OFFSET
1,1
COMMENTS
For terms k > 1, numbers that begin with the digit 4 followed by k-2 occurrences of the digit 5 followed by the digits 61 are prime (see Example section).
a(30) > 2*10^5.
LINKS
Alois P. Heinz, Table of n, a(n) for n = 1..29
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 45w61.
EXAMPLE
3 is in this sequence because (41*10^3 + 49)/9 = 4561 is prime.
Initial terms and associated primes:
a(1) = 2, 461;
a(2) = 3, 4561;
a(3) = 6, 4555561;
a(4) = 20, 455555555555555555561;
a(5) = 26, 455555555555555555555555561, etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(41*10^# + 49)/9] &]
PROG
(PARI) is(n)=ispseudoprime((41*10^n + 49)/9) \\ Charles R Greathouse IV, Jun 13 2017
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Robert Price, Apr 17 2016
STATUS
approved