A102991 Numbers n such that 4*10^n + 5*R_n - 2 is prime, where R_n = 11...1 is the repunit (A002275) of length n.

0, 1, 4, 7, 19, 34, 54, 78, 90, 213, 307, 702, 1456, 2139, 2214, 3066, 9300, 16440, 16894, 28575, 54051

Offset

1,3

Comments

Also numbers n such that (41*10^n-23)/9 is prime.

a(22) > 10^5. Robert Price, May 08 2015

Links

Table of n, a(n) for n=1..21.

Makoto Kamada, Prime numbers of the form 455...553.

Index entries for primes involving repunits.

Formula

a(n) = A101726(n-1) + 1.

Mathematica

Do[ If[ PrimeQ[(41*10^n - 23)/9], Print[n]], {n, 0, 10000}]

Crossrefs

Cf. A002275, A101726.

Sequence in context: A006381 A318099 A274691 * A298350 A291596 A323655

Adjacent sequences: A102988 A102989 A102990 * A102992 A102993 A102994

Keyword

more,nonn

Author

Robert G. Wilson v, Jan 17 2005

Extensions

Addition of a(17)-a(19) from Kamada data by Robert Price, Dec 10 2010

0 added by Arkadiusz Wesolowski, Mar 10 2011

A(21) from Robert Price, May 08 2015

Status

approved