My question concerns Section 2 of the article "The Simplicial Model of Univalent Foundations (after Voevodsky)" (<https://arxiv.org/pdf/1211.2851.pdf>).


Let $\alpha$ be a strongly inaccessible cardinal.


Let $f : X \to Y$ be a map of simplicial sets.


1. We say that $f$ is *well-ordered* if it is equipped with a well-ordering of $Y\_x := f\_n^{-1}(x)$ for each simplex $x\in X\_n$.
2. We say that $f$ is *$\alpha$-small* if $\left\lvert{Y\_x}\right\rvert < \alpha$ for every simplex $x$.


Let $f: X \to Y$ and $g : Z \to Y$ be well-ordered simplicial maps. A *morphism $f \to g$* is a fiber-preserving simplicial map $h : X \to Z$
such that $h\_n : f\_n^{-1}(y) \to g\_n^{-1}(y)$ is order-preserving for every natural number $n$ and every $y\in Y\_n$.


Define the functor $\mathcal{U}\_{\alpha} : \mathbf{sSet}^{\text{op}} \to \mathbf{Set}$ so that $\mathcal{U}\_{\alpha}(X)$ consists of all isomorphism classes of $\alpha$-small well-ordered Kan fibrations $Y\to X$. Also, let
$$
\mathrm{U}\_{\alpha} = \mathcal{U}\_{\alpha}\circ \mathcal{Y}^{\text{op}} : \varDelta^{\text{op}} \to \mathbf{Set}
$$ where $\mathcal{Y} :\varDelta \to \mathbf{sSet}$ denote the Yoneda embedding.


The authors say that if $\beta <\alpha$ is also inaccessible, then the unique map $\mathrm{U}\_{\beta} \to 1$ is $\alpha$-small (p. 23, near the bottom). This amounts to saying that the set


$$
\left(\mathrm{U}\_{\beta}\right)\_n = \mathcal{U}\_{\beta}(\Delta[n])
$$
has cardinality $<\alpha$ for each $n$.


I am unable, however, to see a set-theoretic justification for this. I'd be grateful if someone could provide one!



Showing that a certain simplicial set has levelwise small cardinality
$\mathcal{U}\_\beta(\Delta[n])$ is the set of isomorphism classes of $\beta$-small well-ordered fibrations over $\Delta[n]$. Such a thing is uniquely determined by an isomorphism class of $\beta$-small well-orderings over each element of $\Delta[n]$, together with the face and degeneracy maps between them. There are $\beta$ isomorphism classes of $\beta$-small well-orderings, and there are countably many elements of $\Delta[n]$. Since $\beta \cdot \aleph\_0 = \beta$, there are $\beta$ choices of these fibers. Now for every face and degeneracy map, the domain and codomain are $\beta$-small, hence (since $\beta$ is inaccessible) there are $<\beta$ possible maps. Since there are countably many face and degeneracy maps, there are no more than $\beta \cdot \aleph\_0 = \beta$ choices of the face and degeneracy maps. Thus there are overall $\beta$ choices, and $\beta<\alpha$.


