ScalingStacks

Example 2 . [04QU]

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Example 2.

Let A⊂ℤn+1A\subset\mathbb{Z}^{n+1} be a finite set and let v:A→ℝv:A\to\mathbb{R} be any function. Let Δ⊂ℝn+1\Delta\subset\mathbb{R}^{n+1} be the convex hull of AA. We associate the following polyhedral complex Πv\Pi_{v} to vv.

Take the Legendre transform Lv:ℝn+1→ℝL_{v}:\mathbb{R}^{n+1}\to\mathbb{R} of vv

Lv​(y)=maxx∈A⁡(x​y−v⁡(x)).L_{v}(y)=\max\limits_{x\in A}(xy-v(x)).

Here x,y∈ℝn+1x,y\in\mathbb{R}^{n+1} and x​yxy is their scalar product. Since the maximum is taken over a finite set, the result LvL_{v} is a convex piecewise-linear function. We define Πv\Pi_{v} as the corner locus of LvL_{v} (recall that this is the set of points where LvL_{v} is not smooth).

To present Example 1 as a special case of Example 2 we take the vertices of the standard simplex

(1) Δ1{(x1,…,xn+1)∈ℝn+1|xj≥0,x1+⋯+xn+1≤1}\Delta_{1}\{(x_{1},\dots,x_{n+1})\in\mathbb{R}^{n+1}\ |\ x_{j}\geq 0,x_{1}+\dots+x_{n+1}\leq 1\}

for AA and set v≡0v\equiv 0.

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