OFFSET
1,2
COMMENTS
a(24) > 2*10^5.
Numbers corresponding to terms <= 699 are certified primes. - Klaus Brockhaus, Feb 15 2005
For k > 1, numbers k such that the digit 6 followed by k-2 occurrences of the digit 9 followed by the digits 89 is prime (see Example section).
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 69w89.
EXAMPLE
Initial terms and associated primes:
a(1) = 1, 59;
a(2) = 6, 6999989;
a(3) = 7, 69999989;
a(4) = 9, 6999999989;
a(5) = 10, 69999999989; etc.
MATHEMATICA
Select[Range[1, 100000], PrimeQ[7*10^# - 11] &]
PROG
(PARI) is(n)=ispseudoprime(7*10^n - 11) \\ Charles R Greathouse IV, Jun 13 2017
CROSSREFS
KEYWORD
more,nonn
AUTHOR
Tom Mueller (muel4503(AT)uni-trier.de), Feb 08 2005
EXTENSIONS
a(11)-a(13) from Klaus Brockhaus, Feb 15 2005
a(14)-a(22) from Robert Price, Oct 29 2017
a(23) from Robert Price, Jun 21 2019
STATUS
approved