A259121 Numbers k such that 7*R_k + 10^k - 6 is prime, where R_k = 11...11 is the repunit (A002275) of length k.

1, 6, 48, 102, 192, 366, 1002, 20364, 37446, 56082

Offset

1,2

Comments

Also, numbers k such that (16*10^k - 61)/9 is prime.

Terms from Kamada data.

a(11) > 10^5.

Links

Table of n, a(n) for n=1..10.

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

Makoto Kamada, Prime numbers of the form 177...771.

Index entries for primes involving repunits.

Example

For k=6, 7*R_6 + 10^k - 6 = 777777 + 10000000 - 6 = 1777771 which is prime.

Mathematica

Select[Range[100000], PrimeQ[(16*10^#-61)/9] &]

Crossrefs

Cf. A002275.

Sequence in context: A078237 A274131 A341683 * A052651 A153796 A250226

Adjacent sequences: A259118 A259119 A259120 * A259122 A259123 A259124

Keyword

nonn,more,hard

Author

Robert Price, Jun 18 2015

Status

approved