Simplicial morphisms from degenerate simplices - MathOverflow most recent 30 from http://mathoverflow.net 2013-05-24T00:50:47Z http://mathoverflow.net/feeds/question/90506 http://www.creativecommons.org/licenses/by-nc/2.5/rdf http://mathoverflow.net/questions/90506/simplicial-morphisms-from-degenerate-simplices Simplicial morphisms from degenerate simplices Mark.Neuhaus 2012-03-07T21:21:31Z 2012-03-07T21:21:31Z <p>Frequently I read the following statement on morphisms of simplicial sets:</p> <p>Suppose $X$ and $Y$ are simplicial sets, $f: X \rightarrow Y$ is a simplicial morphism and we write $X_n$ as well as $Y_n$ and $f_n$ for the dimension $n$ parts. Moreover suppose that there is a $n_0 \in \mathbb{N}_0$ such that $X$ has only degenerate simplices $x \in X_n$ for all $n \geq n_0$. (That is each such element is in the image of at least one degeneracy map)</p> <p>Then any simplicial morphism is determined by its dimension $\leq n$ -parts, that is each $f$ is determined by all the $f_m$'s for all $m \leq n$.</p> <p>Unfortunately I never read a proof for this and hence I would like to know how to proof it. I guess that the Eilenberg Zilber lemma that gives a decomposition of any simplex $x$ into a non degenerate simplex a right action of a monotonic map in the simplex category is the basic idea in the proof, but I don't realy know how to work it out.</p> <p>...</p> <p>Since this is my first post here, I don't know if it is custom here to ask for complete proofs, but at least a reference to a proof would be great.</p> <p>Thanks, Marc</p>