A056695 Numbers k such that 80*R_k + 7 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

0, 2, 3, 5, 8, 11, 71, 117, 123, 189, 243, 303, 356, 1421, 2690, 5537, 7580, 21905, 32175, 44357

Offset

1,2

Comments

Also numbers k such that (8*10^(k+1)-17)/9 is prime.

a(21) > 10^5. - Robert Price, May 20 2014

Links

Table of n, a(n) for n=1..20.

Makoto Kamada, Prime numbers of the form 88...887.

Index entries for primes involving repunits

Formula

a(n) = A096846(n) - 1.

Mathematica

Do[ If[ PrimeQ[80*(10^n - 1)/9 + 7], Print[n]], {n, 0, 5000}]

Crossrefs

Cf. A002275, A093171, A096846.

Sequence in context: A259973 A092362 A105766 * A342746 A287006 A020899

Adjacent sequences: A056692 A056693 A056694 * A056696 A056697 A056698

Keyword

hard,nonn,more

Author

Robert G. Wilson v, Aug 10 2000

Extensions

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

a(18)-a(20) derived from A096846 by Robert Price, May 20 2014

Status

approved