The dimension strata of a non-singular analytic space are analytic manifolds #
On a non-singular analytic K-space X, every point x has an open neighbourhood V and a
K-isomorphism (G', ๐_{G'}) โ
X | V with G' โ K^d open and d = dim ๐ช_{X,x}, by Hironaka's
characterization of the simple points [Hir64, Ch. 0, ยง1, p. 121]
(isRegular_stalk_iff_exists_manifold_nhd), and every point of V then has dimension d. Hence
- the dimension is locally constant: the strata
X_d = {x | dim ๐ช_{X,x} = d}are open (isOpen_dimSet) and partitionX(iUnion_dimSet_eq_univ); - the charts
x โฆ q โ G' โ K^dgiven by the inverses of these isomorphisms form an atlas ofX_d(stratumChart, theChartedSpaceinstance), whose chart changes are the underlying maps of theK-isomorphisms(G'_x, ๐) | โฏ โ X | (V_x โฉ V_y) โ (G'_y, ๐) | โฏand hence analytic (contMDiff_baseFun,Hironaka.AnalyticSpace.Manifold.FullyFaithful):X_dis an analytic manifold modelled onK^d(stratumManifold : AnalyticManifold K (Fin d โ K)), Hausdorff and second countable as a subspace ofX.
This is the manifold half of the statement that a smooth space is a manifold, locally
pure-dimensional [BM97, (3.8)(2)]; the K-isomorphism Sp(X_d) โ
X | X_d completing it is
Hironaka.AnalyticSpace.Manifold.StratumIso.
The dimension strata #
The points of local dimension d: X_d = {x | dim ๐ช_{X,x} = d}.
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Every point of a chart neighbourhood X | V โ
(G', ๐_{G'}), G' โ K^d, has dimension d.
At a point of dimension d of a non-singular space there is a chart (G', ๐_{G'}) โ
X | V,
G' โ K^d [Hir64, Ch. 0, ยง1, p. 121].
The dimension is locally constant on a non-singular space: each stratum X_d is open.
The strata cover the space: every stalk of a non-singular space has finite dimension.
The stratum X_d as an open of X.
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The charts #
The chart neighbourhood of a point of the stratum (a choice).
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The open G' โ K^d of the chart.
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The K-isomorphism (G', ๐_{G'}) โ
X | V of the chart.
Equations
- X.chartE hX d x = Classical.choice โฏ
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The homeomorphism G' โโ V underlying the chart isomorphism.
Equations
- X.chartHomeo hX d x = (X.chartE hX d x).homeomorph
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The point of the stratum under a point of the chart's open G'.
Equations
- AnalyticSpace.chartPt x q = โจโ((CategoryTheory.ConcreteCategory.hom (โ(X.chartE hX d x).hom).base) q), โฏโฉ
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The chart coordinates of a point of V, in K^d.
Equations
- AnalyticSpace.stratumCoord x w hw = (โ((X.chartHomeo hX d x).symm โจโw, hwโฉ)).down
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The chart of the stratum at x: on V_x โฉ X_d, the coordinates
w โฆ e_xโปยน(w) โ G'_x โ K^d.
Equations
- One or more equations did not get rendered due to their size.
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The atlas of the stratum.
Equations
- X.instChartedSpaceDimOpens hX d = { atlas := Set.range AnalyticSpace.stratumChart, chartAt := AnalyticSpace.stratumChart, mem_chart_source := โฏ, chart_mem_atlas := โฏ }
Chart changes are analytic #
The overlap V_x โฉ V_y, seen in G'_x.
Equations
- AnalyticSpace.overlapOpens x y = (TopologicalSpace.Opens.map (โ(CategoryTheory.CategoryStruct.comp (X.chartE hX d x).hom (X.ofRestrict (X.chartV hX d x)))).base).obj (X.chartV hX d y)
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The open immersion (G'_x, ๐) | (V_x โฉ V_y) โถ X.
Equations
- One or more equations did not get rendered due to their size.
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The transition K-isomorphism between the overlaps read in the two charts.
Equations
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The trace of the overlap on K^d: an open G'' โค G'_x.
Equations
- AnalyticSpace.overlapTrace x y = โฏ.choose
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The overlap flattened to (G'', ๐_{G''}) (traceIso).
Equations
- AnalyticSpace.overlapTraceIso x y = AnalyticSpace.traceIso K d (X.chartG hX d x) โฏ (AnalyticSpace.overlapOpens x y) โฏ
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The transition, as a K-isomorphism of the spaces of the open submanifolds G''_x, G''_y of
K^d.
Equations
- One or more equations did not get rendered due to their size.
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The underlying map of the transition.
Equations
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The transition preserves the point of X: e_y (ฯ p) = e_x p in X.
The extension by an arbitrary value of the transition to K^d.
Equations
- AnalyticSpace.transitionExt x y w = if hw : w โ AnalyticSpace.overlapTrace x y then โ(AnalyticSpace.transitionFun x y โจw, hwโฉ) else 0
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The chart change of the stratum at a point of its domain is the transition.
The chart changes of the stratum are analytic.
The stratum X_d is an analytic manifold modelled on K^d.
The stratum X_d as a bundled analytic manifold over K^d.
Equations
- X.stratumManifold hX d = { carrier := โฅ(X.dimOpens hX d), top := instTopologicalSpaceSubtype, charted := X.instChartedSpaceDimOpens hX d, manifold := โฏ, t2 := โฏ, secondCountable := โฏ }