# Michael Shulman truncated object

An object $A$ of an $n$-category $K$ is $k$-truncated if $K\left(X,A\right)$ is a $k$-category for every object $X\in K$. Here $n\le 2$ (although the notion makes sense more generally) and $k\le n-1$, but neither need be directed (see n-prefix). For example:

• In an $n$-category every object is $\left(n-1\right)$-truncated.
• In a 1-category (or, actually, in any $n$-category) the (-1)-truncated objects are the subterminals, and a terminal object is the only (-2)-truncated object.
• In a 2-category, the (1,0)-truncated objects are the groupoidal ones, the (0,1)-truncated objects are the posetal ones, and the 0-truncated objects are the discrete ones.

We write ${\mathrm{trunc}}_{k}\left(K\right)$ for the full sub-$n$-category of $K$ spanned by the $k$-truncated objects, which is a $\mathrm{min}\left(n,k+1\right)$-category. If an object $A$ has a reflection into ${\mathrm{trunc}}_{k}\left(K\right)$, we call this reflection the $k$-truncation of $A$ and write it as ${A}_{\le k}$. See truncation in an exact 2-category for ways to construct such truncations.

Revised on February 17, 2009 17:33:11 by Mike Shulman (75.3.140.11)