A256905 Numbers k such that 7*R_k - 10 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

2, 11, 24, 42, 56, 336, 738, 2712, 3498, 8984, 14036, 46439

Offset

1,1

Comments

Also, numbers k such that (7*10^k - 97)/9 is prime.

Terms from Kamada.

a(12) > 30000.

a(13) > 2*10^5. - Tyler Busby, Feb 01 2023

Links

Table of n, a(n) for n=1..12.

Makoto Kamada, Near-repdigit numbers of the form AA...AABA.

Makoto Kamada, Prime numbers of the form 77...7767.

Index entries for primes involving repunits.

Example

For k=11, 7*R_11 - 10 = 77777777777 - 10 = 77777777767 which is prime.

Mathematica

Select[Range[0, 30000], PrimeQ[(7*10^# - 97)/9] &]

Crossrefs

Cf. A002275.

Sequence in context: A041803 A220281 A297545 * A294547 A294557 A009189

Adjacent sequences: A256902 A256903 A256904 * A256906 A256907 A256908

Keyword

more,hard,nonn

Author

Robert Price, Apr 12 2015

Extensions

a(12) from Tyler Busby, Feb 01 2023

Status

approved