OFFSET

1,2

COMMENTS

Also, numbers k such that (31*10^k - 13)/9 is prime.

Terms from Kamada data.

Note that Kamada does not recognize k=0 as 2 is a degenerate case of form ABB..BBA.

a(7) > 2*10^5.

LINKS

EXAMPLE

For k=6, 4*R_6 + 3*10^k - 1 = 444444 + 3000000 - 1 = 3444443 which is prime.

MATHEMATICA

Select[Range[0, 100000], PrimeQ[(31*10^#-13)/9] &]

CROSSREFS

KEYWORD

nonn,more,hard

AUTHOR

Robert Price, Jun 18 2015

STATUS

approved