# [seqfan] Re: The Euler-Fibonacci pseudoprimes

Tomasz Ordowski tomaszordowski at gmail.com
Fri Jun 18 08:16:24 CEST 2021

```Ami, thanks for finding these numbers!

All pseudoprimes m found by Amiram satisfy the congruence
F(m) == 5^{(m-1)/2} == 1 (mod m). They are a subset of A094394.
Are there odd composites n such that F(n) == 5^{(n-1)/2} == -1 (mod n) ?
They are a proper subset (maybe empty) of A094395: https://oeis.org/A094395

Have a successful furter search!

Tom

pt., 18 cze 2021 o 01:22 Ami Eldar <amiram.eldar at gmail.com> napisał(a):

> The odd composites m such  that F(m) == 5^{(m-1)/2} == +-1 (mod m) are:
> 146611, 252601, 399001, 512461, 556421, 852841, 1024651, 1193221, 1314631,
> 1857241, 1909001, 2100901, 2165801, 2603381, 2704801, 3470921, 3828001,
> 3942271, 4504501, 5049001, 5148001, 5481451, 6189121, 6840001, 7267051,
> 7519441, 7879681, 8086231, 8341201, 8719921, 9439201, 9863461, ...
>
> Best,
> Amiram
>
> On Thu, Jun 17, 2021 at 9:48 PM Tomasz Ordowski <tomaszordowski at gmail.com>
> wrote:
>
> >
> > Let F(n) = Fibonacci(n) = A000045(n).
> >
> > As is known, if p <> 5 is prime, then p | F(p-1) or p | F(p+1).
> > By Cassini's identity, if p <> 5 is prime, then p | F(p)^2+(-1)^p.
> > There are composite numbers n | F(n)^2+(-1)^n, namely
> > 231, 323, 377, 442, 1378, 1443, 1551, 1891, 2737, 2834, 2849, ...
> > Such odd numbers are A337231.
> > If p <> 5 is an odd prime, then F(p) == +-1 (mod p).
> > Such odd pseudoprimes are A094394 and A094395.
> >
> > If p <> 5 is an odd prime, then F(p) == 5^{(p-1)/2} == +-1 (mod p).
> > The weak Euler-Fibonacci pseudoprimes can be defined as
> > odd composites k such that F(k) == 5^{(k-1)/2} (mod k),
> > but maybe someone has already done it on the OEIS pages.
> > 25, 75, 125, 425, 555, 625, 1625, 1875, 1891, 3125, 4375, 13161, ...
> > Such pseudoprimes indivisible by 5 are 1891, 13161, 13981, 68101, ...
> > However, this subset is also not in the OEIS. Data from Amiram Eldar.
> > Consider the strong Euler-Fibonacci pseudoprimes with the full condition:
> > F(m) == 5^{(m-1)/2} == +-1 (mod m). Are there such odd composites m?
> > Maybe someone will find such pseudoprimes, if they exist (in the OEIS).
> >
> > Best regards,
> >
> > Thomas Ordowski
> >
> > --
> > Seqfan Mailing list - http://list.seqfan.eu/
> >
>
> --
> Seqfan Mailing list - http://list.seqfan.eu/
>

```