#16553: Clean IncidenceStructure
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Reporter: vdelecroix | Owner:
Type: enhancement | Status: needs_info
Priority: major | Milestone: sage-6.3
Component: combinatorial | Resolution:
designs | Merged in:
Keywords: | Reviewers:
Authors: Nathann Cohen, | Work issues:
Vincent Delecroix | Commit:
Report Upstream: N/A | a5c4dbc9d1b77315d90dd3dd7a8ccea780f59ecf
Branch: public/16553 | Stopgaps:
Dependencies: |
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Comment (by vdelecroix):
Replying to [comment:21 ncohen]:
> Yo !
>
> > Another question: why steiner triple systems are incidence structure
but quadruple ones are tuples of tuples? It is ugly in the doctest
>
> Just because I hate classes. You can make them !BlockDesign if you like,
Dima will be happy too.
>
> By the way, `is_t_design` is wrong as it is written. Turns out that you
cannot write this function without "knowing" the value of `t`. There is no
such thing as "the largest t such that a design is a `t`-design.
Why not? It is well defined as soon as `t` is assumed to be smaller than
the block size. Note that if you have a t-(v,k,lambda) design it is also a
s-(v,k,lambda_s) design with
{{{
lambda_s = lambda binomial(v-s,t-s) / binomial(k-s,t-s).
}}}
(Handbook theorem II.4.8)
I think that we should forbid lambda=0, that's all.
--
Ticket URL: <http://trac.sagemath.org/ticket/16553#comment:22>
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