A098960 Numbers k such that 2*10^k + 7*R_k - 6 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

2, 6, 33, 69, 150, 936, 3135, 5838, 6990, 20786, 57138

Offset

1,1

Comments

Also numbers k such that (25*10^k - 61)/9 is prime.

a(12) > 10^5. - Robert Price, Feb 27 2015

Links

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

Makoto Kamada, Prime numbers of the form 277...771.

Index entries for primes involving repunits.

Formula

a(n) = A101966(n) + 1.

Example

For n = 2 and 6, we get 271 and 2777771 which are primes.

Mathematica

Do[ If[ PrimeQ[(25*10^n - 61)/9], Print[n]], {n, 0, 10000}]

Crossrefs

Cf. A002275, A101966.

Sequence in context: A372902 A083666 A083126 * A162429 A279461 A243324

Adjacent sequences: A098957 A098958 A098959 * A098961 A098962 A098963

Keyword

more,nonn

Author

Julien Peter Benney (jpbenney(AT)ftml.net), Oct 22 2004

Extensions

a(6) from Ray Chandler, Nov 04 2004

a(7), a(8) & a(9) from Sam Handler (sam_5_5_5_0(AT)yahoo.com), Nov 03 2004

Addition of a(10) from Kamada data by Robert Price, Dec 13 2010

a(11) from Robert Price, Feb 27 2015

Status

approved