Example 14.3.4. The simplest example is the constant simplicial object with value $X \in \mathop{\mathrm{Ob}}\nolimits (\mathcal{C})$. In other words, $U_ n = X$ and all maps are $\text{id}_ X$.

In your comment you can use Markdown and LaTeX style mathematics (enclose it like $\pi$). A preview option is available if you wish to see how it works out (just click on the eye in the toolbar).