# Submanifolds of topological manifolds¶

Given a topological manifold $$M$$ over a topological field $$K$$, a topological submanifold of $$M$$ is defined by a topological manifold $$N$$ over the same field $$K$$ of dimension lower than the dimension of $$M$$ and a topological embedding $$\phi$$ from $$N$$ to $$M$$ (i.e. $$\phi$$ is a homeomorphism onto its image).

In the case where the map $$\phi$$ is only an embedding locally, it is called an topological immersion, and defines an immersed submanifold.

The global embedding property cannot be checked in sage, so the immersed or embedded aspect of the manifold must be declared by the user, by calling either set_embedding() or set_immersion() while declaring the map $$\phi$$.

The map $$\phi: N\to M$$ can also depend on one or multiple parameters. As long as $$\phi$$ remains injective in these parameters, it represents a foliation. The dimension of the foliation is defined as the number of parameters.

AUTHORS:

• Florentin Jaffredo (2018): initial version

REFERENCES:

class sage.manifolds.topological_submanifold.TopologicalSubmanifold(n, name, field, structure, ambient=None, base_manifold=None, latex_name=None, start_index=0, category=None, unique_tag=None)

Submanifold of a topological manifold.

Given a topological manifold $$M$$ over a topological field $$K$$, a topological submanifold of $$M$$ is defined by a topological manifold $$N$$ over the same field $$K$$ of dimension lower than the dimension of $$M$$ and a topological embedding $$\phi$$ from $$N$$ to $$M$$ (i.e. $$\phi$$ is an homeomorphism onto its image).

In the case where $$\phi$$ is only an topological immersion (i.e. is only locally an embedding), one says that $$N$$ is an immersed submanifold.

The map $$\phi$$ can also depend on one or multiple parameters. As long as $$\phi$$ remains injective in these parameters, it represents a foliation. The dimension of the foliation is defined as the number of parameters.

INPUT:

• n – positive integer; dimension of the manifold
• name – string; name (symbol) given to the manifold
• field – field $$K$$ on which the manifold is defined; allowed values are
• 'real' or an object of type RealField (e.g., RR) for a manifold over $$\RR$$
• 'complex' or an object of type ComplexField (e.g., CC) for a manifold over $$\CC$$
• an object in the category of topological fields (see Fields and TopologicalSpaces) for other types of manifolds
• structure – manifold structure (see TopologicalStructure or RealTopologicalStructure)
• ambient – (default: None) manifold of destination of the immersion. If None, set to self
• base_manifold – (default: None) if not None, must be a topological manifold; the created object is then an open subset of base_manifold
• latex_name – (default: None) string; LaTeX symbol to denote the manifold; if none are provided, it is set to name
• start_index – (default: 0) integer; lower value of the range of indices used for “indexed objects” on the manifold, e.g., coordinates in a chart
• category – (default: None) to specify the category; if None, Manifolds(field) is assumed (see the category Manifolds)
• unique_tag – (default: None) tag used to force the construction of a new object when all the other arguments have been used previously (without unique_tag, the UniqueRepresentation behavior inherited from ManifoldSubset would return the previously constructed object corresponding to these arguments)

EXAMPLES:

Let $$N$$ be a 2-dimensional submanifold of a 3-dimensional manifold $$M$$:

sage: M = Manifold(3, 'M', structure="topological")
sage: N = Manifold(2, 'N', ambient=M, structure="topological")
sage: N
2-dimensional submanifold N embedded in 3-dimensional manifold M
sage: CM.<x,y,z> = M.chart()
sage: CN.<u,v> = N.chart()


Let us define a 1-dimensional foliation indexed by $$t$$. The inverse map is needed in order to compute the adapted chart in the ambient manifold:

sage: t = var('t')
sage: phi = N.continuous_map(M, {(CN,CM):[u, v, t+u**2+v**2]}); phi
Continuous map from the 2-dimensional submanifold N embedded in
3-dimensional manifold M to the 3-dimensional topological manifold M
sage: phi_inv = M.continuous_map(N, {(CM, CN):[x, y]})
sage: phi_inv_t = M.scalar_field({CM: z-x**2-y**2})


$$\phi$$ can then be declared as an embedding $$N\to M$$:

sage: N.set_embedding(phi, inverse=phi_inv, var=t,
....:                 t_inverse={t: phi_inv_t})


The foliation can also be used to find new charts on the ambient manifold that are adapted to the foliation, i.e. in which the expression of the immersion is trivial. At the same time, the appropriate coordinate changes are computed:

sage: N.adapted_chart()
[Chart (M, (u_M, v_M, t_M))]
sage: len(M.coord_changes())
2


The foliations parameters are always added as the last coordinates.

adapted_chart(index='', latex_index='')

Create charts and changes of charts in the ambient manifold adapted to the foliation.

A manifold $$M$$ of dimension $$m$$ can be foliated by submanifolds $$N$$ of dimension $$n$$. The corresponding embedding needs $$m-n$$ free parameters to describe the whole manifold.

A set of coordinates adapted to a foliation is a set of coordinates $$(x_1,...,x_n,t_1,...t_{m-n})$$ such that $$(x_1,...x_n)$$ are coordinates of $$N$$ and $$(t_1,...t_{m-n})$$ are the $$m-n$$ free parameters of the foliation.

Provided that an embedding with free variables is already defined, this function constructs such charts and coordinates changes whenever it is possible.

If there are restrictions of the coordinates on the starting chart, these restrictions are also propagated.

INPUT:

• index – (default: "") string defining the name of the coordinates in the new chart. This string will be added at the end of the names of the old coordinates. By default, it is replaced by "_"+self._ambient._name
• latex_index – (default: "") string defining the latex name of the coordinates in the new chart. This string will be added at the end of the latex names of the old coordinates. By default, it is replaced by "_"+self._ambient._latex_()

OUTPUT:

• list of charts created from the charts of self

EXAMPLES:

sage: M = Manifold(3, 'M', structure="topological")
sage: N = Manifold(2, 'N', ambient=M, structure="topological")
sage: N
2-dimensional submanifold N embedded in 3-dimensional manifold M
sage: CM.<x,y,z> = M.chart()
sage: CN.<u,v> = N.chart()
sage: t = var('t')
sage: phi = N.continuous_map(M,{(CN,CM):[u,v,t+u**2+v**2]})
sage: phi_inv = M.continuous_map(N,{(CM,CN):[x,y]})
sage: phi_inv_t = M.scalar_field({CM:z-x**2-y**2})
sage: N.set_immersion(phi, inverse=phi_inv, var=t,
....:                 t_inverse={t:phi_inv_t})
sage: N.declare_embedding()
[Chart (M, (u_M, v_M, t_M))]

ambient()

Return the ambient manifold in which self is immersed or embedded.

EXAMPLES:

sage: M = Manifold(3, 'M', structure="topological")
sage: N = Manifold(2, 'N', ambient=M, structure="topological")
sage: N.ambient()
3-dimensional topological manifold M

declare_embedding()

Declare that the immersion provided by set_immersion() is in fact an embedding.

A topological embedding is a continuous map that is a homeomorphism onto its image. A differentiable embedding is a topological embedding that is also a differentiable immersion.

EXAMPLES:

sage: M = Manifold(3, 'M', structure="topological")
sage: N = Manifold(2, 'N', ambient=M, structure="topological")
sage: N
2-dimensional submanifold N embedded in 3-dimensional manifold M
sage: CM.<x,y,z> = M.chart()
sage: CN.<u,v> = N.chart()
sage: t = var('t')
sage: phi = N.continuous_map(M,{(CN,CM):[u,v,t+u**2+v**2]}); phi
Continuous map from the 2-dimensional submanifold N embedded in
3-dimensional manifold M to the 3-dimensional topological
manifold M
sage: phi_inv = M.continuous_map(N,{(CM,CN):[x,y]})
sage: phi_inv_t = M.scalar_field({CM:z-x**2-y**2})
sage: N.set_immersion(phi, inverse=phi_inv, var=t,
....:                 t_inverse={t: phi_inv_t})
sage: N._immersed
True
sage: N._embedded
False
sage: N.declare_embedding()
sage: N._immersed
True
sage: N._embedded
True

embedding()

Return the embedding of the submanifold.

EXAMPLES:

sage: M = Manifold(3, 'M', structure="topological")
sage: N = Manifold(2, 'N', ambient=M, structure="topological")
sage: CM.<x,y,z> = M.chart()
sage: CN.<u,v> = N.chart()
sage: t = var('t')
sage: phi = N.continuous_map(M,{(CN,CM):[u,v,t+u**2+v**2]})
sage: phi_inv = M.continuous_map(N,{(CM,CN):[x,y]})
sage: phi_inv_t = M.scalar_field({CM:z-x**2-y**2})
sage: N.set_embedding(phi, inverse=phi_inv, var=t,
....:                 t_inverse={t: phi_inv_t})
sage: N.embedding()
Homeomorphism from the 2-dimensional submanifold N embedded in
3-dimensional manifold M to the 3-dimensional topological manifold
M

immersion()

Return the immersion of the submanifold.

EXAMPLES:

sage: M = Manifold(3, 'M', structure="topological")
sage: N = Manifold(2, 'N', ambient=M, structure="topological")
sage: CM.<x,y,z> = M.chart()
sage: CN.<u,v> = N.chart()
sage: t = var('t')
sage: phi = N.continuous_map(M,{(CN,CM):[u,v,t+u**2+v**2]})
sage: phi_inv = M.continuous_map(N,{(CM,CN):[x,y]})
sage: phi_inv_t = M.scalar_field({CM:z-x**2-y**2})
sage: N.set_immersion(phi, inverse=phi_inv, var=t,
....:                 t_inverse={t: phi_inv_t})
sage: N.immersion()
Continuous map from the 2-dimensional submanifold N embedded in
3-dimensional manifold M to the 3-dimensional topological
manifold M

plot(param, u, v, chart1=None, chart2=None, **kwargs)

Plot an embedding.

Plot the embedding defined by the foliation and a set of values for the free parameters. This function can only plot 2-dimensional surfaces embedded in 3-dimensional manifolds. It ultimately calls ParametricSurface.

INPUT:

• param – dictionary of values indexed by the free variables appearing in the foliation.
• u – iterable of the values taken by the first coordinate of the surface to plot
• v – iterable of the values taken by the second coordinate of the surface to plot
• chart1 – (default: None) chart in which u and v are considered. By default, the default chart of the submanifold is used
• chart1 – (default: None) destination chart. By default, the default chart of the manifold is used
• **kwargs – other arguments as used in ParametricSurface

EXAMPLES:

sage: M = Manifold(3, 'M', structure="topological")
sage: N = Manifold(2, 'N', ambient = M, structure="topological")
sage: CM.<x,y,z> = M.chart()
sage: CN.<u,v> = N.chart()
sage: t = var('t')
sage: phi = N.continuous_map(M,{(CN,CM):[u,v,t+u**2+v**2]})
sage: phi_inv = M.continuous_map(N,{(CM,CN):[x,y]})
sage: phi_inv_t = M.scalar_field({CM:z-x**2-y**2})
sage: N.set_immersion(phi, inverse=phi_inv, var=t,
....:                 t_inverse = {t:phi_inv_t})
sage: N.declare_embedding()
[Chart (M, (u_M, v_M, t_M))]
sage: P0 = N.plot({t:0}, srange(-1, 1, 0.1), srange(-1, 1, 0.1),
....:             CN, CM, opacity=0.3, mesh=True)
sage: P1 = N.plot({t:1}, srange(-1, 1, 0.1), srange(-1, 1, 0.1),
....:             CN, CM, opacity=0.3, mesh=True)
sage: P2 = N.plot({t:2}, srange(-1, 1, 0.1), srange(-1, 1, 0.1),
....:             CN, CM, opacity=0.3, mesh=True)
sage: P3 = N.plot({t:3}, srange(-1, 1, 0.1), srange(-1, 1, 0.1),
....:             CN, CM, opacity=0.3, mesh=True)
sage: show(P0+P1+P2+P3)

set_embedding(phi, inverse=None, var=None, t_inverse=None)

Register the embedding of an embedded submanifold.

A topological embedding is a continuous map that is a homeomorphism onto its image. A differentiable embedding is a topological embedding that is also a differentiable immersion.

INPUTS:

• phi – continuous map $$\phi$$ from self to self._ambient
• inverse – (default: None) inverse of $$\phi$$ onto its image, used for computing changes of chart from or to adapted charts. No verification is made
• var – (default: None) list of parameters appearing in $$\phi$$
• t_inverse – (default: None) dictionary of scalar field on self._ambient indexed by elements of var representing the missing information in inverse

EXAMPLES:

sage: M = Manifold(3, 'M', structure="topological")
sage: N = Manifold(2, 'N', ambient=M, structure="topological")
sage: N
2-dimensional submanifold N embedded in 3-dimensional manifold M
sage: CM.<x,y,z> = M.chart()
sage: CN.<u,v> = N.chart()
sage: t = var('t')
sage: phi = N.continuous_map(M,{(CN,CM):[u,v,t+u**2+v**2]}); phi
Continuous map from the 2-dimensional submanifold N embedded in
3-dimensional manifold M to the 3-dimensional topological
manifold M
sage: phi_inv = M.continuous_map(N,{(CM,CN):[x,y]})
sage: phi_inv_t = M.scalar_field({CM:z-x**2-y**2})
sage: N.set_embedding(phi, inverse=phi_inv, var=t,
....:                 t_inverse={t: phi_inv_t})

set_immersion(phi, inverse=None, var=None, t_inverse=None)

Register the immersion of the immersed submanifold.

A topological immersion is a continuous map that is locally a topological embedding (i.e. a homeomorphism onto its image). A differentiable immersion is a differentiable map whose differential is injective at each point.

If an inverse of the immersion onto its image exists, it can be registered at the same time. If the immersion depends on parameters, they must also be declared here.

INPUTS:

• phi – continuous map $$\phi$$ from self to self._ambient
• inverse – (default: None) inverse of $$\phi$$ onto its image, used for computing changes of chart from or to adapted charts. No verification is made
• var – (default: None) list of parameters appearing in $$\phi$$
• t_inverse – (default: None) dictionary of scalar field on self._ambient indexed by elements of var representing the missing information in inverse

EXAMPLES:

sage: M = Manifold(3, 'M', structure="topological")
sage: N = Manifold(2, 'N', ambient=M, structure="topological")
sage: N
2-dimensional submanifold N embedded in 3-dimensional manifold M
sage: CM.<x,y,z> = M.chart()
sage: CN.<u,v> = N.chart()
sage: t = var('t')
sage: phi = N.continuous_map(M,{(CN,CM):[u,v,t+u**2+v**2]}); phi
Continuous map from the 2-dimensional submanifold N embedded in
3-dimensional manifold M to the 3-dimensional topological
manifold M
sage: phi_inv = M.continuous_map(N,{(CM,CN):[x,y]})
sage: phi_inv_t = M.scalar_field({CM:z-x**2-y**2})
sage: N.set_immersion(phi, inverse=phi_inv, var=t,
....:                 t_inverse={t: phi_inv_t})