Tensor Field Modules¶
The set of tensor fields along a differentiable manifold \(U\) with values on a differentiable manifold \(M\) via a differentiable map \(\Phi: U \rightarrow M\) (possibly \(U = M\) and \(\Phi = \mathrm{Id}_M\)) is a module over the algebra \(C^k(U)\) of differentiable scalar fields on \(U\). It is a free module if and only if \(M\) is parallelizable. Accordingly, two classes are devoted to tensor field modules:
TensorFieldModule
for tensor fields with values on a generic (in practice, not parallelizable) differentiable manifold \(M\),TensorFieldFreeModule
for tensor fields with values on a parallelizable manifold \(M\).
AUTHORS:
Eric Gourgoulhon, Michal Bejger (2014-2015): initial version
Travis Scrimshaw (2016): review tweaks
REFERENCES:
- class sage.manifolds.differentiable.tensorfield_module.TensorFieldFreeModule(vector_field_module, tensor_type)[source]¶
Bases:
TensorFreeModule
Free module of tensor fields of a given type \((k,l)\) along a differentiable manifold \(U\) with values on a parallelizable manifold \(M\), via a differentiable map \(U \rightarrow M\).
Given two nonnegative integers \(k\) and \(l\) and a differentiable map
\[\Phi:\ U \longrightarrow M,\]the tensor field module \(T^{(k,l)}(U, \Phi)\) is the set of all tensor fields of the type
\[t:\ U \longrightarrow T^{(k,l)} M\](where \(T^{(k,l)}M\) is the tensor bundle of type \((k,l)\) over \(M\)) such that
\[t(p) \in T^{(k,l)}(T_{\Phi(p)}M)\]for all \(p \in U\), i.e. \(t(p)\) is a tensor of type \((k,l)\) on the tangent vector space \(T_{\Phi(p)}M\). Since \(M\) is parallelizable, the set \(T^{(k,l)}(U,\Phi)\) is a free module over \(C^k(U)\), the ring (algebra) of differentiable scalar fields on \(U\) (see
DiffScalarFieldAlgebra
).The standard case of tensor fields on a differentiable manifold corresponds to \(U = M\) and \(\Phi = \mathrm{Id}_M\); we then denote \(T^{(k,l)}(M,\mathrm{Id}_M)\) by merely \(T^{(k,l)}(M)\). Other common cases are \(\Phi\) being an immersion and \(\Phi\) being a curve in \(M\) (\(U\) is then an open interval of \(\RR\)).
Note
If \(M\) is not parallelizable, the class
TensorFieldModule
should be used instead, for \(T^{(k,l)}(U,\Phi)\) is no longer a free module.INPUT:
vector_field_module
– free module \(\mathfrak{X}(U,\Phi)\) of vector fields along \(U\) associated with the map \(\Phi: U \rightarrow M\)tensor_type
– pair \((k,l)\) with \(k\) being the contravariant rank and \(l\) the covariant rank
EXAMPLES:
Module of type-\((2,0)\) tensor fields on \(\RR^3\):
sage: M = Manifold(3, 'R^3') sage: c_xyz.<x,y,z> = M.chart() # Cartesian coordinates sage: T20 = M.tensor_field_module((2,0)) ; T20 Free module T^(2,0)(R^3) of type-(2,0) tensors fields on the 3-dimensional differentiable manifold R^3
>>> from sage.all import * >>> M = Manifold(Integer(3), 'R^3') >>> c_xyz = M.chart(names=('x', 'y', 'z',)); (x, y, z,) = c_xyz._first_ngens(3)# Cartesian coordinates >>> T20 = M.tensor_field_module((Integer(2),Integer(0))) ; T20 Free module T^(2,0)(R^3) of type-(2,0) tensors fields on the 3-dimensional differentiable manifold R^3
\(T^{(2,0)}(\RR^3)\) is a module over the algebra \(C^k(\RR^3)\):
sage: T20.category() Category of tensor products of finite dimensional modules over Algebra of differentiable scalar fields on the 3-dimensional differentiable manifold R^3 sage: T20.base_ring() is M.scalar_field_algebra() True
>>> from sage.all import * >>> T20.category() Category of tensor products of finite dimensional modules over Algebra of differentiable scalar fields on the 3-dimensional differentiable manifold R^3 >>> T20.base_ring() is M.scalar_field_algebra() True
\(T^{(2,0)}(\RR^3)\) is a free module:
sage: from sage.tensor.modules.finite_rank_free_module import FiniteRankFreeModule_abstract sage: isinstance(T20, FiniteRankFreeModule_abstract) True
>>> from sage.all import * >>> from sage.tensor.modules.finite_rank_free_module import FiniteRankFreeModule_abstract >>> isinstance(T20, FiniteRankFreeModule_abstract) True
because \(M = \RR^3\) is parallelizable:
sage: M.is_manifestly_parallelizable() True
>>> from sage.all import * >>> M.is_manifestly_parallelizable() True
The zero element:
sage: z = T20.zero() ; z Tensor field zero of type (2,0) on the 3-dimensional differentiable manifold R^3 sage: z[:] [0 0 0] [0 0 0] [0 0 0]
>>> from sage.all import * >>> z = T20.zero() ; z Tensor field zero of type (2,0) on the 3-dimensional differentiable manifold R^3 >>> z[:] [0 0 0] [0 0 0] [0 0 0]
A random element:
sage: t = T20.an_element() ; t Tensor field of type (2,0) on the 3-dimensional differentiable manifold R^3 sage: t[:] [2 0 0] [0 0 0] [0 0 0]
>>> from sage.all import * >>> t = T20.an_element() ; t Tensor field of type (2,0) on the 3-dimensional differentiable manifold R^3 >>> t[:] [2 0 0] [0 0 0] [0 0 0]
The module \(T^{(2,0)}(\RR^3)\) coerces to any module of type-\((2,0)\) tensor fields defined on some subdomain of \(\RR^3\):
sage: U = M.open_subset('U', coord_def={c_xyz: x>0}) sage: T20U = U.tensor_field_module((2,0)) sage: T20U.has_coerce_map_from(T20) True sage: T20.has_coerce_map_from(T20U) # the reverse is not true False sage: T20U.coerce_map_from(T20) Coercion map: From: Free module T^(2,0)(R^3) of type-(2,0) tensors fields on the 3-dimensional differentiable manifold R^3 To: Free module T^(2,0)(U) of type-(2,0) tensors fields on the Open subset U of the 3-dimensional differentiable manifold R^3
>>> from sage.all import * >>> U = M.open_subset('U', coord_def={c_xyz: x>Integer(0)}) >>> T20U = U.tensor_field_module((Integer(2),Integer(0))) >>> T20U.has_coerce_map_from(T20) True >>> T20.has_coerce_map_from(T20U) # the reverse is not true False >>> T20U.coerce_map_from(T20) Coercion map: From: Free module T^(2,0)(R^3) of type-(2,0) tensors fields on the 3-dimensional differentiable manifold R^3 To: Free module T^(2,0)(U) of type-(2,0) tensors fields on the Open subset U of the 3-dimensional differentiable manifold R^3
The coercion map is actually the restriction of tensor fields defined on \(\RR^3\) to \(U\).
There is also a coercion map from fields of tangent-space automorphisms to tensor fields of type \((1,1)\):
sage: T11 = M.tensor_field_module((1,1)) ; T11 Free module T^(1,1)(R^3) of type-(1,1) tensors fields on the 3-dimensional differentiable manifold R^3 sage: GL = M.automorphism_field_group() ; GL General linear group of the Free module X(R^3) of vector fields on the 3-dimensional differentiable manifold R^3 sage: T11.has_coerce_map_from(GL) True
>>> from sage.all import * >>> T11 = M.tensor_field_module((Integer(1),Integer(1))) ; T11 Free module T^(1,1)(R^3) of type-(1,1) tensors fields on the 3-dimensional differentiable manifold R^3 >>> GL = M.automorphism_field_group() ; GL General linear group of the Free module X(R^3) of vector fields on the 3-dimensional differentiable manifold R^3 >>> T11.has_coerce_map_from(GL) True
An explicit call to this coercion map is:
sage: id = GL.one() ; id Field of tangent-space identity maps on the 3-dimensional differentiable manifold R^3 sage: tid = T11(id) ; tid Tensor field Id of type (1,1) on the 3-dimensional differentiable manifold R^3 sage: tid[:] [1 0 0] [0 1 0] [0 0 1]
>>> from sage.all import * >>> id = GL.one() ; id Field of tangent-space identity maps on the 3-dimensional differentiable manifold R^3 >>> tid = T11(id) ; tid Tensor field Id of type (1,1) on the 3-dimensional differentiable manifold R^3 >>> tid[:] [1 0 0] [0 1 0] [0 0 1]
- Element[source]¶
alias of
TensorFieldParal
- class sage.manifolds.differentiable.tensorfield_module.TensorFieldModule(vector_field_module, tensor_type, category=None)[source]¶
Bases:
UniqueRepresentation
,ReflexiveModule_tensor
Module of tensor fields of a given type \((k,l)\) along a differentiable manifold \(U\) with values on a differentiable manifold \(M\), via a differentiable map \(U \rightarrow M\).
Given two nonnegative integers \(k\) and \(l\) and a differentiable map
\[\Phi:\ U \longrightarrow M,\]the tensor field module \(T^{(k,l)}(U,\Phi)\) is the set of all tensor fields of the type
\[t:\ U \longrightarrow T^{(k,l)} M\](where \(T^{(k,l)} M\) is the tensor bundle of type \((k,l)\) over \(M\)) such that
\[t(p) \in T^{(k,l)}(T_{\Phi(p)}M)\]for all \(p \in U\), i.e. \(t(p)\) is a tensor of type \((k,l)\) on the tangent vector space \(T_{\Phi(p)} M\). The set \(T^{(k,l)}(U,\Phi)\) is a module over \(C^k(U)\), the ring (algebra) of differentiable scalar fields on \(U\) (see
DiffScalarFieldAlgebra
).The standard case of tensor fields on a differentiable manifold corresponds to \(U = M\) and \(\Phi = \mathrm{Id}_M\); we then denote \(T^{(k,l)}(M,\mathrm{Id}_M)\) by merely \(T^{(k,l)}(M)\). Other common cases are \(\Phi\) being an immersion and \(\Phi\) being a curve in \(M\) (\(U\) is then an open interval of \(\RR\)).
Note
If \(M\) is parallelizable, the class
TensorFieldFreeModule
should be used instead.INPUT:
vector_field_module
– module \(\mathfrak{X}(U,\Phi)\) of vector fields along \(U\) associated with the map \(\Phi: U \rightarrow M\)tensor_type
– pair \((k,l)\) with \(k\) being the contravariant rank and \(l\) the covariant rank
EXAMPLES:
Module of type-\((2,0)\) tensor fields on the 2-sphere:
sage: M = Manifold(2, 'M') # the 2-dimensional sphere S^2 sage: U = M.open_subset('U') # complement of the North pole sage: c_xy.<x,y> = U.chart() # stereographic coordinates from the North pole sage: V = M.open_subset('V') # complement of the South pole sage: c_uv.<u,v> = V.chart() # stereographic coordinates from the South pole sage: M.declare_union(U,V) # S^2 is the union of U and V sage: xy_to_uv = c_xy.transition_map(c_uv, (x/(x^2+y^2), y/(x^2+y^2)), ....: intersection_name='W', restrictions1= x^2+y^2!=0, ....: restrictions2= u^2+v^2!=0) sage: uv_to_xy = xy_to_uv.inverse() sage: W = U.intersection(V) sage: T20 = M.tensor_field_module((2,0)); T20 Module T^(2,0)(M) of type-(2,0) tensors fields on the 2-dimensional differentiable manifold M
>>> from sage.all import * >>> M = Manifold(Integer(2), 'M') # the 2-dimensional sphere S^2 >>> U = M.open_subset('U') # complement of the North pole >>> c_xy = U.chart(names=('x', 'y',)); (x, y,) = c_xy._first_ngens(2)# stereographic coordinates from the North pole >>> V = M.open_subset('V') # complement of the South pole >>> c_uv = V.chart(names=('u', 'v',)); (u, v,) = c_uv._first_ngens(2)# stereographic coordinates from the South pole >>> M.declare_union(U,V) # S^2 is the union of U and V >>> xy_to_uv = c_xy.transition_map(c_uv, (x/(x**Integer(2)+y**Integer(2)), y/(x**Integer(2)+y**Integer(2))), ... intersection_name='W', restrictions1= x**Integer(2)+y**Integer(2)!=Integer(0), ... restrictions2= u**Integer(2)+v**Integer(2)!=Integer(0)) >>> uv_to_xy = xy_to_uv.inverse() >>> W = U.intersection(V) >>> T20 = M.tensor_field_module((Integer(2),Integer(0))); T20 Module T^(2,0)(M) of type-(2,0) tensors fields on the 2-dimensional differentiable manifold M
\(T^{(2,0)}(M)\) is a module over the algebra \(C^k(M)\):
sage: T20.category() Category of tensor products of modules over Algebra of differentiable scalar fields on the 2-dimensional differentiable manifold M sage: T20.base_ring() is M.scalar_field_algebra() True
>>> from sage.all import * >>> T20.category() Category of tensor products of modules over Algebra of differentiable scalar fields on the 2-dimensional differentiable manifold M >>> T20.base_ring() is M.scalar_field_algebra() True
\(T^{(2,0)}(M)\) is not a free module:
sage: from sage.tensor.modules.finite_rank_free_module import FiniteRankFreeModule_abstract sage: isinstance(T20, FiniteRankFreeModule_abstract) False
>>> from sage.all import * >>> from sage.tensor.modules.finite_rank_free_module import FiniteRankFreeModule_abstract >>> isinstance(T20, FiniteRankFreeModule_abstract) False
because \(M = S^2\) is not parallelizable:
sage: M.is_manifestly_parallelizable() False
>>> from sage.all import * >>> M.is_manifestly_parallelizable() False
On the contrary, the module of type-\((2,0)\) tensor fields on \(U\) is a free module, since \(U\) is parallelizable (being a coordinate domain):
sage: T20U = U.tensor_field_module((2,0)) sage: isinstance(T20U, FiniteRankFreeModule_abstract) True sage: U.is_manifestly_parallelizable() True
>>> from sage.all import * >>> T20U = U.tensor_field_module((Integer(2),Integer(0))) >>> isinstance(T20U, FiniteRankFreeModule_abstract) True >>> U.is_manifestly_parallelizable() True
The zero element:
sage: z = T20.zero() ; z Tensor field zero of type (2,0) on the 2-dimensional differentiable manifold M sage: z is T20(0) True sage: z[c_xy.frame(),:] [0 0] [0 0] sage: z[c_uv.frame(),:] [0 0] [0 0]
>>> from sage.all import * >>> z = T20.zero() ; z Tensor field zero of type (2,0) on the 2-dimensional differentiable manifold M >>> z is T20(Integer(0)) True >>> z[c_xy.frame(),:] [0 0] [0 0] >>> z[c_uv.frame(),:] [0 0] [0 0]
The module \(T^{(2,0)}(M)\) coerces to any module of type-\((2,0)\) tensor fields defined on some subdomain of \(M\), for instance \(T^{(2,0)}(U)\):
sage: T20U.has_coerce_map_from(T20) True
>>> from sage.all import * >>> T20U.has_coerce_map_from(T20) True
The reverse is not true:
sage: T20.has_coerce_map_from(T20U) False
>>> from sage.all import * >>> T20.has_coerce_map_from(T20U) False
The coercion:
sage: T20U.coerce_map_from(T20) Coercion map: From: Module T^(2,0)(M) of type-(2,0) tensors fields on the 2-dimensional differentiable manifold M To: Free module T^(2,0)(U) of type-(2,0) tensors fields on the Open subset U of the 2-dimensional differentiable manifold M
>>> from sage.all import * >>> T20U.coerce_map_from(T20) Coercion map: From: Module T^(2,0)(M) of type-(2,0) tensors fields on the 2-dimensional differentiable manifold M To: Free module T^(2,0)(U) of type-(2,0) tensors fields on the Open subset U of the 2-dimensional differentiable manifold M
The coercion map is actually the restriction of tensor fields defined on \(M\) to \(U\):
sage: t = M.tensor_field(2,0, name='t') sage: eU = c_xy.frame() ; eV = c_uv.frame() sage: t[eU,:] = [[2,0], [0,-3]] sage: t.add_comp_by_continuation(eV, W, chart=c_uv) sage: T20U(t) # the conversion map in action Tensor field t of type (2,0) on the Open subset U of the 2-dimensional differentiable manifold M sage: T20U(t) is t.restrict(U) True
>>> from sage.all import * >>> t = M.tensor_field(Integer(2),Integer(0), name='t') >>> eU = c_xy.frame() ; eV = c_uv.frame() >>> t[eU,:] = [[Integer(2),Integer(0)], [Integer(0),-Integer(3)]] >>> t.add_comp_by_continuation(eV, W, chart=c_uv) >>> T20U(t) # the conversion map in action Tensor field t of type (2,0) on the Open subset U of the 2-dimensional differentiable manifold M >>> T20U(t) is t.restrict(U) True
There is also a coercion map from fields of tangent-space automorphisms to tensor fields of type-\((1,1)\):
sage: T11 = M.tensor_field_module((1,1)) ; T11 Module T^(1,1)(M) of type-(1,1) tensors fields on the 2-dimensional differentiable manifold M sage: GL = M.automorphism_field_group() ; GL General linear group of the Module X(M) of vector fields on the 2-dimensional differentiable manifold M sage: T11.has_coerce_map_from(GL) True
>>> from sage.all import * >>> T11 = M.tensor_field_module((Integer(1),Integer(1))) ; T11 Module T^(1,1)(M) of type-(1,1) tensors fields on the 2-dimensional differentiable manifold M >>> GL = M.automorphism_field_group() ; GL General linear group of the Module X(M) of vector fields on the 2-dimensional differentiable manifold M >>> T11.has_coerce_map_from(GL) True
Explicit call to the coercion map:
sage: a = GL.one() ; a Field of tangent-space identity maps on the 2-dimensional differentiable manifold M sage: a.parent() General linear group of the Module X(M) of vector fields on the 2-dimensional differentiable manifold M sage: ta = T11.coerce(a) ; ta Tensor field Id of type (1,1) on the 2-dimensional differentiable manifold M sage: ta.parent() Module T^(1,1)(M) of type-(1,1) tensors fields on the 2-dimensional differentiable manifold M sage: ta[eU,:] # ta on U [1 0] [0 1] sage: ta[eV,:] # ta on V [1 0] [0 1]
>>> from sage.all import * >>> a = GL.one() ; a Field of tangent-space identity maps on the 2-dimensional differentiable manifold M >>> a.parent() General linear group of the Module X(M) of vector fields on the 2-dimensional differentiable manifold M >>> ta = T11.coerce(a) ; ta Tensor field Id of type (1,1) on the 2-dimensional differentiable manifold M >>> ta.parent() Module T^(1,1)(M) of type-(1,1) tensors fields on the 2-dimensional differentiable manifold M >>> ta[eU,:] # ta on U [1 0] [0 1] >>> ta[eV,:] # ta on V [1 0] [0 1]
- Element[source]¶
alias of
TensorField
- base_module()[source]¶
Return the vector field module on which
self
is constructed.OUTPUT:
a
VectorFieldModule
representing the module on whichself
is defined
EXAMPLES:
sage: M = Manifold(2, 'M') sage: T13 = M.tensor_field_module((1,3)) sage: T13.base_module() Module X(M) of vector fields on the 2-dimensional differentiable manifold M sage: T13.base_module() is M.vector_field_module() True sage: T13.base_module().base_ring() Algebra of differentiable scalar fields on the 2-dimensional differentiable manifold M
>>> from sage.all import * >>> M = Manifold(Integer(2), 'M') >>> T13 = M.tensor_field_module((Integer(1),Integer(3))) >>> T13.base_module() Module X(M) of vector fields on the 2-dimensional differentiable manifold M >>> T13.base_module() is M.vector_field_module() True >>> T13.base_module().base_ring() Algebra of differentiable scalar fields on the 2-dimensional differentiable manifold M
- tensor_type()[source]¶
Return the tensor type of
self
.OUTPUT: pair \((k,l)\) of nonnegative integers such that the tensor fields belonging to this module are of type \((k,l)\)
EXAMPLES:
sage: M = Manifold(2, 'M') sage: T13 = M.tensor_field_module((1,3)) sage: T13.tensor_type() (1, 3) sage: T20 = M.tensor_field_module((2,0)) sage: T20.tensor_type() (2, 0)
>>> from sage.all import * >>> M = Manifold(Integer(2), 'M') >>> T13 = M.tensor_field_module((Integer(1),Integer(3))) >>> T13.tensor_type() (1, 3) >>> T20 = M.tensor_field_module((Integer(2),Integer(0))) >>> T20.tensor_type() (2, 0)