ScalingStacks

A.6 Clemens cones and paths [03XU]

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A.6 Clemens cones and paths

For a model ๐’ณ{\cal X} we can interpret elements of CXaโ€‹nโ€‹(๐™)C_{X^{an}}({\bf Z}) as paths in ๐’ณ{\cal X}, i.e. equivalence classes of maps

ฯ•:Sโ€‹pโ€‹eโ€‹cโ€‹(๐’ชL)โ†’๐’ณ,\phi:Spec({{\cal O}}_{L})\to{\cal X},

where ๐’ชL{{\cal O}}_{L} is the ring of integers in a field LL with discrete valuation in ๐™{\bf Z}, such that the image of ฯ•\phi does not lie in ๐’ณ{\cal X}. We define the map

p๐’ณ๐™:CXaโ€‹nโ€‹(๐™)โ†’C๐’ณโ€‹(๐™)p_{\cal X}^{\bf Z}\,:\,C_{X^{an}}({\bf Z})\to C_{\cal X}({\bf Z})

as

p๐’ณ๐™โ€‹([ฯ•]):=โˆ‘iaiโ€‹โŸจDiโŸฉ,p_{\cal X}^{\bf Z}([\phi]):=\sum_{i}a_{i}\langle D_{i}\rangle,

where aiโˆˆ๐™โ‰ฅ0a_{i}\in{\bf Z}_{\geq 0} is the multiplicity of the intersection of the path ฯ•\phi with the divisor Di,iโˆˆI๐’ณD_{i},\,\,i\in I_{\cal X}.

The following proposition can be derived from [Be1].

Proposition 10

The map p๐’ณ๐™p_{\cal X}^{\bf Z} extends uniquely to a continuous ๐‘+ร—{\bf R}_{+}^{\times}-equivariant map p๐’ณ๐‘:CXaโ€‹nโ€‹(๐‘)โ†’C๐’ณโ€‹(๐‘)p_{\cal X}^{\bf R}\,:\,C_{X^{an}}({\bf R})\to C_{\cal X}({\bf R}). The map p๐’ณ๐‘p_{\cal X}^{\bf R} is a surjection.

We denote by p๐’ณ:Xaโ€‹nโ†’S๐’ณp_{\cal X}:X^{an}\to S_{\cal X} the map induced by p๐’ณ๐‘p_{\cal X}^{\bf R}.

Let f:๐’ณโ€ฒโ†’๐’ณf:{\cal X}^{\prime}\to{\cal X} be a dominating map of models. Let us denote by mi,iโ€ฒโˆˆ๐™โ‰ฅ0m_{i,i^{\prime}}\in{{\bf Z}}_{\geq 0} the multiplicity of a divisor Diโ€ฒ,iโ€ฒโˆˆI๐’ณโ€ฒD_{i^{\prime}},i^{\prime}\in I_{{\cal X}^{\prime}} in the proper pull-back of Di,iโˆˆI๐’ณD_{i},i\in I_{\cal X}. We define p๐’ณโ€ฒ,๐’ณ๐™:CXaโ€‹nโ€‹(๐™)โ†’C๐’ณโ€‹(๐™)p_{{\cal X}^{\prime},{\cal X}}^{{\bf Z}}:C_{X^{an}}({\bf Z})\to C_{\cal X}({\bf Z}) by the formulas โˆ‘iโ€ฒaiโ€ฒโ€‹โŸจDiโ€ฒโŸฉโ†ฆโˆ‘imi,iโ€ฒโ€‹aiโ€ฒโ€‹โŸจDiโŸฉ\sum_{i^{\prime}}a_{i^{\prime}}\langle D_{i^{\prime}}\rangle\mapsto\sum_{i}m_{i,i^{\prime}}a_{i^{\prime}}\langle D_{i}\rangle. Let p๐’ณโ€ฒ,๐’ณ:SXaโ€‹nโ€‹(๐‘)โ†’S๐’ณโ€‹(๐‘)p_{{\cal X}^{\prime},{\cal X}}:S_{X^{an}}({\bf R})\to S_{\cal X}({\bf R}) be the corresponding by map of Clemens polytopes.

Then we have the following result, which is easy to prove.

Lemma 8

For any dominating map of models ๐’ณโ€ฒโ†’๐’ณ{\cal X}^{\prime}\to{\cal X} we have

p๐’ณ๐™=p๐’ณโ€ฒ,๐’ณ๐™โˆ˜p๐’ณโ€ฒ๐™.p_{{\cal X}}^{\bf Z}=p_{{\cal X}^{\prime},{\cal X}}^{\bf Z}\circ p_{{\cal X}^{\prime}}^{{\bf Z}}\,\,.
Corollary 4

For dominating maps ๐’ณโ€ฒโ€ฒโ‰ฅ๐’ณโ€ฒโ‰ฅ๐’ณ{\cal X}^{\prime\prime}\geq{\cal X}^{\prime}\geq{\cal X} we have

p๐’ณโ€ฒโ€ฒ,๐’ณ=p๐’ณโ€ฒ,๐’ณโˆ˜p๐’ณโ€ฒโ€ฒ,๐’ณโ€ฒ.p_{{\cal X}^{\prime\prime},{\cal X}}=p_{{\cal X}^{\prime},{\cal X}}\circ p_{{\cal X}^{\prime\prime},{\cal X}^{\prime}}\,\,.
Theorem 10

For any algebraic XX the analytic space Xaโ€‹nX^{an} is a projective limit over the partially ordered set of snc models ๐’ณ{\cal X} of Clemens polytopes S๐’ณS_{\cal X}. The connecting maps are p๐’ณโ€ฒ,๐’ณp_{{\cal X}^{\prime},{\cal X}}.

With any meromorphic at t=0t=0 family of smooth complex projective varieties Xt,โ€‰โ€‰โ€‰0<|t|<ฯตX_{t},\,\,\,0<|t|<\epsilon one can associate a variety XX over the field ๐‚โก((t)){\bf C}((t)). It is easy to see that for any snc model ๐’ณ\cal X one can canonically complete the family XtX_{t} by adding S๐’ณS_{\cal X} as the fiber over t=0t=0. The total space is not a complex manifold by just a Hausdorff locally compact space which maps properly to the dick {tโˆˆ๐‚||t|<ฯต}\{t\in{\bf C}\,|\,|t|<\epsilon\}. Passing to the projective limit we see that one can compactify the family XtX_{t} at t=0t=0 by Xaโ€‹nX^{an}.

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