A256375 Numbers k such that 3*R_(k+2) - 10^k is prime, where R_k = 11...1 is the repunit (A002275) of length k.

4, 5, 6, 8, 13, 14, 25, 30, 44, 58, 383, 455, 463, 504, 511, 1358, 2293, 4096, 4374, 8664, 13676, 23993, 84134, 90155, 115794, 127858

Offset

1,1

Comments

Also, numbers k such that (97*10^k - 1)/3 is prime.

Terms from Kamada.

a(27) > 250000.

Links

Table of n, a(n) for n=1..26.

Makoto Kamada, Near-repdigit numbers of the form ABAA...AA.

Makoto Kamada, Prime numbers of the form 3233...33.

Index entries for primes involving repunits.

Example

For k=4, 3*R_6 - 10^4 = 333333 - 10000 = 323333 which is prime.

Mathematica

Select[Range[0, 250000], PrimeQ[(97*10^#-1)/3 ] &]

Crossrefs

Cf. A002275.

Sequence in context: A310572 A310573 A158650 * A105829 A143903 A006756

Adjacent sequences: A256372 A256373 A256374 * A256376 A256377 A256378

Keyword

more,hard,nonn

Author

Robert Price, Apr 14 2015

Status

approved