A259122 Numbers k such that 5*R_k + 10^k - 4 is prime, where R_k = 11...11 is the repunit (A002275) of length k.

1, 2, 4, 20, 32, 400, 562, 7016, 37684

Offset

1,2

Comments

Also, numbers k such that (14*10^k - 41)/9 is prime.

Terms from Kamada data.

a(10) > 10^5.

Per Kamada link, 211262, 222718, 250336 are also terms, found by Serge Batalov - Michael S. Branicky, Sep 13 2024

Links

Table of n, a(n) for n=1..9.

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

Makoto Kamada, Prime numbers of the form 155...551.

Index entries for primes involving repunits.

Example

For k=4, 5*R_4 + 10^k - 4 = 5555 + 10000 - 4 = 15551 which is prime.

Mathematica

Select[Range[100000], PrimeQ[(14*10^#-41)/9] &]

Crossrefs

Cf. A002275.

Sequence in context: A201502 A069535 A108866 * A059206 A294230 A332626

Adjacent sequences: A259119 A259120 A259121 * A259123 A259124 A259125

Keyword

nonn,more,hard

Author

Robert Price, Jun 18 2015

Status

approved