Example 67.53.3. This is a continuation of Remark 67.19.4. Consider the algebraic space $X = \mathbf{A}^1_ k/\{ x \sim -x \mid x \not= 0\} $. There are morphisms
\[ \mathbf{A}^1_ k \longrightarrow X \longrightarrow \mathbf{A}^1_ k \]
such that the first arrow is étale surjective, the second arrow is universally injective, and the composition is the map $x \mapsto x^2$. Hence the composition is universally closed. Thus it follows that the map $X \to \mathbf{A}^1_ k$ is a universal homeomorphism, but $X \to \mathbf{A}^1_ k$ is not separated.
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