OFFSET
1,1
COMMENTS
For k > 1, numbers k such that the digits 20 followed by k-1 occurrences of the digit 2 followed by the digit 7 is prime (see Example section).
a(23) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 202w7.
EXAMPLE
2 is in this sequence because (182*10^2 + 43)/9 = 2027 is prime.
Initial terms and associated primes:
a(1) = 2, 2027;
a(2) = 6, 20222227;
a(3) = 14, 2022222222222227;
a(4) = 17, 2022222222222222227;
a(5) = 27, 20222222222222222222222222227; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(182*10^# + 43)/9] &]
PROG
(PARI) isok(k) = isprime((182*10^k + 43)/9); \\ Michel Marcus, Nov 20 2017
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Nov 19 2017
EXTENSIONS
a(22) from Robert Price, Feb 01 2020
STATUS
approved