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I think my reasoning is correct, but I want to run through it here because having the right intuition will make similar problems easier in future.

A 2-simplex is homeomorphic to a closed disc, and a closed disc is homeomorphic to a hemisphere, so we can "build" $S^2$ out of two 2-simplices. However we need to have 3 specified vertices, say $v_0, v_1$ and $v_2$, on the circumference where the two hemispheres meet. This also means three 1-simplices joining these vertices.

Is this the simplest $\Delta$-structure possible?

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  • $\begingroup$ You can get a simpler structure by modeling the sphere as a CW-complex by using a single 0-cell and a single 2-cell. $\endgroup$
    – wckronholm
    May 19, 2011 at 18:53
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    $\begingroup$ The OP wants a delta-complex structure, not a cell-complex structure. $\endgroup$
    – Josh
    May 19, 2011 at 20:44
  • $\begingroup$ @Josh: Of course. My comment was not meant to be an answer, but merely meant to illuminate the fact that in other settings things can be simpler. $\endgroup$
    – wckronholm
    May 19, 2011 at 22:43
  • $\begingroup$ @wchronholm: Ah, that makes sense. $\endgroup$
    – Josh
    May 19, 2011 at 23:55

1 Answer 1

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You need at least two $2$-simplices, but you can glue them up in another way to get an equally simple structure. Take a simplex and glue two of its sides together to get a cone. Do this for another simplex, and then glue the boundary circles together. This is a $\Delta$ complex structure on the sphere with 3 vertices 3 edges and 2 faces, just like yours, but glued together differently.

To see that you can't get away with just one $2$-simplex, you just have to notice that there's no way of gluing the sides of a triangle together to get a sphere. (or even a surface).

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  • $\begingroup$ +1 Great, this is exactly what I wanted to know. That there isn't anything simpler than 3 vertices, 3 edges and 2 faces was the hunch I had. Interesting alternative structure there too.. $\endgroup$
    – Sputnik
    May 19, 2011 at 18:39

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