A256326 Numbers k such that 3*R_(k+2) - 2*10^k is prime, where R_k = 11...1 is the repunit (A002275) of length k.

0, 1, 3, 4, 6, 70, 73, 109, 237, 264, 337, 496, 1029, 1077, 1254, 1288, 2049, 3606, 5035, 9163, 35899

Offset

1,3

Comments

Also, numbers k such that (94*10^k - 1)/3 is prime.

Terms from Kamada.

a(22) > 250000.

Links

Table of n, a(n) for n=1..21.

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

Makoto Kamada, Prime numbers of the form 3133...33.

Index entries for primes involving repunits.

Example

For k=3, 3*R_5 - 2*10^3 = 33333 - 2000 = 31333 which is prime.

Mathematica

Select[Range[0, 250000], PrimeQ[(94*10^#-1)/3 ] &]

Crossrefs

Cf. A002275.

Sequence in context: A019120 A263940 A358936 * A239244 A267943 A066466

Adjacent sequences: A256323 A256324 A256325 * A256327 A256328 A256329

Keyword

more,hard,nonn

Author

Robert Price, Apr 14 2015

Status

approved