A056678 Numbers k such that 20*R_k + 9 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

1, 2, 4, 13, 175, 415, 2504, 2758, 7924, 9400, 10390, 12104, 19615

Offset

1,2

Comments

Also numbers k such that (2*10^(k+1)+61)/9 is prime.

a(14) > 10^5. - Robert Price, Nov 01 2014

Links

Table of n, a(n) for n=1..13.

Makoto Kamada, Prime numbers of the form 22...229.

Index entries for primes involving repunits

Formula

a(n) = A099410(n+1) - 1. - Robert Price, Nov 01 2014

Mathematica

Do[ If[ PrimeQ[20*(10^n - 1)/9 + 9], Print[n]], {n, 5000}]

Crossrefs

Cf. A002275, A093401 (corresponding primes), A099410.

Sequence in context: A369313 A091957 A327443 * A218655 A369998 A369606

Adjacent sequences: A056675 A056676 A056677 * A056679 A056680 A056681

Keyword

hard,nonn

Author

Robert G. Wilson v, Aug 10 2000

Extensions

Two more terms from Rick L. Shepherd, Mar 30 2004

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

a(11)-a(13) derived from A099410 by Robert Price, Nov 01 2014

Status

approved