OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 4 followed by k-2 occurrences of the digit 7 followed by the digits 33 is prime (see Example section).
a(26) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 47w33.
EXAMPLE
3 is in this sequence because (43*10^3 - 403)/9 = 4733 is prime.
Initial terms and associated primes:
a(1) = 1, 3;
a(2) = 2, 433;
a(3) = 3, 4733;
a(4) = 6, 4777733;
a(5) = 11, 477777777733; etc.
MAPLE
select(k->isprime((43*10^k-403)/9), [$1..1000]); # Muniru A Asiru, Jan 05 2019
MATHEMATICA
Select[Range[1, 100000], PrimeQ[(43*10^# - 403)/9] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Apr 24 2017
EXTENSIONS
a(25) from Robert Price, Jan 04 2019
STATUS
approved