OFFSET
1,2
COMMENTS
For k>1, numbers k such that the digit 9 followed by k-2 occurrences of the digit 4 followed by the digits 77 is prime (see Example section).
a(25) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 94w77.
EXAMPLE
2 is in this sequence because (85*10^2 + 293)/9 = 977 is prime.
Initial terms and associated primes:
a(1) = 1, 127;
a(2) = 2, 977;
a(3) = 4, 94477;
a(4) = 10, 94444444477;
a(5) = 23, 944444444444444444444477; etc.
MATHEMATICA
Select[Range[1, 100000], PrimeQ[(85*10^# + 293)/9] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Nov 06 2017
EXTENSIONS
a(22)-a(24) from Robert Price, Dec 13 2019
STATUS
approved