OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 7 followed by k-2 occurrences of the digit 0 followed by the digits 87 is prime (see Example section).
a(32) > 3*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 70w87.
EXAMPLE
5 is in this sequence because 7*10^5 + 87 = 700087 is prime.
Initial terms and associated primes:
a(1) = 1, 157;
a(2) = 2, 787;
a(3) = 5, 700087;
a(4) = 6, 7000087;
a(5) = 18, 7000000000000000087, etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[7*10^# + 87] &]
PROG
(PARI) is(n)=ispseudoprime(7*10^n + 87) \\ Charles R Greathouse IV, Jun 13 2017
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Robert Price, Nov 11 2016
EXTENSIONS
a(28)-a(29) from Robert Price, Jul 27 2019
a(30)-a(31) from Robert Price, May 31 2023
STATUS
approved