login
The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A298758 Numbers k such that both k and 2k-1 are Poulet numbers (Fermat pseudoprimes to base 2). 4
15709, 65281, 20770621, 104484601, 112037185, 196049701, 425967301, 2593182901, 16923897871, 32548281361, 45812984491, 52035130951, 55897227751, 82907336737, 90003640021, 92010062101, 138016057141, 204082130071, 310026150211, 620006892121, 622333751509 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

2*a(n) - 1 = A303531(n) belongs to A217465. - Max Alekseyev, Apr 24 2018

Numbers k such that both k and 2k+1 are Poulet numbers are listed in A303447.

LINKS

Table of n, a(n) for n=1..21.

MATHEMATICA

s = Import["b001567.txt", "Data"][[All, -1]]; n = Length[s];

aQ[n_] := ! PrimeQ[n] && PowerMod[2, (n - 1), n] == 1;

a = {}; Do[p = 2*s[[k]] - 1; If[aQ[p], AppendTo[a, s[[k]]]], {k, 1, n}]; a (* using the b-File from A001567 *)

PROG

(PARI) isP(n) = (Mod(2, n)^n==2) && !isprime(n) && (n>1);

isok(n) = isP(n) && isP(2*n-1); \\ Michel Marcus, Mar 09 2018

CROSSREFS

Cf. A001567, A005382, A303447, A303448.

Sequence in context: A134121 A177949 A204728 * A205289 A205897 A190834

Adjacent sequences:  A298755 A298756 A298757 * A298759 A298760 A298761

KEYWORD

nonn

AUTHOR

Amiram Eldar, Jan 26 2018

STATUS

approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified September 16 07:19 EDT 2021. Contains 347469 sequences. (Running on oeis4.)