OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 7 followed by k-2 occurrences of the digit 2 followed by the digits 99 is prime (see Example section).
a(27) > 10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 72w99.
EXAMPLE
5 is in this sequence because (65*10^5+691)/9 = 722299 is prime.
Initial terms and associated primes:
a(1) = 1, 149;
a(2) = 5, 722299;
a(3) = 7, 72222299;
a(4) = 17, 722222222222222299;
a(5) = 35, 722222222222222222222222222222222299, etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(65*10^#+691)/9] &]
PROG
(PARI) is(n)=ispseudoprime((65*10^n+691)/9) \\ Charles R Greathouse IV, Jun 13 2017
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Robert Price, Aug 11 2016
STATUS
approved