ScalingStacks

Lemma 3.3 . [03DT]

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Lemma 3.3.

In certain special cases of later interest we can describe Q(I∣J)λ​(0)Q^{\lambda}_{(I\mid J)}(0) as follows:

  1. (1)

    If J=∅J=\emptyset, then QIλ​(0)Q^{\lambda}_{I}(0) is non-empty if and only if II is the set of vertices of some simplex σ¯∈S∗{0}\overline{\sigma}\in S\ast\{0\}, in which case QIλ​(0)=Qσ¯λQ^{\lambda}_{I}(0)=Q^{\lambda}_{\overline{\sigma}}.

  2. (2)

    Q({0}∣v)λ​(0)Q^{\lambda}_{(\{0\}\mid v)}(0) contains the relative interior of the facet Gv⊂Δλ∨G_{v}\subset\Delta^{\vee}_{\lambda}.

  3. (3)

    Q({0}∣w⟂)λ​(0)Q^{\lambda}_{(\{0\}\mid w^{\perp})}(0) is the Minkowski sum of the polytope Δλ∨\Delta^{\vee}_{\lambda} and the normal cone to carrierΔ\w⟂⁡{0}\operatorname{carrier}_{\Delta\backslash w^{\perp}}\{0\}:

    Q({0}∣w⟂)λ​(0)=Δλ∨+NCΔ\w⟂⁡({0}).Q^{\lambda}_{(\{0\}\mid w^{\perp})}(0)=\Delta^{\vee}_{\lambda}+\operatorname{NC}_{\Delta\backslash w^{\perp}}(\{0\}).

    In particular, it contains Δλ∨+cone⁡(w)\Delta^{\vee}_{\lambda}+\operatorname{cone}(w).

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