OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 5 followed by k-2 occurrences of the digit 0 followed by the digits 21 is prime (see Example section).
a(27) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 50w21.
EXAMPLE
4 is in this sequence because 5*10^4 + 21 = 50021 is prime.
Initial terms and associated primes:
a(1) = 1, 71;
a(2) = 2, 521;
a(3) = 3, 5021;
a(4) = 4, 50021;
a(5) = 7, 50000021; etc.
MAPLE
select(k->isprime(5*10^k+21), [$1..1000]); # Muniru A Asiru, Jan 05 2019
MATHEMATICA
Select[Range[0, 100000], PrimeQ[5*10^# + 21] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Jan 31 2017
STATUS
approved