Definition 14.11.1. Let $U$ be a simplicial set. We say $x$ is an *$n$-simplex of $U$* to signify that $x$ is an element of $U_ n$. We say that $y$ is the $j$the *face of $x$* to signify that $d^ n_ jx = y$. We say that $z$ is the $j$th *degeneracy of $x$* if $z = s^ n_ jx$. A simplex is called *degenerate* if it is the degeneracy of another simplex.

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## Comments (1)

Comment #8516 by Takagi Benseki on