OFFSET

1,1

COMMENTS

For k > 1, numbers k such that the digit 8 followed by k-2 occurrences of the digit 1 followed by the digits 23 is prime (see Example section).

a(26) > 10^5.

LINKS

Makoto Kamada, Factorization of near-repdigit-related numbers.

Makoto Kamada, Search for 81w23.

EXAMPLE

3 is in this sequence because (73*10^3+107)/9 = 8123 is prime.

Initial terms and associated primes:

a(1) = 2, 823;

a(2) = 3, 8123;

a(3) = 5, 811123;

a(4) = 6, 8111123;

a(5) = 11, 811111111123, etc.

MATHEMATICA

Select[Range[0, 100000], PrimeQ[(73*10^#+107)/9] &]

PROG

(PARI) lista(nn) = for(n=1, nn, if(ispseudoprime((73*10^n+107)/9), print1(n, ", "))); \\ Altug Alkan, Aug 11 2016

CROSSREFS

KEYWORD

nonn,more

AUTHOR

Robert Price, Aug 11 2016

STATUS

approved