OFFSET
1,2
COMMENTS
For k > 1, numbers k such that k-2 occurrences of the digit 3 followed by the digits 97 is prime (see Example section).
a(25) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 3w97.
EXAMPLE
2 is in this sequence because (10^2 + 191)/3 = 97 is prime.
Initial terms and associated primes:
a(1) = 1, 67;
a(2) = 2, 97;
a(3) = 3, 397;
a(4) = 6, 333397;
a(5) = 7, 3333397; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(10^# + 191)/3] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Nov 10 2017
EXTENSIONS
a(23)-a(24) from Robert Price, Feb 14 2018
STATUS
approved