Triangle commutative diagram does not work here at MO

Question

I frequently tried to draw a triangle commutative diagram similar to the definition of projective modules for this question.

A module associated to an endomorphism of a vector bundle

so I searched in latex stackexchange and I find some relevant post.

then I copy paste the corresponding commands and I observed that that does not work for MO.

I copied the commands in this post:

https://tex.stackexchange.com/questions/191462/creating-triangle-commutative-diagram-with-tikz-cd

(I prefer $\gamma$ lies on a Dots arrows.

can I ask you to kindly help me to draw this diagram. such a diagram can present my meaning more precisely.

thank you in advance for your help.

P.S.: Even when I try a simple command as $$\begin{CD} A \\ @VVV \\ B \end{CD}$$

It does not work at MO.

Answer 1

This is a community wiki answer made explicitly for testing.

$$\require{AMScd}\begin{CD}
A \\
@VVV \\
B
\end{CD}$$

$$\require{AMScd}\begin{CD} A \\ @VVV \\ B \end{CD}$$

$$\require{AMScd}
\def\diaguparrow#1{\smash{
  \raise.6em\rlap{\scriptstyle #1}
  \lower.3em{\mathord{\diagup}}
  \raise.52em{\!\mathord{\nearrow}}
}}
\begin{CD}
&& E\\
& \diaguparrow{\gamma} @VV \alpha V \\
F @>> \beta> G
\end{CD}$$

$$\require{AMScd} \def\diaguparrow#1{\smash{ \raise.6em\rlap{\scriptstyle #1} \lower.3em{\mathord{\diagup}} \raise.52em{\!\mathord{\nearrow}} }} \begin{CD} && E\\ & \diaguparrow{\gamma} @VV \alpha V \\ F @>> \beta> G \end{CD}$$ This comes from taking the three characters $\gamma \diagup \nearrow$, adjusting their heights, using rlap so that the first two characters overlap, using $\backslash !$ to back up the third character, using smash to make them all spill out of a $0\times 0$ box, and then putting that box in the right place.

Answer 2

Added in 2022. The website mentioned below does not work now - so this is no longer an option. (It seems that it has been down for some time. And the corresponding GitHub repository seems to be inactive, too. There are only a few posts on MO with such links. There are more such posts on Mathematics Stack Exchange.)

---

Especially for more complicated diagrams (where it's difficult to draw them using AMScd), a possible solution is to use presheaf website and include the picture into the post. I have learned about this site from Bruno Stonek's answer to How to draw a commutative diagram? (Mathematics Meta).

Advantages of this website:

• It is easy to use for people who are already familiar with xypic. (Of course there are also other LaTeX packages for commutative diagrams, the most popular probably are tikz-cd and xy-pic.)
• The code is compiled online and it generates picture which can be used elsewhere. (Naturally, some users might prefer to do this locally on their computer and convert the output from LaTeX to picture by themselves.)
• If you add link to the website, you simultaneously provide the xypic code.

I will explicitly mention that images uploaded through the editor should be stable and relatively immune to link rot, see: Permanent Picture Uploads (Mathematics Meta).

^{I am fully aware that this is not optimal from the viewpoint of screen readers and visually impaired users. However, web accessibility is probably quite a challenging issue when using commutative diagrams and various other visualizations.}

Some examples:

http://presheaf.com/?d=d5f3h332d491j3m2v5j4g5o193hr503v

http://presheaf.com/?d=d564x6z6o2c383r6t3r55r375c4v4l1h

http://presheaf.com/?d=d4l86n4i40s4n18675m3rw6cye1p

http://presheaf.com/?d=d6213453at1p2gd324i21h3i4g3v32