OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 1 followed by k-2 occurrences of the digit 3 followed by the digits 81 is prime (see Example section).
a(27) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 13w81.
EXAMPLE
2 is in this sequence because (4*10^2 + 143) / 3 = 1381 is prime.
Initial terms and associated primes:
a(1) = 1, 61;
a(2) = 2, 181;
a(3) = 3, 1381;
a(4) = 4, 13381;
a(5) = 7, 13333381, etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(4*10^# + 143) / 3] &]
PROG
(PARI) is(n) = ispseudoprime((4*10^n + 143) / 3); \\ Altug Alkan, Sep 20 2016
(Magma) [n: n in [0..500] | IsPrime((4*10^n+143) div 3)]; // Vincenzo Librandi, Sep 22 2016
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Robert Price, Sep 20 2016
EXTENSIONS
a(26) from Robert Price, Mar 05 2018
STATUS
approved