A259120 Numbers k such that 9*R_k + 10^k - 8 is prime, where R_k = 11...11 is the repunit (A002275) of length k.

1, 2, 4, 8, 40, 86, 200, 730, 1460, 23672, 28630

Offset

1,2

Comments

Also, numbers k such that 2*10^k - 9 is prime.

Terms from Kamada data.

a(12) > 2*10^5.

Links

Table of n, a(n) for n=1..11.

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

Makoto Kamada, Prime numbers of the form 199...991.

Index entries for primes involving repunits.

Example

For k=4, 9*R_4 + 10^k - 2 = 9999 + 10000 - 8 = 19991 which is prime.

Mathematica

Select[Range[200000], PrimeQ[2*10^n-9] &]

Crossrefs

Cf. A002275.

Sequence in context: A172977 A018380 A058236 * A319595 A285632 A102918

Adjacent sequences: A259117 A259118 A259119 * A259121 A259122 A259123

Keyword

nonn,more,hard

Author

Robert Price, Jun 18 2015

Status

approved