A056662 Numbers k such that 90*R_k + 7 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

0, 1, 2, 16, 139, 989, 1886, 3529, 5995, 13819, 21872, 26044, 87719, 232598

Offset

1,3

Comments

Also numbers k such that 10^(k+1) - 3 is a prime number.

Links

Table of n, a(n) for n=1..14.

Makoto Kamada, Prime numbers of the form 99...997.

Index entries for primes involving repunits

Formula

a(n) = A089675(n) - 1. - Robert Price, Nov 01 2014

Mathematica

Do[ If[ PrimeQ[ 90*(10^n - 1)/9 + 7 ], Print[n]], {n, 0, 1250}]

Crossrefs

Cf. A002275, A089675.

Sequence in context: A378461 A348803 A369580 * A348618 A151402 A365526

Adjacent sequences: A056659 A056660 A056661 * A056663 A056664 A056665

Keyword

hard,nonn

Author

Robert G. Wilson v, Aug 09 2000

Extensions

More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008

a(13)-a(14) derived from A089675 by Robert Price, Nov 01 2014

Status

approved