• Adjoint Equivalence. We have an adjoint equivalence of categories
    witnessed by a bijection of sets
    \begin{align*} \mathsf{Sets}\webleft (X^{-},Y\webright )\cong \mathsf{Sets}_{*}\webleft (X,Y^{+}\webright ),\end{align*}

    natural in $X\in \text{Obj}\webleft (\mathsf{Sets}_{*}\webright )$ and $Y\in \text{Obj}\webleft (\mathsf{Sets}\webright )$, and by isomorphisms

    \begin{align*} \webleft (X^{-}\webright )^{+} & \cong X,\\ \webleft (Y^{+}\webright )^{-} & \cong Y, \end{align*}

    once again natural in $X\in \text{Obj}\webleft (\mathsf{Sets}_{*}\webright )$ and $Y\in \text{Obj}\webleft (\mathsf{Sets}\webright )$.


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