A259136 Numbers k such that 4*R_k + 7*10^k + 3 is prime, where R_k = 11...11 is the repunit (A002275) of length k.

10, 30, 120, 484, 1486, 1578, 13672, 13810, 15094, 72772, 94212

Offset

1,1

Comments

Also, numbers k such that (67*10^k + 23)/9 is prime.

Terms from Kamada data.

a(12) > 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 744...447.

Index entries for primes involving repunits.

Example

For k=10, 4*R_2 + 7*10^k + 3 = 4444444444 + 7000000000 + 3 = 74444444447 which is prime.

Mathematica

Select[Range[0, 100000], PrimeQ[(67*10^#+23)/9] &]

Crossrefs

Cf. A002275.

Sequence in context: A185829 A301664 A335803 * A098684 A268117 A256879

Adjacent sequences: A259133 A259134 A259135 * A259137 A259138 A259139

Keyword

nonn,more,hard

Author

Robert Price, Jun 18 2015

Status

approved