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    • CommentRowNumber1.
    • CommentAuthorUrs
    • CommentTimeJul 9th 2010

    have a quick suggestion for a definition at embedding,

    This was the first idea that came to mind when reading Toby’s initial remark there, haven’t really thought much about it.

    • CommentRowNumber2.
    • CommentAuthorUrs
    • CommentTimeJul 9th 2010

    Ah, what I am describing is of course just effective monomorphism (in the sense remarked at effective epimorphism)

    • CommentRowNumber3.
    • CommentAuthorUrs
    • CommentTimeJul 9th 2010

    wrote effective monomorphism (which was in fact requested already by a handful of entries) and linked back and forth to embedding.

    We should really have

    • effectivemonoembeddingeffective mono \leftrightarrow embedding

    • effectiveepicoveringeffective epi \leftrightarrow covering

    • CommentRowNumber4.
    • CommentAuthorTodd_Trimble
    • CommentTimeJul 9th 2010

    In a category with finite limits and finite colimits, effective monomorphisms are the same thing as regular monomorphisms. I recorded this fact (with proof) at regular monomorphism.

    But regular monomorphisms are slightly more general than effective monomorphisms, since there are notable examples where we don’t have all limits and colimits. A relevant example is the category of smooth manifolds. Here I believe it is a fact that smooth embeddings i:MNi: M \to N are indeed regular monomorphisms, but not effective since there is no pushout N+ MNN +_M N.

    Thus I propose that it should be

    • regular mono \leftrightarrow embedding

    • regular epi \leftrightarrow cover

    and in fact I think this is a fairly widely adopted convention among category theorists, to define embedding and cover in this way.

    • CommentRowNumber5.
    • CommentAuthorUrs
    • CommentTimeJul 9th 2010

    Thanks, Todd. I rephrased embedding a bit to reflect this better.

    • CommentRowNumber6.
    • CommentAuthorUrs
    • CommentTimeJul 9th 2010
    • (edited Jul 9th 2010)

    okay, so there is evidently some polishing indicated on some nnLab entries here.

    i did brief edits to make the pattern

    be reflected better in the nnLab entries. But have to rush off now.

    • CommentRowNumber7.
    • CommentAuthorTodd_Trimble
    • CommentTimeJul 9th 2010

    At some point I’ll see whether I can prove that claim about smooth embeddings of manifolds being regular monomorphisms, and stick it somewhere in the Lab, but I’m not sure where yet.

    • CommentRowNumber8.
    • CommentAuthorMike Shulman
    • CommentTimeJul 9th 2010

    In the absence of kernel pairs, there is also the notion which the nLab currently calls a strict epimorphism. I have seen some sources define “regular epi” or “effective epi” to mean this notion in the absence of kernel pairs. In particular I’m thinking of the remarks after C2.1.11 in the Elephant which define a sieve to be “effective-epimorphic” just when the morphisms composing it are jointly “strictly epimorphic” in the sense used here. It would be interesting to see examples separating strict epis from regular ones.

    • CommentRowNumber9.
    • CommentAuthorDavidRoberts
    • CommentTimeJan 7th 2011

    Regarding regular epimorphisms of smooth manifolds: these are more general that surjective submersions, and include all split epis, which are not very useful (in general) when working with manifolds. It seems regular epis really are only good, as Mike says, in the presence of kernel pairs.

    • CommentRowNumber10.
    • CommentAuthorzskoda
    • CommentTimeJan 7th 2011
    • (edited Jan 7th 2011)

    I do not like that nnLab defines “covering” in a specific way topos theorists and alike strains of category theorists do. Covering/cover is a much more wide notion which can take in various functorial setups different versions.

    If somebody wants to make an equality between regular epi and covering, than say regular epi, without assuming local conventions from a narrow subfield of mathematics, and without incorporating them in addition to a number of other entries. It is for example, inappropriate for categorical situations related to “sheaves” on noncommutative spaces.

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