A259134 Numbers k such that 9*R_k + 7*10^k - 2 is prime, where R_k = 11...11 is the repunit (A002275) of length k.

0, 2, 4, 28, 156, 322, 352, 1212, 1284, 7984, 15192, 84772, 119930, 148860

Offset

1,2

Comments

Also, numbers k such that 8*10^k - 3 is prime.

Terms from Kamada data.

Note that Kamada does not recognize k=0 as 5 is a degenerate case of form ABB..BBA.

a(15) > 2*10^5.

Links

Table of n, a(n) for n=1..14.

Makoto Kamada, Near-repdigit numbers of the form ABB...BBA.

Makoto Kamada, Prime numbers of the form 799...997.

Index entries for primes involving repunits.

Example

For k=2, 9*R_2 + 7*10^k - 2 = 99 + 700 - 2 = 797 which is prime.

Mathematica

Select[Range[0, 200000], PrimeQ[8*10^#-3] &]

Crossrefs

Cf. A002275.

Sequence in context: A264667 A356700 A357158 * A117443 A095858 A062792

Adjacent sequences: A259131 A259132 A259133 * A259135 A259136 A259137

Keyword

nonn,more,hard

Author

Robert Price, Jun 18 2015

Status

approved