When does adding inverses of morphisms preserve commutativity of a diagram? - MathOverflow most recent 30 from http://mathoverflow.net 2013-05-18T17:09:34Z http://mathoverflow.net/feeds/question/21591 http://www.creativecommons.org/licenses/by-nc/2.5/rdf http://mathoverflow.net/questions/21591/when-does-adding-inverses-of-morphisms-preserve-commutativity-of-a-diagram When does adding inverses of morphisms preserve commutativity of a diagram? Steven Gubkin 2010-04-16T16:56:32Z 2010-04-16T18:26:58Z <p>Here is the essence of a problem I have run in to: I have a finite poset D with a terminal object. If I formally invert all of the morphisms, and add these into my diagram, does the new diagram D' still commute?</p> <p>I think that the resulting diagram will still commute basically because I have done a lot of examples. Working out a few examples you can see that it basically follows by doing it for the "commutative triangle", and applying this finitely many times. It feels like I should be able to do some kind of messy induction, but I do not really want such a proof cluttering up my work.</p> <p>Is there a reference I could quote for a result like this? It seems like if it is true it should be a "folk lemma".</p> <p>Of course, if you have more relaxed criteria for when the result will hold, that would be helpful too. </p> <p>Also if you know of a conceptual proof which does not fall back on some messy induction, that would be wonderful!</p> <p>EDIT: An example might help to clarify my question. (How do you draw diagrams on MO?)</p> <pre><code> a--&gt;b ^ ^ | | c--&gt;d </code></pre> <p>is my poset. b is the terminal object. Now say someone told you that this was actually a subcategory of a larger category, and in that larger category all of the arrows were invertible. Now consider the larger diagram consisting of the 4 original arrows and their inverses. Is this diagram also commutative? Yes! It is just one or two lines of formal manipulation. </p> http://mathoverflow.net/questions/21591/when-does-adding-inverses-of-morphisms-preserve-commutativity-of-a-diagram/21596#21596 Answer by Reid Barton for When does adding inverses of morphisms preserve commutativity of a diagram? Reid Barton 2010-04-16T18:01:43Z 2010-04-16T18:26:58Z <p>There is an easy conceptual proof using the fact that the category obtained by formally inverting all the arrows in a category C is equivalent to the fundamental groupoid of the nerve NC of C, and that the nerve of a category with a final object is contractible. Without the assumption of a final object your assertion is false in general, e.g., reverse the arrows from c in your example.</p> <p>But it should also be easy to prove by induction: for any zigzag of arrows between a and b, the corresponding map in the category with all arrows inverted, when composed with the map from b to the original terminal object, is equal to the map from a to the original terminal object (this is by induction); and so any two maps from a to b in the category with all arrows inverted are equal. In symbols: let me write $t_x$ for the unique morphism in C from $x$ to the terminal object and $[f]$ for the image of $f$ in the category with all arrows inverted. Suppose $[f_1]^{\pm 1} \cdots [f_n]^{\pm 1}$ is a typical map in the category with all arrows inverted with domain $a$ and target $b$. Then the inductive claim is that $[t_b] [f_1]^{\pm 1} \cdots [f_n]^{\pm 1} = [t_a]$, and so $[f_1]^{\pm 1} \cdots [f_n]^{\pm 1} = [t_b]^{-1} [t_a]$.</p>