OFFSET
1,1
COMMENTS
For k > 1, numbers k such that the digit 1 followed by k-2 occurrences of the digit 2 followed by the digits 07 is prime (see Example section).
a(20) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 12w07.
EXAMPLE
2 is in this sequence because (11*10^2 - 137)/9 = 107 is prime.
Initial terms and associated primes:
a(1) = 2, 107;
a(2) = 5, 122207;
a(3) = 21, 1222222222222222222207;
a(4) = 23, 122222222222222222222207;
a(5) = 47, 122222222222222222222222222222222222222222222207; etc.
MATHEMATICA
Select[Range[2, 100000], PrimeQ[(11*10^# - 137)/9] &]
PROG
(PARI) isok(k) = isprime((11*10^k - 137)/9); \\ Michel Marcus, Nov 15 2017
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Nov 15 2017
EXTENSIONS
a(19) from Robert Price, Jan 26 2018
STATUS
approved