A097402 Numbers k such that 6*10^k + 2*R_k + 5 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

0, 1, 13, 73, 127

Offset

1,3

Comments

Also numbers k such that (56*10^k + 43)/9 is prime.

a(6) > 10^5. - Robert Price, Sep 12 2015

Links

Table of n, a(n) for n=1..5.

Makoto Kamada, Prime numbers of the form 622...227.

Index entries for primes involving repunits.

Formula

a(n) = A101523(n-1) + 1 for n>1.

Example

1, 13 are members since 67 and 62222222222227 are primes.

Mathematica

Do[ If[ PrimeQ[(56*10^n + 43)/9], Print[n]], {n, 0, 10000}]
Select[Range[0, 130], PrimeQ[10*FromDigits[PadRight[{6}, #, 2]]+7]&] (* Harvey P. Dale, Feb 17 2018 *)

Prog

(PARI) is(n)=isprime(6*10^n + 2*(10^n-1)/9 + 5) \\ Anders Hellström, Sep 12 2015

Crossrefs

Cf. A002275, A101523.

Sequence in context: A097460 A336796 A138353 * A255416 A201788 A139873

Adjacent sequences: A097399 A097400 A097401 * A097403 A097404 A097405

Keyword

more,nonn

Author

Julien Peter Benney (jpbenney(AT)ftml.net), Nov 11 2004

Extensions

Inserted a(1)=0 by Robert Price, Sep 12 2015

Status

approved