A056706 Numbers k such that 4*10^k + 1*R_k is prime, where R_k = 11...1 is the repunit (A002275) of length k.

1, 3, 13, 25, 72, 108, 375, 393, 589, 973

Offset

1,2

Comments

Also numbers k such that (37*10^k - 1)/9 is prime.

a(11) > 10^5. - Robert Price, Nov 30 2014

Links

Table of n, a(n) for n=1..10.

Makoto Kamada, Prime numbers of the form 411...11.

Index entries for primes involving repunits

Mathematica

Do[ If[ PrimeQ[ 4*10^n + (10^n-1)/9], Print[n]], {n, 0, 10000}]

Crossrefs

Cf. A002275, A068815.

Sequence in context: A146379 A056714 A120074 * A052454 A330718 A284108

Adjacent sequences: A056703 A056704 A056705 * A056707 A056708 A056709

Keyword

hard,nonn

Author

Robert G. Wilson v, Aug 10 2000

Status

approved