ScalingStacks

Example 3.65 . [02MF]

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Example 3.65.

Let Λ\Lambda be a convex polyhedron in NℝN_{\mathbb{R}}. Then both the indicator function ιΛ\iota_{\Lambda} and the support function ΨΛ\Psi_{\Lambda} are concave and piecewise affine. We have ΨΛ∨=ιΛ\Psi_{\Lambda}^{\vee}=\iota_{\Lambda}. In particular, if we fix an isomorphism Nℝ≃ℝnN_{\mathbb{R}}\simeq\mathbb{R}^{n}, the function

ΨΔn:Nℝ⟶ℝ,(u1,…,un)⟼min⁡{0,u1,…,un}\Psi_{\Delta^{n}}\colon N_{\mathbb{R}}\longrightarrow\mathbb{R},\quad(u_{1},\dots,u_{n})\longmapsto\min\{0,u_{1},\dots,u_{n}\}

is the support function of the standard simplex Δn=conv⁡(𝟎,e1∨,…,en∨)⊂Mℝ\Delta^{n}=\operatorname{conv}(\boldsymbol{0},e^{\vee}_{1},\dots,e^{\vee}_{n})\subset M_{\mathbb{R}}, where {e1,…,en}\{e_{1},\dots,e_{n}\} is the standard basis of ℝn\mathbb{R}^{n} and {e1∨,…,en∨}\{e_{1}^{\vee},\dots,e_{n}^{\vee}\} is the dual basis. Hence, stab⁡(ΨΔn)=Δn\operatorname{stab}(\Psi_{\Delta^{n}})=\Delta^{n} and ΨΔn∨=ιΔn\Psi_{\Delta^{n}}^{\vee}=\iota_{\Delta^{n}}.

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