Isomorphisms of nodal curves presented by flag-pair data #
A nodal curve presented by flag-pair data d : FlagPairData ฮน Flag X is the quotient of the
disjoint union ฮฃ i, X i of its normalization components by the flag pairs. An isomorphism of
two such curves is given on the normalizations: it is a diffeomorphism of the disjoint unions of
the components which carries the two points of a flag pair exactly to the two points of a flag
pair. For complex curves (๐ = โ and n = โ) the diffeomorphism is a biholomorphism, so an
isomorphism consists of component biholomorphisms together with the compatible maps of the dual
graph.
This file defines these isomorphisms, extracts the combinatorial maps they induce, descends them to the glued curves and assembles the automorphism group.
Main definitions #
NodalGluing.Iso I n d d': aC^ndiffeomorphism(ฮฃ i, X i) โโ^nโฎI, Iโฏ (ฮฃ i, X' i)such that two points are the two points of a flag pair ofd'exactly when their preimages are the two points of a flag pair ofd.NodalGluing.Iso.branchEquiv: the induced bijection of branchesFlag ร Bool โ Flag' ร Bool, characterized byflagPoint d' (e.branchEquiv p) = e (flagPoint d p). It commutes with exchanging the two branches of a node (branchEquiv_swap), so it may interchange them.NodalGluing.Iso.edgeEquiv: the induced bijection of nodesFlag โ Flag'.NodalGluing.Iso.vertexEquiv: for connected components, the induced bijection of componentsฮน โ ฮน', compatible with the branches (fst_flagPoint_branchEquiv).NodalGluing.Iso.gluedCurveHomeomorph: the descended homeomorphism of glued curves.NodalGluing.Aut I n d: the automorphism group, with its actions on branches (Aut.branchPerm), nodes (Aut.edgePerm), components (Aut.vertexPerm) and the glued curve (Aut.toGluedCurveHomeomorph).
Main results #
NodalGluing.Iso.gluedCurveHomeomorph_mk,gluedCurveHomeomorph_nodeOfFlag: descent commutes with the quotient maps and sends the node offto the node ofe.edgeEquiv f; in particular it preserves the node set (image_gluedCurveHomeomorph_nodeSet).NodalGluing.Iso.gluedCurveHomeomorph_injective: descent is faithful as soon as the points which are not flag points are dense in the normalization, as they are for curves. So an isomorphism of normalizations is determined by the map of glued curves it induces.
References #
- Joel W. Robbin and Dietmar A. Salamon, A construction of the DeligneโMumford orbifold, J. Eur. Math. Soc. 8 (2006), 611โ699: a nodal Riemann surface is a surface with a set of unordered pairs of distinct points, and an isomorphism is a biholomorphism of the surfaces carrying the nodal pairs exactly to the nodal pairs.
- Enrico Arbarello, Maurizio Cornalba, Phillip A. Griffiths, Geometry of Algebraic Curves, Volume II, Chapter X, for the dual graph of a nodal curve and the action of automorphisms on it.
An isomorphism of nodal curves presented by flag-pair data. A C^n diffeomorphism of the
disjoint unions of the normalization components which carries the two points of a flag pair of
d exactly to the two points of a flag pair of d'. Over โ with n = โ this is a
biholomorphism of the normalizations matching the nodes.
- toDiffeomorph : Diffeomorph I I ((i : ฮน) ร X i) ((i : ฮน') ร X' i) n
The diffeomorphism of the normalizations.
- flagRel_apply_iff' (x y : (i : ฮน) ร X i) : flagRel d' (self.toDiffeomorph x) (self.toDiffeomorph y) โ flagRel d x y
Two points are the two points of a flag pair of
dexactly when their images are the two points of a flag pair ofd'.
Instances For
The underlying diffeomorphism of an isomorphism acts as the isomorphism.
An isomorphism built from a diffeomorphism acts as that diffeomorphism.
Two isomorphisms agreeing at every point of the normalization are equal.
An isomorphism carries the two points of a flag pair exactly to the two points of a flag pair.
An isomorphism is continuous on the normalizations.
The identity isomorphism.
Instances For
The identity isomorphism fixes every point.
The underlying diffeomorphism of the identity isomorphism is the identity.
The inverse isomorphism.
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The inverse isomorphism undoes the isomorphism.
The isomorphism undoes its inverse.
The inverse of the inverse is the original isomorphism.
The underlying diffeomorphism of the inverse is the inverse diffeomorphism.
The inverse of the identity isomorphism is the identity.
The inverse isomorphism sends y to x exactly when the isomorphism sends x to y.
The isomorphism sends x to y exactly when the inverse sends y to x.
The composite of two isomorphisms.
Instances For
The composite acts by applying the first isomorphism, then the second.
The underlying diffeomorphism of a composite is the composite of the diffeomorphisms.
The inverse of a composite is the composite of the inverses in the opposite order.
An isomorphism followed by its inverse is the identity.
The inverse of an isomorphism followed by it is the identity.
Composing with the identity on the right does nothing.
Composing with the identity on the left does nothing.
Composition of isomorphisms is associative.
An isomorphism maps flag points exactly to flag points.
An isomorphism respects the equivalence relation generated by the flag pairs.
Branches, nodes and components #
The branch map. The bijection of branches induced by an isomorphism: the flag point
selected by p is carried to the flag point selected by e.branchEquiv p.
Instances For
The characteristic property of the branch map.
The branch map sends p to q exactly when the isomorphism sends the flag point of p
to the flag point of q.
The identity isomorphism fixes every branch.
The branch map of the inverse is the inverse of the branch map.
The branch map of a composite is the composite of the branch maps.
The branch map respects the nodes. The two branches of a node are carried to the two branches of a node: exchanging the branches commutes with the branch map.
The node map. The bijection of nodes (flag pairs) induced by an isomorphism.
Instances For
The branch map lies over the node map.
The node map sends a node to the node of the image of either of its branches.
The identity isomorphism fixes every node.
The node map of a composite is the composite of the node maps.
The node map of the inverse is the inverse of the node map.
An isomorphism maps each connected component of the normalization into a single component.
The component map. For connected components, the bijection of components induced by an
isomorphism: the component i is carried onto the component e.vertexEquiv i.
Instances For
The characteristic property of the component map.
The graph maps are compatible. The component carrying the image of a branch is the image of the component carrying that branch.
The component map sends i to j exactly when some point of the component i is sent
into the component j.
The identity isomorphism fixes every component.
The component map of a composite is the composite of the component maps.
The component map of the inverse is the inverse of the component map.
Descent to the glued curves #
Descent. The homeomorphism of glued curves induced by an isomorphism of nodal curves.
Instances For
The descended homeomorphism commutes with the quotient maps.
The descended homeomorphism sends the node of f to the node of e.edgeEquiv f.
The descended homeomorphism maps nodes exactly to nodes.
The descended homeomorphism maps the node set onto the node set.
Descent sends the identity to the identity.
Descent sends a composite to the composite of the descended homeomorphisms.
Descent sends the inverse to the inverse homeomorphism.
Descent is faithful. If the points of the normalization which are not flag points are
dense, then an isomorphism of nodal curves is determined by the homeomorphism of glued curves it
induces. The density hypothesis holds for curves, whose components have no isolated points, by
Dense.sdiff_finite.
The automorphism group of a nodal curve presented by flag-pair data: its isomorphisms with itself, with composition as multiplication.
Instances For
Multiplication of automorphisms is composition, the right factor acting first.
The unit automorphism is the identity.
The inverse automorphism is the inverse isomorphism.
A product of automorphisms acts by composition.
The unit automorphism fixes every point.
The inverse automorphism acts as the inverse isomorphism.
The branch action of the automorphism group: its action on the branches at the nodes.
Instances For
The branch action of an automorphism is its branch map.
The action of the automorphism group on the nodes.
Instances For
The node action of an automorphism is its node map.
The action of the automorphism group on the components, when they are connected.
Instances For
The component action of an automorphism is its component map.
The action of the automorphism group on the glued curve, by descent.
Instances For
An automorphism acts on the glued curve by its descended homeomorphism.
The automorphism group acts faithfully on the glued curve when the points which are not flag points are dense in the normalization.