Tangent-Space Automorphism Fields#
The class AutomorphismField
implements fields of automorphisms of
tangent spaces to a generic (a priori not parallelizable) differentiable
manifold, while the class AutomorphismFieldParal
is devoted to fields of automorphisms of tangent spaces to a parallelizable
manifold. The latter play the important role of transitions between vector
frames sharing the same domain on a differentiable manifold.
AUTHORS:
Eric Gourgoulhon (2015): initial version
Travis Scrimshaw (2016): review tweaks
- class sage.manifolds.differentiable.automorphismfield.AutomorphismField(vector_field_module, name=None, latex_name=None)#
Bases:
TensorField
Field of automorphisms of tangent spaces to a generic (a priori not parallelizable) differentiable manifold.
Given a differentiable manifold \(U\) and a differentiable map \(\Phi: U \rightarrow M\) to a differentiable manifold \(M\), a field of tangent-space automorphisms along \(U\) with values on \(M \supset\Phi(U)\) is a differentiable map
\[a:\ U \longrightarrow T^{(1,1)} M,\]with \(T^{(1,1)} M\) being the tensor bundle of type \((1,1)\) over \(M\), such that
\[\forall p \in U,\ a(p) \in \mathrm{Aut}(T_{\Phi(p)} M),\]i.e. \(a(p)\) is an automorphism of the tangent space to \(M\) at the point \(\Phi(p)\).
The standard case of a field of tangent-space automorphisms on a manifold corresponds to \(U = M\) and \(\Phi = \mathrm{Id}_M\). 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, then
AutomorphismFieldParal
must be used instead.INPUT:
vector_field_module
– module \(\mathfrak{X}(U,\Phi)\) of vector fields along \(U\) with values on \(M\) via the map \(\Phi\)name
– (default:None
) name given to the fieldlatex_name
– (default:None
) LaTeX symbol to denote the field; if none is provided, the LaTeX symbol is set toname
is_identity
– (default:False
) determines whether the constructed object is a field of identity automorphisms
EXAMPLES:
Field of tangent-space automorphisms on a non-parallelizable 2-dimensional manifold:
sage: M = Manifold(2, 'M') sage: U = M.open_subset('U') ; V = M.open_subset('V') sage: M.declare_union(U,V) # M is the union of U and V sage: c_xy.<x,y> = U.chart() ; c_uv.<u,v> = V.chart() sage: transf = c_xy.transition_map(c_uv, (x+y, x-y), intersection_name='W', ....: restrictions1= x>0, restrictions2= u+v>0) sage: inv = transf.inverse() sage: a = M.automorphism_field(name='a') ; a Field of tangent-space automorphisms a 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
We first define the components of \(a\) with respect to the coordinate frame on \(U\):
sage: eU = c_xy.frame() ; eV = c_uv.frame() sage: a[eU,:] = [[1,x], [0,2]]
It is equivalent to pass the components while defining \(a\):
sage: a = M.automorphism_field({eU: [[1,x], [0,2]]}, name='a')
We then set the components with respect to the coordinate frame on \(V\) by extending the expressions of the components in the corresponding subframe on \(W = U \cap V\):
sage: W = U.intersection(V) sage: a.add_comp_by_continuation(eV, W, c_uv)
At this stage, the automorphism field \(a\) is fully defined:
sage: a.display(eU) a = ∂/∂x⊗dx + x ∂/∂x⊗dy + 2 ∂/∂y⊗dy sage: a.display(eV) a = (1/4*u + 1/4*v + 3/2) ∂/∂u⊗du + (-1/4*u - 1/4*v - 1/2) ∂/∂u⊗dv + (1/4*u + 1/4*v - 1/2) ∂/∂v⊗du + (-1/4*u - 1/4*v + 3/2) ∂/∂v⊗dv
In particular, we may ask for its inverse on the whole manifold \(M\):
sage: ia = a.inverse() ; ia Field of tangent-space automorphisms a^(-1) on the 2-dimensional differentiable manifold M sage: ia.display(eU) a^(-1) = ∂/∂x⊗dx - 1/2*x ∂/∂x⊗dy + 1/2 ∂/∂y⊗dy sage: ia.display(eV) a^(-1) = (-1/8*u - 1/8*v + 3/4) ∂/∂u⊗du + (1/8*u + 1/8*v + 1/4) ∂/∂u⊗dv + (-1/8*u - 1/8*v + 1/4) ∂/∂v⊗du + (1/8*u + 1/8*v + 3/4) ∂/∂v⊗dv
Equivalently, one can use the power minus one to get the inverse:
sage: ia is a^(-1) True
or the operator
~
:sage: ia is ~a True
- add_comp(basis=None)#
Return the components of
self
w.r.t. a given module basis for assignment, keeping the components w.r.t. other bases.To delete the components w.r.t. other bases, use the method
set_comp()
instead.INPUT:
basis
– (default:None
) basis in which the components are defined; if none is provided, the components are assumed to refer to the module’s default basis
Warning
If the automorphism field has already components in other bases, it is the user’s responsibility to make sure that the components to be added are consistent with them.
OUTPUT:
components in the given basis, as an instance of the class
Components
; if such components did not exist previously, they are created
EXAMPLES:
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: e_uv = c_uv.frame() sage: a= M.automorphism_field(name='a') sage: a.add_comp(e_uv) 2-indices components w.r.t. Coordinate frame (V, (∂/∂u,∂/∂v)) sage: a.add_comp(e_uv)[0,0] = u+v sage: a.add_comp(e_uv)[1,1] = u+v sage: a.display(e_uv) a = (u + v) ∂/∂u⊗du + (u + v) ∂/∂v⊗dv
Setting the components in a new frame:
sage: e = V.vector_frame('e') sage: a.add_comp(e) 2-indices components w.r.t. Vector frame (V, (e_0,e_1)) sage: a.add_comp(e)[0,1] = u*v sage: a.add_comp(e)[1,0] = u*v sage: a.display(e) a = u*v e_0⊗e^1 + u*v e_1⊗e^0
The components with respect to
e_uv
are kept:sage: a.display(e_uv) a = (u + v) ∂/∂u⊗du + (u + v) ∂/∂v⊗dv
Since the identity map is a special element, its components cannot be changed:
sage: id = M.tangent_identity_field() sage: id.add_comp(e)[0,1] = u*v Traceback (most recent call last): ... ValueError: the components of an immutable element cannot be changed
- copy(name=None, latex_name=None)#
Return an exact copy of the automorphism field
self
.INPUT:
name
– (default:None
) name given to the copylatex_name
– (default:None
) LaTeX symbol to denote the copy; if none is provided, the LaTeX symbol is set toname
Note
The name and the derived quantities are not copied.
EXAMPLES:
sage: M = Manifold(2, 'M') sage: U = M.open_subset('U') ; V = M.open_subset('V') sage: M.declare_union(U,V) # M is the union of U and V sage: c_xy.<x,y> = U.chart() ; c_uv.<u,v> = V.chart() sage: xy_to_uv = c_xy.transition_map(c_uv, (x+y, x-y), ....: intersection_name='W', restrictions1= x>0, ....: restrictions2= u+v>0) sage: uv_to_xy = xy_to_uv.inverse() sage: Id = M.tangent_identity_field(); Id Field of tangent-space identity maps on the 2-dimensional differentiable manifold M sage: one = Id.copy('1'); one Field of tangent-space automorphisms 1 on the 2-dimensional differentiable manifold M
- inverse()#
Return the inverse automorphism of
self
.EXAMPLES:
Inverse of a field of tangent-space automorphisms on a non-parallelizable 2-dimensional manifold:
sage: M = Manifold(2, 'M') sage: U = M.open_subset('U') ; V = M.open_subset('V') sage: M.declare_union(U,V) # M is the union of U and V sage: W = U.intersection(V) sage: c_xy.<x,y> = U.chart() ; c_uv.<u,v> = V.chart() sage: transf = c_xy.transition_map(c_uv, (x+y, x-y), ....: intersection_name='W', restrictions1= x>0, restrictions2= u+v>0) sage: inv = transf.inverse() sage: eU = c_xy.frame() ; eV = c_uv.frame() sage: a = M.automorphism_field({eU: [[1,x], [0,2]]}, name='a') sage: a.add_comp_by_continuation(eV, W, c_uv) sage: ia = a.inverse() ; ia Field of tangent-space automorphisms a^(-1) on the 2-dimensional differentiable manifold M sage: a[eU,:], ia[eU,:] ( [1 x] [ 1 -1/2*x] [0 2], [ 0 1/2] ) sage: a[eV,:], ia[eV,:] ( [ 1/4*u + 1/4*v + 3/2 -1/4*u - 1/4*v - 1/2] [ 1/4*u + 1/4*v - 1/2 -1/4*u - 1/4*v + 3/2], [-1/8*u - 1/8*v + 3/4 1/8*u + 1/8*v + 1/4] [-1/8*u - 1/8*v + 1/4 1/8*u + 1/8*v + 3/4] )
Let us check that ia is indeed the inverse of a:
sage: s = a.contract(ia) sage: s[eU,:], s[eV,:] ( [1 0] [1 0] [0 1], [0 1] ) sage: s = ia.contract(a) sage: s[eU,:], s[eV,:] ( [1 0] [1 0] [0 1], [0 1] )
The result is cached:
sage: a.inverse() is ia True
Instead of
inverse()
, one can use the power minus one to get the inverse:sage: ia is a^(-1) True
or the operator
~
:sage: ia is ~a True
- restrict(subdomain, dest_map=None)#
Return the restriction of
self
to some subdomain.This is a redefinition of
sage.manifolds.differentiable.tensorfield.TensorField.restrict()
to take into account the identity map.INPUT:
subdomain
–DifferentiableManifold
open subset \(V\) ofself._domain
dest_map
– (default:None
)DiffMap
; destination map \(\Phi:\ V \rightarrow N\), where \(N\) is a subdomain ofself._codomain
; ifNone
, the restriction ofself.base_module().destination_map()
to \(V\) is used
OUTPUT:
a
AutomorphismField
representing the restriction
EXAMPLES:
Restrictions of an automorphism field on the 2-sphere:
sage: M = Manifold(2, 'S^2', start_index=1) sage: U = M.open_subset('U') # the complement of the North pole sage: stereoN.<x,y> = U.chart() # stereographic coordinates from the North pole sage: eN = stereoN.frame() # the associated vector frame sage: V = M.open_subset('V') # the complement of the South pole sage: stereoS.<u,v> = V.chart() # stereographic coordinates from the South pole sage: eS = stereoS.frame() # the associated vector frame sage: transf = stereoN.transition_map(stereoS, (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: inv = transf.inverse() # transformation from stereoS to stereoN sage: W = U.intersection(V) # the complement of the North and South poles sage: stereoN_W = W.atlas()[0] # restriction of stereo. coord. from North pole to W sage: stereoS_W = W.atlas()[1] # restriction of stereo. coord. from South pole to W sage: eN_W = stereoN_W.frame() ; eS_W = stereoS_W.frame() sage: a = M.automorphism_field({eN: [[1, atan(x^2+y^2)], [0,3]]}, ....: name='a') sage: a.add_comp_by_continuation(eS, W, chart=stereoS); a Field of tangent-space automorphisms a on the 2-dimensional differentiable manifold S^2 sage: a.restrict(U) Field of tangent-space automorphisms a on the Open subset U of the 2-dimensional differentiable manifold S^2 sage: a.restrict(U)[eN,:] [ 1 arctan(x^2 + y^2)] [ 0 3] sage: a.restrict(V) Field of tangent-space automorphisms a on the Open subset V of the 2-dimensional differentiable manifold S^2 sage: a.restrict(V)[eS,:] [ (u^4 + 10*u^2*v^2 + v^4 + 2*(u^3*v - u*v^3)*arctan(1/(u^2 + v^2)))/(u^4 + 2*u^2*v^2 + v^4) -(4*u^3*v - 4*u*v^3 + (u^4 - 2*u^2*v^2 + v^4)*arctan(1/(u^2 + v^2)))/(u^4 + 2*u^2*v^2 + v^4)] [ 4*(u^2*v^2*arctan(1/(u^2 + v^2)) - u^3*v + u*v^3)/(u^4 + 2*u^2*v^2 + v^4) (3*u^4 - 2*u^2*v^2 + 3*v^4 - 2*(u^3*v - u*v^3)*arctan(1/(u^2 + v^2)))/(u^4 + 2*u^2*v^2 + v^4)] sage: a.restrict(W) Field of tangent-space automorphisms a on the Open subset W of the 2-dimensional differentiable manifold S^2 sage: a.restrict(W)[eN_W,:] [ 1 arctan(x^2 + y^2)] [ 0 3]
Restrictions of the field of tangent-space identity maps:
sage: id = M.tangent_identity_field() ; id Field of tangent-space identity maps on the 2-dimensional differentiable manifold S^2 sage: id.restrict(U) Field of tangent-space identity maps on the Open subset U of the 2-dimensional differentiable manifold S^2 sage: id.restrict(U)[eN,:] [1 0] [0 1] sage: id.restrict(V) Field of tangent-space identity maps on the Open subset V of the 2-dimensional differentiable manifold S^2 sage: id.restrict(V)[eS,:] [1 0] [0 1] sage: id.restrict(W)[eN_W,:] [1 0] [0 1] sage: id.restrict(W)[eS_W,:] [1 0] [0 1]
- set_comp(basis=None)#
Return the components of
self
w.r.t. a given module basis for assignment.The components with respect to other bases are deleted, in order to avoid any inconsistency. To keep them, use the method
add_comp()
instead.INPUT:
basis
– (default:None
) basis in which the components are defined; if none is provided, the components are assumed to refer to the module’s default basis
OUTPUT:
components in the given basis, as an instance of the class
Components
; if such components did not exist previously, they are created.
EXAMPLES:
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: e_uv = c_uv.frame() sage: a= M.automorphism_field(name='a') sage: a.set_comp(e_uv) 2-indices components w.r.t. Coordinate frame (V, (∂/∂u,∂/∂v)) sage: a.set_comp(e_uv)[0,0] = u+v sage: a.set_comp(e_uv)[1,1] = u+v sage: a.display(e_uv) a = (u + v) ∂/∂u⊗du + (u + v) ∂/∂v⊗dv
Setting the components in a new frame:
sage: e = V.vector_frame('e') sage: a.set_comp(e) 2-indices components w.r.t. Vector frame (V, (e_0,e_1)) sage: a.set_comp(e)[0,1] = u*v sage: a.set_comp(e)[1,0] = u*v sage: a.display(e) a = u*v e_0⊗e^1 + u*v e_1⊗e^0
Since the frames
e
ande_uv
are defined on the same domain, the components w.r.t.e_uv
have been erased:sage: a.display(c_uv.frame()) Traceback (most recent call last): ... ValueError: no basis could be found for computing the components in the Coordinate frame (V, (∂/∂u,∂/∂v))
Since the identity map is an immutable element, its components cannot be changed:
sage: id = M.tangent_identity_field() sage: id.add_comp(e)[0,1] = u*v Traceback (most recent call last): ... ValueError: the components of an immutable element cannot be changed
- class sage.manifolds.differentiable.automorphismfield.AutomorphismFieldParal(vector_field_module, name=None, latex_name=None)#
Bases:
FreeModuleAutomorphism
,TensorFieldParal
Field of tangent-space automorphisms with values on a parallelizable manifold.
Given a differentiable manifold \(U\) and a differentiable map \(\Phi: U \rightarrow M\) to a parallelizable manifold \(M\), a field of tangent-space automorphisms along \(U\) with values on \(M\supset\Phi(U)\) is a differentiable map
\[a:\ U \longrightarrow T^{(1,1)}M\](\(T^{(1,1)}M\) being the tensor bundle of type \((1,1)\) over \(M\)) such that
\[\forall p \in U,\ a(p) \in \mathrm{Aut}(T_{\Phi(p)} M)\]i.e. \(a(p)\) is an automorphism of the tangent space to \(M\) at the point \(\Phi(p)\).
The standard case of a field of tangent-space automorphisms on a manifold corresponds to \(U=M\) and \(\Phi = \mathrm{Id}_M\). 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
AutomorphismField
must be used instead.INPUT:
vector_field_module
– free module \(\mathfrak{X}(U,\Phi)\) of vector fields along \(U\) with values on \(M\) via the map \(\Phi\)name
– (default:None
) name given to the fieldlatex_name
– (default:None
) LaTeX symbol to denote the field; if none is provided, the LaTeX symbol is set toname
EXAMPLES:
A \(\pi/3\)-rotation in the Euclidean 2-plane:
sage: M = Manifold(2, 'R^2') sage: c_xy.<x,y> = M.chart() sage: rot = M.automorphism_field([[sqrt(3)/2, -1/2], [1/2, sqrt(3)/2]], ....: name='R'); rot Field of tangent-space automorphisms R on the 2-dimensional differentiable manifold R^2 sage: rot.parent() General linear group of the Free module X(R^2) of vector fields on the 2-dimensional differentiable manifold R^2
The inverse automorphism is obtained via the method
inverse()
:sage: inv = rot.inverse() ; inv Field of tangent-space automorphisms R^(-1) on the 2-dimensional differentiable manifold R^2 sage: latex(inv) R^{-1} sage: inv[:] [1/2*sqrt(3) 1/2] [ -1/2 1/2*sqrt(3)] sage: rot[:] [1/2*sqrt(3) -1/2] [ 1/2 1/2*sqrt(3)] sage: inv[:] * rot[:] # check [1 0] [0 1]
Equivalently, one can use the power minus one to get the inverse:
sage: inv is rot^(-1) True
or the operator
~
:sage: inv is ~rot True
- at(point)#
Value of
self
at a given point.If the current field of tangent-space automorphisms is
\[a:\ U \longrightarrow T^{(1,1)} M\]associated with the differentiable map
\[\Phi:\ U \longrightarrow M,\]where \(U\) and \(M\) are two manifolds (possibly \(U = M\) and \(\Phi = \mathrm{Id}_M\)), then for any point \(p \in U\), \(a(p)\) is an automorphism of the tangent space \(T_{\Phi(p)}M\).
INPUT:
point
–ManifoldPoint
; point \(p\) in the domain of the field of automorphisms \(a\)
OUTPUT:
the automorphism \(a(p)\) of the tangent vector space \(T_{\Phi(p)}M\)
EXAMPLES:
Automorphism at some point of a tangent space of a 2-dimensional manifold:
sage: M = Manifold(2, 'M') sage: c_xy.<x,y> = M.chart() sage: a = M.automorphism_field([[1+exp(y), x*y], [0, 1+x^2]], ....: name='a') sage: a.display() a = (e^y + 1) ∂/∂x⊗dx + x*y ∂/∂x⊗dy + (x^2 + 1) ∂/∂y⊗dy sage: p = M.point((-2,3), name='p') ; p Point p on the 2-dimensional differentiable manifold M sage: ap = a.at(p) ; ap Automorphism a of the Tangent space at Point p on the 2-dimensional differentiable manifold M sage: ap.display() a = (e^3 + 1) ∂/∂x⊗dx - 6 ∂/∂x⊗dy + 5 ∂/∂y⊗dy sage: ap.parent() General linear group of the Tangent space at Point p on the 2-dimensional differentiable manifold M
The identity map of the tangent space at point
p
:sage: id = M.tangent_identity_field() ; id Field of tangent-space identity maps on the 2-dimensional differentiable manifold M sage: idp = id.at(p) ; idp Identity map of the Tangent space at Point p on the 2-dimensional differentiable manifold M sage: idp is M.tangent_space(p).identity_map() True sage: idp.display() Id = ∂/∂x⊗dx + ∂/∂y⊗dy sage: idp.parent() General linear group of the Tangent space at Point p on the 2-dimensional differentiable manifold M sage: idp * ap == ap True
- inverse()#
Return the inverse automorphism of
self
.EXAMPLES:
sage: M = Manifold(2, 'M') sage: X.<x,y> = M.chart() sage: a = M.automorphism_field([[0, 2], [-1, 0]], name='a') sage: b = a.inverse(); b Field of tangent-space automorphisms a^(-1) on the 2-dimensional differentiable manifold M sage: b[:] [ 0 -1] [1/2 0] sage: a[:] [ 0 2] [-1 0]
The result is cached:
sage: a.inverse() is b True
Instead of
inverse()
, one can use the power minus one to get the inverse:sage: b is a^(-1) True
or the operator
~
:sage: b is ~a True
- restrict(subdomain, dest_map=None)#
Return the restriction of
self
to some subset of its domain.If such restriction has not been defined yet, it is constructed here.
This is a redefinition of
sage.manifolds.differentiable.tensorfield_paral.TensorFieldParal.restrict()
to take into account the identity map.INPUT:
subdomain
–DifferentiableManifold
; open subset \(V\) ofself._domain
dest_map
– (default:None
)DiffMap
destination map \(\Phi:\ V \rightarrow N\), where \(N\) is a subset ofself._codomain
; ifNone
, the restriction ofself.base_module().destination_map()
to \(V\) is used
OUTPUT:
a
AutomorphismFieldParal
representing the restriction
EXAMPLES:
Restriction of an automorphism field defined on \(\RR^2\) to a disk:
sage: M = Manifold(2, 'R^2') sage: c_cart.<x,y> = M.chart() # Cartesian coordinates on R^2 sage: D = M.open_subset('D') # the unit open disc sage: c_cart_D = c_cart.restrict(D, x^2+y^2<1) sage: a = M.automorphism_field([[1, x*y], [0, 3]], name='a'); a Field of tangent-space automorphisms a on the 2-dimensional differentiable manifold R^2 sage: a.restrict(D) Field of tangent-space automorphisms a on the Open subset D of the 2-dimensional differentiable manifold R^2 sage: a.restrict(D)[:] [ 1 x*y] [ 0 3]
Restriction to the disk of the field of tangent-space identity maps:
sage: id = M.tangent_identity_field() ; id Field of tangent-space identity maps on the 2-dimensional differentiable manifold R^2 sage: id.restrict(D) Field of tangent-space identity maps on the Open subset D of the 2-dimensional differentiable manifold R^2 sage: id.restrict(D)[:] [1 0] [0 1] sage: id.restrict(D) == D.tangent_identity_field() True