# [seqfan] Re: Worry about old sequence, A030077, paths in K_n

israel at math.ubc.ca israel at math.ubc.ca
Mon Apr 4 06:17:20 CEST 2022

```Maybe what you're missing is the word "squarefree".  Without it, you do
have "trivial" equivalences, for example 2*sqrt(x) = sqrt(4*x) (which
is exactly what gives your relation between the diagonal and edge of
a hexagon).

Cheers,
Robert

On Apr 3 2022, Allan Wechsler wrote:

>I must be missing something very dumb. If the different diagonals are all
>incommensurate then why are we doing high-precision floating point
>arithmetic at all, and why are we worrying about whether two "cyclic
>polylines" might or might not be equal? If the diagonals are
>incommensurate, all we need to know is the number of each kind of diagonal.
>That can't be right; there must be linear equivalences. (Note, for example,
>that the long diagonal of a hexagon is exactly twice an edge.)
>
>On Sun, Apr 3, 2022 at 3:16 PM <israel at math.ubc.ca> wrote:
>
>> There are no nontrivial equivalences: the square roots of squarefree
>> integers are linearly independent over the rationals. See e.g.
>>
>>
>>
>> https://qchu.wordpress.com/2009/07/02/square-roots-have-no-unexpected-linear-relationships/
>>
>> Cheers,
>> Robert
>>
>> On Apr 3 2022, Allan Wechsler wrote:
>>
>> > Are there any good examples of nontrivial equivalences? I'm thinking
>> > that we might be able to *characterize* nontrivial equivalences, and
>> > thus be able to prove two paths to be unequal in a sort of
>> > "combinatorial" way, without resorting to extended arithmetic.
>> >
>> >On Sat, Apr 2, 2022 at 3:03 PM Sean A. Irvine <sairvin at gmail.com> wrote:
>> >
>> >> By the way, my code is using computable reals not floating-point, but
>> >> it still faces the problem of deciding equality. I've been using 50
>> >> decimal digits for this problem. I could easily rerun with higher
>> >> precision, but I would like to remove the need for the approximation
>> >> altogether.
>> >>
>> >> Sean.
>> >>
>> >>
>> >> On Sun, 3 Apr 2022 at 02:04, David Applegate <david at bcda.us> wrote:
>> >>
>> >> > I worry about using floating point (extended or not) to check if
>> >> > different sums of square roots are equal or not. Using finite
>> >> > precision for this is extremely tricky This is a notoriously hard
>> >> > problem in general. For example, to see that
>> >> > sqrt(7)+sqrt(14)+sqrt(39)+sqrt(70)+sqrt(72)+sqrt(76)+sqrt(85) !=
>> >> > sqrt(13)+sqrt(16)+sqrt(55)+sqrt(67)+sqrt(73)+sqrt(79) you already
>> need
>> >> > more than double-precision floating point (their difference is
>> 10^-19,
>> >> > see
>> >> >
>> >> >
>> >>
>> >>
>> >>
>>
>>
>> >> > ). There is a randomized polynomial time algorithm for testing
>> >> > equality, but it isn't just compute the result to enough precision.
>> >> >
>> >> >
>> >> >
>> >>
>> >>
>> >>
>>
>>
>> https://cstheory.stackexchange.com/questions/79/problems-between-p-and-npc/4010#4010
>> >> > https://topp.openproblem.net/p33
>> >> >
>> >> >
>> >>
>> >>
>> >>
>>
>>
>> https://refubium.fu-berlin.de/bitstream/handle/fub188/18449/1993-13.pdf;jsessionid=89F489131478714C98250CFA34AFBFE7?sequence=1
>> >> >
>> >> > -Dave
>> >> >
>> >> > On 4/2/2022 5:05 AM, Sean A. Irvine wrote:
>> >> > > So far I can verify to A030077(14):
>> >> > >
>> >> > > 2022-04-02 21:46:54 1
>> >> > > 2022-04-02 21:46:54 1
>> >> > > 2022-04-02 21:46:54 1
>> >> > > 2022-04-02 21:46:54 3
>> >> > > 2022-04-02 21:46:54 5
>> >> > > 2022-04-02 21:46:54 17
>> >> > > 2022-04-02 21:46:54 28
>> >> > > 2022-04-02 21:46:54 105
>> >> > > 2022-04-02 21:46:54 161
>> >> > > 2022-04-02 21:46:54 670
>> >> > > 2022-04-02 21:46:55 1001
>> >> > > 2022-04-02 21:47:00 2869
>> >> > > 2022-04-02 21:47:58 6188
>> >> > > 2022-04-02 22:01:58 26565
>> >> > >
>> >> > > I adapted my existing program for A007874 to this case.  I'm not
>> sure
>> >> > why I
>> >> > > skipped over it before, but perhaps the dihedral group confused
>> >> > > me.
>> >> The
>> >> > > program uses 50 digits of precision for the length
>> >> > > determination. I
>> >> feel
>> >> > > like it should be possible to do this without any kind of
>> >> > > precision
>> >> > limit,
>> >> > > but I don't have the time for that now.  I'll leave it running
>> >> overnight.
>> >> > >
>> >> > > I'll also start a run for A007874 itself which also has four
>> >> > > additional terms with a(15) looking a little suspicious.
>> >> > >
>> >> > > Sean.
>> >> > >
>> >> > >
>> >> > >
>> >> > > On Sat, 2 Apr 2022 at 16:22, Neil Sloane <njasloane at gmail.com>
>> wrote:
>> >> > >
>> >> > >> To Seq Fans, The creator of an interesting sequence, A030077,
>> >> > >> Daniel Gittelson, submitted 12 terms in 1999, and 4 more terms
>> were
>> >> > >> added in
>> >> > 2007
>> >> > >> by a former editor. Daniel G. wrote to me today, expressing doubt
>> >> > the
>> >> > >> 4 additional terms.
>> >> > >>
>> >> > >> He says he interrupted his study of sequences to pursue a medical
>> >> > career,
>> >> > >> but now that he is retired, he can return to combinatorics.
>> >> > >>
>> >> > >> It would be nice if someone could verify the terms! (It is not at
>> >> > >> all obvious to me how to do this.)
>> >> > >>
>> >> > >> Neil Sloane
>> >> > >>
>> >> > >> --
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>> >
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```