A099413 Numbers k such that 4*R_k + 3 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

0, 1, 2, 4, 10, 20, 26, 722, 1310, 3170, 28934, 66284, 67796

Offset

1,3

Comments

Also numbers k such that (4*10^k + 23)/9 is prime.

a(14) > 10^5. - Robert Price, Nov 26 2014

Links

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

Makoto Kamada, Prime numbers of the form 44...447.

Index entries for primes involving repunits

Formula

a(n) = A056682(n-1) + 1. - Robert Price, Nov 26 2014

Mathematica

Do[ If[ PrimeQ[ 4(10^n - 1)/9 + 3], Print[n]], {n, 5000}]

Crossrefs

Cf. A002275, A056682.

Sequence in context: A026554 A259132 A369391 * A328761 A271624 A283090

Adjacent sequences: A099410 A099411 A099412 * A099414 A099415 A099416

Keyword

nonn,hard,more

Author

Robert G. Wilson v, Oct 14 2004

Extensions

a(11)-a(13) from Robert Price, Nov 26 2014

Status

approved