OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 7 followed by k-2 occurrences of the digit 9 followed by the digits 49 is prime (see Example section).
a(28) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 79w49.
EXAMPLE
3 is in this sequence because 8*10^3 - 51 = 7949 is prime.
Initial terms and associated primes:
a(1) = 1, 29;
a(2) = 3, 7949;
a(3) = 5, 799949;
a(4) = 28, 79999999999999999999999999949; etc.
MATHEMATICA
Select[Range[1, 100000], PrimeQ[8*10^# - 51] &]
PROG
(PARI) is(n) = ispseudoprime(8*10^n-51) \\ Felix Fröhlich, Aug 12 2019
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Oct 06 2017
EXTENSIONS
a(27) from Robert Price, Aug 12 2019
STATUS
approved