OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 4 followed by k-2 occurrences of the digit 9 followed by the digits 87 is prime (see Example section).
a(28) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 49w87.
EXAMPLE
2 is in this sequence because 5*10^2 - 13 = 487 is prime.
Initial terms and associated primes:
a(1) = 1, 37;
a(2) = 2, 487;
a(3) = 3, 4987;
a(4) = 15, 4999999999999987;
a(5) = 20, 499999999999999999987; etc.
MAPLE
select(k->isprime(5*10^k-13), [$1..1000]); # Muniru A Asiru, Jan 03 2019
MATHEMATICA
Select[Range[1, 100000], PrimeQ[5*10^# - 13] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Oct 23 2017
EXTENSIONS
a(27) from Robert Price, Jan 03 2019
STATUS
approved