ScalingStacks

4.2.1 Logarithmic map [03U4]

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4.2.1 Logarithmic map

This is a basic example

π=πc​a​n=log|⋅|:X=(𝐆ma​n)n→B0=B=𝐑n\pi=\pi_{can}=\log|\cdot|:X=({\bf G}_{m}^{an})^{n}\to B_{0}=B={{\bf R}}^{n}

described in details in Appendix A. For any algebraic (or analytic) subvariety Z⊂(𝐆ma​n)nZ\subset({\bf G}_{m}^{an})^{n} of dimension m≤nm\leq n its image π⁡(Z)\pi(Z) is a non-compact piecewise-linear closed subset of 𝐑n{\bf R}^{n} of real dimension mm. Smooth points for π|Z\pi_{|Z} are dense in π⁡(Z)\pi(Z).

In particular, if ZZ is a curve then π⁡(Z)\pi(Z) is a graph in BB with straight edges having rational directions. One can try to make a dictionary which translates the properties of the algebraic variety Z⊂𝐆mnZ\subset{\bf G}_{m}^{n} to the properties of the PL set π⁡(Za​n)\pi(Z^{an}) which is the closure of π⁡(Z⁡(K¯))\pi(Z(\overline{K})) in 𝐑n{\bf R}^{n}. This circle of ideas is a subject of the so-called “tropical geometry” (see e.g. [Mi]).

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