Re: Monoidal structure, take II

Re: Monoidal structure, take II

[Marco Grandis]

On Francois Lamarche's question.

If I understand correctly, the tensor product  X tensor Y  has the obvious
objects  (x, y)  and arrows of three types

(a, y): (x, y) -->  (x', y),  for  a: x -> x'  in  X,  y  in  Y,

(x, b): (x, y) -->  (x, y'),  for  x  in  X,  b: y -> y'  in  Y,

(a, b): (x, y) -->  (x', y'),  for  a  and  b  as above.

***

Remark 1.
We are thus simulating "identities" of  X  and  Y  (which are not given).
In other words, we are considering the cartesian product  X'xY'  of the
free reflexive graphs over  X  and  Y,  and taking out its identities.

Would it not be simpler to work with  REFLEXIVE GRAPHS  and their cartesian
closed structure?
In my opinion, reflexive graphs are more natural than graphs:

reflexive graph = 1-truncated simplicial set = 1-truncated cubical set

It is the topos of presheaves over a FULL subcategory of non-empty ordinals
(or cardinals as well), actually the initial segment  1,  2.

Remark 2.
Roughly speaking, a category enriched over reflexive graphs (wrt the cc
structure) is a "2-category without vertical composition". It has cells  a:
f -> g: X -> Y,  with a categorical horizontal composition; it also has
trivial cells  f -> f: X -> Y  ("vertical identities").

All this is clearly related to homotopy and its abstract settings in
"2-dimensional categories" (in some sense).
And indeed topological spaces, with continuous maps and homotopies, form a
rather obvious example.
The horizontal composition of homotopies

   a: f -> g: X -> Y,    b: h -> k: Y -> Z

is     b(a(x, t), t)               t  in  [0, 1],

which is indeed categorical.

Remark 3.
[The sequel is relevant for homotopy; I do not know if it may be relevant
in CS, but I always had the impression that abstract homotopy should be of
use there, eg with respect to deformations of processes, in some sense.]

I do not think that the latter is the "right" 2-dimensional categorical
setting for abstract homotopy (even as a starting point).
The previous horizontal composition of homotopies is rather artificial; it
is what you get from the "double homotopy"

b(a(x, t), t')       (t, t')  in  [0, 1]^2

through the diagonal  t = t'  of the square.
(The "double homotopy" itself is quite natural, as produced by the cubical
enrichment due to the cylinder functor; it is also important in homotopy.)

When the diagonal of the "standard interval" is missing (eg for chain
complexes of abelian groups), there is no canonical horizontal composition
of homotopies (working with the vertical composition, you get two of them;
the middle four interchange does not hold). But there still are canonical
horizontal compositions of "maps with homotopies" and "homotopies with
maps".

This is why I think that the basic 2-dimensional categorical setting for
abstract homotopy should only treat such "reduced horizontal composition":
arrows with cells, cells with arrows, but NOT cells with cells.
Formally, it is again a category enriched over reflexive graphs, BUT wrt
the following monoidal closed structure:

X tensor Y:
 -  the subgraph of  XxY  whose arrows are pairs  (a, b),  where  a  or  b
is an identity;

[X, Y]:
-   vertices: the graph morhisms;
-   arrows: their transformations (without "diagonals").

References:

a) The last enrichment (with further developments) has been used for
abstract homotopy in:

M. Grandis, On the categorical foundations of homological and homotopical
algebra, Cahiers Top. Geom. Diff. Categ. 33 (1992), 135-175.   [sketch]

 - , Homotopical algebra in homotopical categories, Appl. Categ. Structures
2 (1994), 351-406.

b) A notion equivalent to a category enriched in the same sense had already
been studied in:

K.H. Kamps, Ueber einige formale Eigenschaften von Faserungen und
h-Faserungen, Manuscripta Math. 3 (1970), 237-255.

c) For homotopy in groupoid-enriched categories, see Gabriel-Zisman's text
(1967).

***

With best regards

Marco Grandis

Dipartimento di Matematica
Universita' di Genova
via Dodecaneso 35
16146 GENOVA, Italy

e-mail: grandis@dima.unige.it
tel: +39.010.353 6805   fax: +39.010.353 6752

http://www.dima.unige.it/STAFF/GRANDIS/
ftp://pitagora.dima.unige.it/WWW/FTP/GRANDIS/

Monoidal structure, take II

[Francois Lamarche]

Given the private replies I got to yesterday's queries, it is obvious I
was not clear enough, and indeed there was an unhelpful typo.

> 
> I'm wondering, if anybody has ever described the following monoidal
> structure on the category of oriented multigraphs, what MacLane calls
> graphs, the most common kind of graph in category theory (but not

OK Saunders, from now on they're graphs. This what happens when you hang
out with combinatorists AND category theorists.

> 
> Given graphs X, Y, the set |X-oY| of vertices on  X-oY  is the set of graph
> morphisms
> X --> Y.

So right from the start this is not the usual presheaf CC structure,
where the set of vertices is the set of all functions |X| --> |Y| .

So in what follows I use categorical notation for vertices, arrows, etc.
 
> Given f,g : X --> Y the set of arrows f-->g is the set of pairs
> (p_0,p_1) of functions such that
> 
> forall  x in |X|, p_0(x) : f(x)-->g(x)
> 
> forall  k: x-->y in X, p_1(k) : f(x)-->g(y)

Now the typo has been corrected. So an arrow f --> g is like a natural
transformation, with p_0 the usual familly of arrows indexed by the
vertices/objects of X, but since things don't compose, you add the
diagonal p_1 as part of the information. There is some kinship to
homotopies, as M. Barr has remarked.

> This co-contra bifunctor has a tensor left adjoint, which is symmetric
> and monoidal.
> 

Is this more understandable?

Thanks again

Francois

Re: Monoidal structure, take II

[Michael Barr]

I did not quite understand Francois' construction.  However, my first
reaction to a question like that is that it ought to be a homotopy.  So
I will say what a homotopy reduces to in this case and leave it to
Francois to decide if this is what he has.

I suspect that rather few people know what a simplicial homotopy is and,
of those, rather few have ever actually verified one.  I am in that
minority^2, so perhaps I tend to see them where they are not the most
natural, but I think it quite remarkable that they can arise where no
real topology (but some combinatorics) is present.

I have to begin by saying how a graph becomes a simplicial set.
Actually, that is a lie, since unless you are dealing with reflexive
graphs--that are equipped with a selected loop at each vertex--you will
only get a face complex.  But homotopies are still definable.  A
category is a simplicial set by taking for n-simplexes composable
n-tuples of arrows.  This doesn't work for graphs, since the "interior
faces" (all except the lowest and highest numbered) all depend on
composition.  But there is a face complex in which an n-simplex is
simply an n-simplex in the graph.  So a 2-simplex is a
triangle--obviously non-commutative and a 3-simplex is a tetrahedron and
so on.  You can describe a composable n-tuple in a category as
commutative n-simplex, so this isn't so different.  Now given this, if
f,g:  X --> Y are graph morphisms, what is a homotopy?  Well, write X as
d^0,d^1:  X_1 --> X_0 and similarly for Y. Then f consists of f_0:  X_0
--> Y_0 and f_1:  X_1 --> Y_1 giving a serially commutative square.
Just a homomtopy between functors turns out to be simply a natural
transformation, a homotopy in this case turns out to consist of a
function p_0:  X_0 --> Y_1 and a function p_1:  X_1 --> Y_1 such that
there
is a diagram (not, of course commutative; what a diagram does is specify
source and target) as follows.  In this diagram I assume x:  x^0 --> x^1
in X, and f(x):  y^0 --> y^1 and g(x):  z^0 --> z^1 in Y.
                            f(x)
                    y^0 -----------> y^1
                     | \              |
                     |  \             |
                     |   \            |
                     |    \           |
                     |     \          |
                     |      \         |
                     |       \        |
             p_0(x^0)|  p_1(x)\       |p_0(x^1)
                     |         \      |
                     |          \     |
                     |           \    |
                     |            \   |
                     |             \  |
                     |              \ |
                     v      g(x)     vv
                    z^0 -----------> z^1

So if this is what Francois was saying, then the answer is it a homotopy
of face complexes.  Of course, if you replaced X_1 by X_1 + X_0, you
would have a reflexive graph and I assume (I have not checked this) you
would then get a simplicial homotopy.

BTW, homotopies do not generally compose--and the ones described here do
not appear to either.  Categories are special because of their internal
composition.  It makes me wonder if the well-known failure of dinats to
compose could be related to this in some way.

Having seen Francois' clarification, I think this is exactly what he
had.

Michael



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