#18972: twographs and Seidel switching
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Reporter: dimpase | Owner:
Type: defect | Status: new
Priority: major | Milestone: sage-6.9
Component: graph theory | Resolution:
Keywords: | Merged in:
Authors: | Reviewers:
Report Upstream: N/A | Work issues:
Branch: | Commit:
u/dimpase/seidelsw | b76c9a0369a5babc98481d6b96125c9b86f1adc9
Dependencies: | Stopgaps:
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Comment (by dimpase):
Replying to [comment:3 ncohen]:
> > why, why... Tutte won't complain from the grave, but, you know, to me
this feels like dancing on his tombstone...
>
> Well, I don't like much that all graphs constructors end with "Graph"
but it's like a 'standard' problem. We either change them all, or none of
them. And in the case of upper cases for family names, I expect that Sage
has many occurrences of that.
I just posted a question on sage-devel: perhaps it's only me, and then
I'll have to live with this :-)
>
> > Seidel adj.mat. has interesting spectral properties, and that's why it
is there.
> > (one gets srgs out of it if it has just 2 distinct eigenvalues; as
well, it is a means to construct cospectral graphs, by switching---which
is matrix conjugation, and this is why you preserve the spectrum...)
> >
> > So it is very useful on its own right.
>
> I personally don't see the added value with respect to the adjacency
matrix. For sure it is not needed as far as the switching method is
concerned.
In a similar vein you can say that Laplacian matrix should not be there,
as it is a simple variation of the adjacency matrix, too...
It is there because we want to have readable code with readable
documentation. It is syntactic sugar, if you wish. Of course you can say
that all syntactic sugar is a waste of time and space, but without it you
end up with obscure unreadable implementation...
--
Ticket URL: <http://trac.sagemath.org/ticket/18972#comment:4>
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