OFFSET
1,1
COMMENTS
For k > 1, numbers k such that the digit 3 followed by k-2 occurrences of the digit 8 followed by the digits 07 is prime (see Example section).
a(23) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 38w07.
EXAMPLE
2 is in this sequence because (35*10^2 - 737)/9 = 307 is prime.
Initial terms and associated primes:
a(1) = 2, 307;
a(2) = 7, 38888807;
a(3) = 13, 38888888888807;
a(4) = 20, 388888888888888888807;
a(5) = 26, 388888888888888888888888807; etc.
MATHEMATICA
Select[Range[2, 100000], PrimeQ[(35*10^# - 737)/9] &]
PROG
(PARI) isok(k) = isprime((35*10^k - 737)/9); \\ Michel Marcus, Nov 22 2017
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Nov 21 2017
EXTENSIONS
a(22) from Robert Price, Jul 16 2018
STATUS
approved