Hi Nicolas, On Mar 3, 7:00 am, "Nicolas M. Thiery" <nicolas.thi...@u-psud.fr> wrote: > Hi Andrey, > > On Sun, Feb 13, 2011 at 01:30:03PM -0800, Andrey Novoseltsev wrote: > > While writing an enhancement proposal on > >http://trac.sagemath.org/sage_trac/ticket/10777 > > I thought that it could be convenient to create a (finite) poset > > without specifying its elements but perhaps adding them one-by-one as/ > > if they are created later. > > > It seems to me that it is not possible to create such posets in Sage > > now, is it? > > Nope. > > > Is it actually of any interest to anyone or my situation was too > > specific? (I want to create polytopes which are faces of other > > polytopes without necessarily constructing the whole face lattice.) > > We definitely have interest for lazy posets, where the list of > vertices/edges is not necessarily constructed explicitly at the > creation of the poset. For example, many of us would like to > manipulate infinite posets, like Bruhat order on an infinite Coxeter > group. A first step in this direction would be to create the category > of posets, and progressively factor out there the code from Poset. > Then one could create other concrete classes for posets, such as > LazyPoset, .. > > Just a note: one should assume that a given LazyPoset P always model > the the same (large/infinite/not yet completely calculated) > mathematical poset. In other words P is semantically immutable. Does > this indeed cover your needs?
Absolutely, all current polyhedral classes are immutable and I don't think that there are any plans to change it. This means that face lattices are also immutable, but, while they are finite, they may be quite large and should not be created unless necessary. On the other hand, one may be interested in some particular faces (e.g. all facets and a few of their intersections), in which case it would make sense to create them as elements of such a LazyPoset. > Speaking about posets: one indeed cannot yet create "facade posets" > whose elements are not wrapped. That's also a feature desired by many > of us! > > For more about "facades" see also (with the Sage-Combinat patches applied): > > sage: F = Sets().Facades() > sage: F? Thank you, will take a look! Andrey > > Cheers, > Nicolas > -- > Nicolas M. Thi�ry "Isil" <nthi...@users.sf.net>http://Nicolas.Thiery.name/ -- You received this message because you are subscribed to the Google Groups "sage-combinat-devel" group. To post to this group, send email to sage-combinat-devel@googlegroups.com. To unsubscribe from this group, send email to sage-combinat-devel+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/sage-combinat-devel?hl=en.
