OFFSET
1,3
COMMENTS
For k > 0, numbers k such that the digits 42 followed by k - 1 occurrences of the digit 0 followed by the digit 1 is prime (see the Example section).
a(28) > 10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 420w1.
EXAMPLE
4 is in this sequence because 42*10^4+1 = 420001 is prime.
Initial terms and associated primes:
a(1) = 0, 43;
a(2) = 1, 421;
a(3) = 2, 4201;
a(4) = 4, 420001;
a(5) = 13, 420000000000001, etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[42 * 10^# + 1] &]
PROG
(PARI) is(n)=ispseudoprime(42*10^n+1) \\ Charles R Greathouse IV, Jun 13 2017
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Robert Price, Aug 20 2016
STATUS
approved