6 followed by k-2 occurrences of the digit 1 followed by the digits 21 is prime (see Example section).
a(25) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 61w21.
EXAMPLE
3 is in this sequence because (55*10^3 + 89)/9 = 6121 is prime.
Initial terms and associated primes:
a(1) = 1, 71;
a(2) = 3, 6121;
a(3) = 4, 61121;
a(4) = 7, 61111121;
a(5) = 10, 61111111121; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(55*10^# + 89)/9] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Sep 04 2017
EXTENSIONS
a(24) from Robert Price, Mar 01 2019
STATUS
approved