Vector Frames¶
The class VectorFrame
implements vector frames on differentiable
manifolds.
By vector frame, it is meant a field on some
differentiable manifold
endowed with a differentiable map
to a differentiable manifold
such that for each
,
is a vector basis of the tangent space
.
The standard case of a vector frame on corresponds to
and
. Other common cases are
being an
immersion and
being a curve in
(
is then an open
interval of
).
A derived class of VectorFrame
is CoordFrame
;
it regards the vector frames associated with a chart, i.e. the
so-called coordinate bases.
The vector frame duals, i.e. the coframes, are implemented via the class
CoFrame
. The derived class CoordCoFrame
is devoted to
coframes deriving from a chart.
AUTHORS:
- Eric Gourgoulhon, Michal Bejger (2013-2015): initial version
- Travis Scrimshaw (2016): review tweaks
REFERENCES:
- [Lee2013]
EXAMPLES:
Setting a vector frame on a 3-dimensional manifold:
sage: M = Manifold(3, 'M')
sage: c_xyz.<x,y,z> = M.chart()
sage: e = M.vector_frame('e') ; e
Vector frame (M, (e_0,e_1,e_2))
sage: latex(e)
\left(M, \left(e_0,e_1,e_2\right)\right)
The first frame defined on a manifold is its default frame; in the present
case it is the coordinate frame defined when introducing the chart
c_xyz
:
sage: M.default_frame()
Coordinate frame (M, (d/dx,d/dy,d/dz))
The default frame can be changed via the method
set_default_frame()
:
sage: M.set_default_frame(e)
sage: M.default_frame()
Vector frame (M, (e_0,e_1,e_2))
The elements of a vector frame are vector fields on the manifold:
sage: [e[i] for i in M.irange()]
[Vector field e_0 on the 3-dimensional differentiable manifold M,
Vector field e_1 on the 3-dimensional differentiable manifold M,
Vector field e_2 on the 3-dimensional differentiable manifold M]
Each element can be accessed by its index:
sage: e[0]
Vector field e_0 on the 3-dimensional differentiable manifold M
The index range depends on the starting index defined on the manifold:
sage: M = Manifold(3, 'M', start_index=1)
sage: c_xyz.<x,y,z> = M.chart()
sage: e = M.vector_frame('e')
sage: [e[i] for i in M.irange()]
[Vector field e_1 on the 3-dimensional differentiable manifold M,
Vector field e_2 on the 3-dimensional differentiable manifold M,
Vector field e_3 on the 3-dimensional differentiable manifold M]
sage: e[1], e[2], e[3]
(Vector field e_1 on the 3-dimensional differentiable manifold M,
Vector field e_2 on the 3-dimensional differentiable manifold M,
Vector field e_3 on the 3-dimensional differentiable manifold M)
Let us check that the vector fields e[i]
are the frame vectors from
their components with respect to the frame :
sage: e[1].comp(e)[:]
[1, 0, 0]
sage: e[2].comp(e)[:]
[0, 1, 0]
sage: e[3].comp(e)[:]
[0, 0, 1]
Defining a vector frame on a manifold automatically creates the dual
coframe, which bares the same name (here ):
sage: M.coframes()
[Coordinate coframe (M, (dx,dy,dz)), Coframe (M, (e^1,e^2,e^3))]
sage: f = M.coframes()[1] ; f
Coframe (M, (e^1,e^2,e^3))
Each element of the coframe is a 1-form:
sage: f[1], f[2], f[3]
(1-form e^1 on the 3-dimensional differentiable manifold M,
1-form e^2 on the 3-dimensional differentiable manifold M,
1-form e^3 on the 3-dimensional differentiable manifold M)
sage: latex(f[1]), latex(f[2]), latex(f[3])
(e^1, e^2, e^3)
Let us check that the coframe is indeed the dual of the vector
frame
:
sage: f[1](e[1]) # the 1-form e^1 applied to the vector field e_1
Scalar field e^1(e_1) on the 3-dimensional differentiable manifold M
sage: f[1](e[1]).expr() # the explicit expression of e^1(e_1)
1
sage: f[1](e[1]).expr(), f[1](e[2]).expr(), f[1](e[3]).expr()
(1, 0, 0)
sage: f[2](e[1]).expr(), f[2](e[2]).expr(), f[2](e[3]).expr()
(0, 1, 0)
sage: f[3](e[1]).expr(), f[3](e[2]).expr(), f[3](e[3]).expr()
(0, 0, 1)
The coordinate frame associated to spherical coordinates of the
sphere :
sage: M = Manifold(2, 'S^2', start_index=1) # Part of S^2 covered by spherical coord.
sage: c_spher.<th,ph> = M.chart(r'th:[0,pi]:\theta ph:[0,2*pi):\phi')
sage: b = M.default_frame() ; b
Coordinate frame (S^2, (d/dth,d/dph))
sage: b[1]
Vector field d/dth on the 2-dimensional differentiable manifold S^2
sage: b[2]
Vector field d/dph on the 2-dimensional differentiable manifold S^2
The orthonormal frame constructed from the coordinate frame:
sage: change_frame = M.automorphism_field()
sage: change_frame[:] = [[1,0], [0, 1/sin(th)]]
sage: e = b.new_frame(change_frame, 'e') ; e
Vector frame (S^2, (e_1,e_2))
sage: e[1][:]
[1, 0]
sage: e[2][:]
[0, 1/sin(th)]
The change-of-frame automorphisms and their matrices:
sage: M.change_of_frame(c_spher.frame(), e)
Field of tangent-space automorphisms on the 2-dimensional
differentiable manifold S^2
sage: M.change_of_frame(c_spher.frame(), e)[:]
[ 1 0]
[ 0 1/sin(th)]
sage: M.change_of_frame(e, c_spher.frame())
Field of tangent-space automorphisms on the 2-dimensional
differentiable manifold S^2
sage: M.change_of_frame(e, c_spher.frame())[:]
[ 1 0]
[ 0 sin(th)]
-
class
sage.manifolds.differentiable.vectorframe.
CoFrame
(frame, symbol, latex_symbol=None)¶ Bases:
sage.tensor.modules.free_module_basis.FreeModuleCoBasis
Coframe on a differentiable manifold.
By coframe, it is meant a field
on some differentiable manifold
endowed with a differentiable map
to a differentiable manifold
such that for each
,
is a basis of the vector space
(the dual to the tangent space
).
The standard case of a coframe on
corresponds to
and
. Other common cases are
being an immersion and
being a curve in
(
is then an open interval of
).
INPUT:
frame
– the vector frame dual to the coframesymbol
– a letter (of a few letters) to denote a generic 1-form in the coframelatex_symbol
– (default:None
) symbol to denote a generic 1-form in the coframe; ifNone
, the value ofsymbol
is used
EXAMPLES:
Coframe on a 3-dimensional manifold:
sage: M = Manifold(3, 'M', start_index=1) sage: c_xyz.<x,y,z> = M.chart() sage: v = M.vector_frame('v') sage: from sage.manifolds.differentiable.vectorframe import CoFrame sage: e = CoFrame(v, 'e') ; e Coframe (M, (e^1,e^2,e^3))
Instead of importing CoFrame in the global namespace, the coframe can be obtained by means of the method
dual_basis()
; the symbol is then the same as that of the frame:sage: a = v.dual_basis() ; a Coframe (M, (v^1,v^2,v^3)) sage: a[1] == e[1] True sage: a[1] is e[1] False sage: e[1].display(v) e^1 = v^1
The 1-forms composing the coframe are obtained via the operator
[]
:sage: e[1], e[2], e[3] (1-form e^1 on the 3-dimensional differentiable manifold M, 1-form e^2 on the 3-dimensional differentiable manifold M, 1-form e^3 on the 3-dimensional differentiable manifold M)
Checking that
is the dual of
:
sage: e[1](v[1]).expr(), e[1](v[2]).expr(), e[1](v[3]).expr() (1, 0, 0) sage: e[2](v[1]).expr(), e[2](v[2]).expr(), e[2](v[3]).expr() (0, 1, 0) sage: e[3](v[1]).expr(), e[3](v[2]).expr(), e[3](v[3]).expr() (0, 0, 1)
-
at
(point)¶ Return the value of
self
at a given point on the manifold, this value being a basis of the dual of the tangent space at the point.INPUT:
point
–ManifoldPoint
; pointin the domain
of the coframe (denoted
hereafter)
OUTPUT:
FreeModuleCoBasis
representing the basisof the vector space
, dual to the tangent space
, where
is the differentiable map associated with
(possibly
)
EXAMPLES:
Cobasis of a tangent space on a 2-dimensional manifold:
sage: M = Manifold(2, 'M') sage: X.<x,y> = M.chart() sage: p = M.point((-1,2), name='p') sage: f = X.coframe() ; f Coordinate coframe (M, (dx,dy)) sage: fp = f.at(p) ; fp Dual basis (dx,dy) on the Tangent space at Point p on the 2-dimensional differentiable manifold M sage: type(fp) <class 'sage.tensor.modules.free_module_basis.FreeModuleCoBasis'> sage: fp[0] Linear form dx on the Tangent space at Point p on the 2-dimensional differentiable manifold M sage: fp[1] Linear form dy on the Tangent space at Point p on the 2-dimensional differentiable manifold M sage: fp is X.frame().at(p).dual_basis() True
-
class
sage.manifolds.differentiable.vectorframe.
CoordCoFrame
(coord_frame)¶ Bases:
sage.manifolds.differentiable.vectorframe.CoFrame
Coordinate coframe on a differentiable manifold.
By coordinate coframe, it is meant the
-tuple of the differentials of the coordinates of some chart on the manifold, with
being the manifold’s dimension.
INPUT:
coord_frame
– coordinate frame dual to the coordinate coframe
EXAMPLES:
Coordinate coframe on a 3-dimensional manifold:
sage: M = Manifold(3, 'M', start_index=1) sage: c_xyz.<x,y,z> = M.chart() sage: M.frames() [Coordinate frame (M, (d/dx,d/dy,d/dz))] sage: M.coframes() [Coordinate coframe (M, (dx,dy,dz))] sage: dX = M.coframes()[0] ; dX Coordinate coframe (M, (dx,dy,dz))
The 1-forms composing the coframe are obtained via the operator
[]
:sage: dX[1] 1-form dx on the 3-dimensional differentiable manifold M sage: dX[2] 1-form dy on the 3-dimensional differentiable manifold M sage: dX[3] 1-form dz on the 3-dimensional differentiable manifold M sage: dX[1][:] [1, 0, 0] sage: dX[2][:] [0, 1, 0] sage: dX[3][:] [0, 0, 1]
The coframe is the dual of the coordinate frame:
sage: e = c_xyz.frame() ; e Coordinate frame (M, (d/dx,d/dy,d/dz)) sage: dX[1](e[1]).expr(), dX[1](e[2]).expr(), dX[1](e[3]).expr() (1, 0, 0) sage: dX[2](e[1]).expr(), dX[2](e[2]).expr(), dX[2](e[3]).expr() (0, 1, 0) sage: dX[3](e[1]).expr(), dX[3](e[2]).expr(), dX[3](e[3]).expr() (0, 0, 1)
Each 1-form of a coordinate coframe is closed:
sage: dX[1].exterior_derivative() 2-form ddx on the 3-dimensional differentiable manifold M sage: dX[1].exterior_derivative() == 0 True
-
class
sage.manifolds.differentiable.vectorframe.
CoordFrame
(chart)¶ Bases:
sage.manifolds.differentiable.vectorframe.VectorFrame
Coordinate frame on a differentiable manifold.
By coordinate frame, it is meant a vector frame on a differentiable manifold
that is associated to a coordinate chart on
.
INPUT:
chart
– the chart defining the coordinates
EXAMPLES:
The coordinate frame associated to spherical coordinates of the sphere
:
sage: M = Manifold(2, 'S^2', start_index=1) # Part of S^2 covered by spherical coord. sage: M.chart(r'th:[0,pi]:\theta ph:[0,2*pi):\phi') Chart (S^2, (th, ph)) sage: b = M.default_frame() sage: b Coordinate frame (S^2, (d/dth,d/dph)) sage: b[1] Vector field d/dth on the 2-dimensional differentiable manifold S^2 sage: b[2] Vector field d/dph on the 2-dimensional differentiable manifold S^2 sage: latex(b) \left(S^2, \left(\frac{\partial}{\partial {\theta} },\frac{\partial}{\partial {\phi} }\right)\right)
-
chart
()¶ Return the chart defining this coordinate frame.
EXAMPLES:
sage: M = Manifold(2, 'M') sage: X.<x,y> = M.chart() sage: e = X.frame() sage: e.chart() Chart (M, (x, y)) sage: U = M.open_subset('U', coord_def={X: x>0}) sage: e.restrict(U).chart() Chart (U, (x, y))
-
structure_coeff
()¶ Return the structure coefficients associated to
self
.being the manifold’s dimension, the structure coefficients of the frame
are the
scalar fields
defined by
In the present case, since
is a coordinate frame,
.
OUTPUT:
- the structure coefficients
, as a vanishing instance of
CompWithSym
with 3 indices ordered as
EXAMPLES:
Structure coefficients of the coordinate frame associated to spherical coordinates in the Euclidean space
:
sage: M = Manifold(3, 'R^3', r'\RR^3', start_index=1) # Part of R^3 covered by spherical coord. sage: c_spher = M.chart(r'r:(0,+oo) th:(0,pi):\theta ph:(0,2*pi):\phi') sage: b = M.default_frame() ; b Coordinate frame (R^3, (d/dr,d/dth,d/dph)) sage: c = b.structure_coeff() ; c 3-indices components w.r.t. Coordinate frame (R^3, (d/dr,d/dth,d/dph)), with antisymmetry on the index positions (1, 2) sage: c == 0 True
- the structure coefficients
-
class
sage.manifolds.differentiable.vectorframe.
VectorFrame
(vector_field_module, symbol, latex_symbol=None, from_frame=None)¶ Bases:
sage.tensor.modules.free_module_basis.FreeModuleBasis
Vector frame on a differentiable manifold.
By vector frame, it is meant a field
on some differentiable manifold
endowed with a differentiable map
to a differentiable manifold
such that for each
,
is a vector basis of the tangent space
.
The standard case of a vector frame on
corresponds to
and
. Other common cases are
being an immersion and
being a curve in
(
is then an open interval of
).
For each instanciation of a vector frame, a coframe is automatically created, as an instance of the class
CoFrame
. It is returned by the methodcoframe()
.INPUT:
vector_field_module
– free moduleof vector fields along
with values on
symbol
– a letter (of a few letters) to denote a generic vector of the frame; can be set to None if the parameterfrom_frame
is filledlatex_symbol
– (default:None
) symbol to denote a generic vector of the frame; ifNone
, the value ofsymbol
is usedfrom_frame
– (default:None
) vector frameon the codomain
of the destination map
; the constructed frame
is then such that
EXAMPLES:
Setting a vector frame on a 3-dimensional manifold:
sage: M = Manifold(3, 'M') sage: c_xyz.<x,y,z> = M.chart() sage: e = M.vector_frame('e') ; e Vector frame (M, (e_0,e_1,e_2)) sage: latex(e) \left(M, \left(e_0,e_1,e_2\right)\right)
The LaTeX symbol can be specified:
sage: e = M.vector_frame('E', r"\epsilon") sage: latex(e) \left(M, \left(\epsilon_0,\epsilon_1,\epsilon_2\right)\right)
Example with a non-trivial map
; a vector frame along a curve:
sage: U = Manifold(1, 'U') # open interval (-1,1) as a 1-dimensional manifold sage: T.<t> = U.chart('t:(-1,1)') # canonical chart on U sage: Phi = U.diff_map(M, [cos(t), sin(t), t], name='Phi', ....: latex_name=r'\Phi') sage: Phi Differentiable map Phi from the 1-dimensional differentiable manifold U to the 3-dimensional differentiable manifold M sage: f = U.vector_frame('f', dest_map=Phi) ; f Vector frame (U, (f_0,f_1,f_2)) with values on the 3-dimensional differentiable manifold M sage: f.domain() 1-dimensional differentiable manifold U sage: f.ambient_domain() 3-dimensional differentiable manifold M
The value of the vector frame at a given point is a basis of the corresponding tangent space:
sage: p = U((0,), name='p') ; p Point p on the 1-dimensional differentiable manifold U sage: f.at(p) Basis (f_0,f_1,f_2) on the Tangent space at Point Phi(p) on the 3-dimensional differentiable manifold M
Vector frames are bases of free modules formed by vector fields:
sage: e.module() Free module X(M) of vector fields on the 3-dimensional differentiable manifold M sage: e.module().base_ring() Algebra of differentiable scalar fields on the 3-dimensional differentiable manifold M sage: e.module() is M.vector_field_module() True sage: e in M.vector_field_module().bases() True
sage: f.module() Free module X(U,Phi) of vector fields along the 1-dimensional differentiable manifold U mapped into the 3-dimensional differentiable manifold M sage: f.module().base_ring() Algebra of differentiable scalar fields on the 1-dimensional differentiable manifold U sage: f.module() is U.vector_field_module(dest_map=Phi) True sage: f in U.vector_field_module(dest_map=Phi).bases() True
-
along
(mapping)¶ Return the vector frame deduced from the current frame via a differentiable map, the codomain of which is included in the domain of of the current frame.
If
is the current vector frame,
its domain and if
is a differentiable map from some differentiable manifold
to
, the returned object is a vector frame
along
with values on
such that
INPUT:
mapping
– differentiable map
OUTPUT:
- vector frame
along
defined above.
EXAMPLES:
Vector frame along a curve:
sage: M = Manifold(2, 'M') sage: X.<x,y> = M.chart() sage: R = Manifold(1, 'R') # R as a 1-dimensional manifold sage: T.<t> = R.chart() # canonical chart on R sage: Phi = R.diff_map(M, {(T,X): [cos(t), t]}, name='Phi', ....: latex_name=r'\Phi') ; Phi Differentiable map Phi from the 1-dimensional differentiable manifold R to the 2-dimensional differentiable manifold M sage: e = X.frame() ; e Coordinate frame (M, (d/dx,d/dy)) sage: te = e.along(Phi) ; te Vector frame (R, (d/dx,d/dy)) with values on the 2-dimensional differentiable manifold M
Check of the formula
:
sage: p = R((pi,)) ; p Point on the 1-dimensional differentiable manifold R sage: te[0].at(p) == e[0].at(Phi(p)) True sage: te[1].at(p) == e[1].at(Phi(p)) True
The result is cached:
sage: te is e.along(Phi) True
-
ambient_domain
()¶ Return the differentiable manifold in which
self
takes its values.The ambient domain is the codomain
of the differentiable map
associated with the frame.
OUTPUT:
- a
DifferentiableManifold
representing
EXAMPLES:
sage: M = Manifold(2, 'M') sage: e = M.vector_frame('e') sage: e.ambient_domain() 2-dimensional differentiable manifold M
In the present case, since
is the identity map:
sage: e.ambient_domain() == e.domain() True
An example with a non trivial map
:
sage: U = Manifold(1, 'U') sage: T.<t> = U.chart() sage: X.<x,y> = M.chart() sage: Phi = U.diff_map(M, {(T,X): [cos(t), t]}, name='Phi', ....: latex_name=r'\Phi') ; Phi Differentiable map Phi from the 1-dimensional differentiable manifold U to the 2-dimensional differentiable manifold M sage: f = U.vector_frame('f', dest_map=Phi); f Vector frame (U, (f_0,f_1)) with values on the 2-dimensional differentiable manifold M sage: f.ambient_domain() 2-dimensional differentiable manifold M sage: f.domain() 1-dimensional differentiable manifold U
- a
-
at
(point)¶ Return the value of
self
at a given point, this value being a basis of the tangent vector space at the point.INPUT:
point
–ManifoldPoint
; pointin the domain
of the vector frame (denoted
hereafter)
OUTPUT:
FreeModuleBasis
representing the basisof the tangent vector space
, where
is the differentiable map associated with
(possibly
)
EXAMPLES:
Basis of a tangent space to a 2-dimensional manifold:
sage: M = Manifold(2, 'M') sage: X.<x,y> = M.chart() sage: p = M.point((-1,2), name='p') sage: e = X.frame() ; e Coordinate frame (M, (d/dx,d/dy)) sage: ep = e.at(p) ; ep Basis (d/dx,d/dy) on the Tangent space at Point p on the 2-dimensional differentiable manifold M sage: type(ep) <class 'sage.tensor.modules.free_module_basis.FreeModuleBasis'> sage: ep[0] Tangent vector d/dx at Point p on the 2-dimensional differentiable manifold M sage: ep[1] Tangent vector d/dy at Point p on the 2-dimensional differentiable manifold M
Note that the symbols used to denote the vectors are same as those for the vector fields of the frame. At this stage,
ep
is the unique basis on the tangent space atp
:sage: Tp = M.tangent_space(p) sage: Tp.bases() [Basis (d/dx,d/dy) on the Tangent space at Point p on the 2-dimensional differentiable manifold M]
Let us consider a vector frame that is a not a coordinate one:
sage: aut = M.automorphism_field() sage: aut[:] = [[1+y^2, 0], [0, 2]] sage: f = e.new_frame(aut, 'f') ; f Vector frame (M, (f_0,f_1)) sage: fp = f.at(p) ; fp Basis (f_0,f_1) on the Tangent space at Point p on the 2-dimensional differentiable manifold M
There are now two bases on the tangent space:
sage: Tp.bases() [Basis (d/dx,d/dy) on the Tangent space at Point p on the 2-dimensional differentiable manifold M, Basis (f_0,f_1) on the Tangent space at Point p on the 2-dimensional differentiable manifold M]
Moreover, the changes of bases in the tangent space have been computed from the known relation between the frames
e
andf
(field of automorphismsaut
defined above):sage: Tp.change_of_basis(ep, fp) Automorphism of the Tangent space at Point p on the 2-dimensional differentiable manifold M sage: Tp.change_of_basis(ep, fp).display() 5 d/dx*dx + 2 d/dy*dy sage: Tp.change_of_basis(fp, ep) Automorphism of the Tangent space at Point p on the 2-dimensional differentiable manifold M sage: Tp.change_of_basis(fp, ep).display() 1/5 d/dx*dx + 1/2 d/dy*dy
The dual bases:
sage: e.coframe() Coordinate coframe (M, (dx,dy)) sage: ep.dual_basis() Dual basis (dx,dy) on the Tangent space at Point p on the 2-dimensional differentiable manifold M sage: ep.dual_basis() is e.coframe().at(p) True sage: f.coframe() Coframe (M, (f^0,f^1)) sage: fp.dual_basis() Dual basis (f^0,f^1) on the Tangent space at Point p on the 2-dimensional differentiable manifold M sage: fp.dual_basis() is f.coframe().at(p) True
-
coframe
()¶ Return the coframe of
self
.EXAMPLES:
sage: M = Manifold(2, 'M') sage: e = M.vector_frame('e') sage: e.coframe() Coframe (M, (e^0,e^1)) sage: X.<x,y> = M.chart() sage: X.frame().coframe() Coordinate coframe (M, (dx,dy))
-
destination_map
()¶ Return the differential map associated to this vector frame.
Let
denote the vector frame; the differential map associated to it is the map
such that for each
,
is a vector basis of the tangent space
.
OUTPUT:
- a
DiffMap
representing the differential map
EXAMPLES:
sage: M = Manifold(2, 'M') sage: e = M.vector_frame('e') sage: e.destination_map() Identity map Id_M of the 2-dimensional differentiable manifold M
An example with a non trivial map
:
sage: U = Manifold(1, 'U') sage: T.<t> = U.chart() sage: X.<x,y> = M.chart() sage: Phi = U.diff_map(M, {(T,X): [cos(t), t]}, name='Phi', ....: latex_name=r'\Phi') ; Phi Differentiable map Phi from the 1-dimensional differentiable manifold U to the 2-dimensional differentiable manifold M sage: f = U.vector_frame('f', dest_map=Phi); f Vector frame (U, (f_0,f_1)) with values on the 2-dimensional differentiable manifold M sage: f.destination_map() Differentiable map Phi from the 1-dimensional differentiable manifold U to the 2-dimensional differentiable manifold M
- a
-
domain
()¶ Return the domain on which
self
is defined.OUTPUT:
- a
DifferentiableManifold
; representing the domain of the vector frame
EXAMPLES:
sage: M = Manifold(2, 'M') sage: e = M.vector_frame('e') sage: e.domain() 2-dimensional differentiable manifold M sage: U = M.open_subset('U') sage: f = e.restrict(U) sage: f.domain() Open subset U of the 2-dimensional differentiable manifold M
- a
-
new_frame
(change_of_frame, symbol, latex_symbol=None)¶ Define a new vector frame from
self
.The new vector frame is defined from a field of tangent-space automorphisms; its domain is the same as that of the current frame.
INPUT:
change_of_frame
–AutomorphismFieldParal
; the field of tangent space automorphismsthat relates the current frame
to the new frame
according to
symbol
– a letter (of a few letters) to denote a generic vector of the framelatex_symbol
– (default:None
) symbol to denote a generic vector of the frame; ifNone
, the value ofsymbol
is used
OUTPUT:
- the new frame
, as an instance of
VectorFrame
EXAMPLES:
Frame resulting from a
-rotation in the Euclidean plane:
sage: M = Manifold(2, 'R^2') sage: c_xy.<x,y> = M.chart() sage: e = M.vector_frame('e') ; M.set_default_frame(e) sage: M._frame_changes {} sage: rot = M.automorphism_field() sage: rot[:] = [[sqrt(3)/2, -1/2], [1/2, sqrt(3)/2]] sage: n = e.new_frame(rot, 'n') sage: n[0][:] [1/2*sqrt(3), 1/2] sage: n[1][:] [-1/2, 1/2*sqrt(3)] sage: a = M.change_of_frame(e,n) sage: a[:] [1/2*sqrt(3) -1/2] [ 1/2 1/2*sqrt(3)] sage: a == rot True sage: a is rot False sage: a._components # random (dictionary output) {Vector frame (R^2, (e_0,e_1)): 2-indices components w.r.t. Vector frame (R^2, (e_0,e_1)), Vector frame (R^2, (n_0,n_1)): 2-indices components w.r.t. Vector frame (R^2, (n_0,n_1))} sage: a.comp(n)[:] [1/2*sqrt(3) -1/2] [ 1/2 1/2*sqrt(3)] sage: a1 = M.change_of_frame(n,e) sage: a1[:] [1/2*sqrt(3) 1/2] [ -1/2 1/2*sqrt(3)] sage: a1 == rot.inverse() True sage: a1 is rot.inverse() False sage: e[0].comp(n)[:] [1/2*sqrt(3), -1/2] sage: e[1].comp(n)[:] [1/2, 1/2*sqrt(3)]
-
restrict
(subdomain)¶ Return the restriction of
self
to some open subset of its domain.If the restriction has not been defined yet, it is constructed here.
INPUT:
subdomain
– open subsetof the current frame domain
OUTPUT:
- the restriction of the current frame to
as a
VectorFrame
EXAMPLES:
Restriction of a frame defined on
to the unit disk:
sage: M = Manifold(2, 'R^2', start_index=1) sage: c_cart.<x,y> = M.chart() # Cartesian coordinates on R^2 sage: a = M.automorphism_field() sage: a[:] = [[1-y^2,0], [1+x^2, 2]] sage: e = c_cart.frame().new_frame(a, 'e') ; e Vector frame (R^2, (e_1,e_2)) sage: U = M.open_subset('U', coord_def={c_cart: x^2+y^2<1}) sage: e_U = e.restrict(U) ; e_U Vector frame (U, (e_1,e_2))
The vectors of the restriction have the same symbols as those of the original frame:
sage: e_U[1].display() e_1 = (-y^2 + 1) d/dx + (x^2 + 1) d/dy sage: e_U[2].display() e_2 = 2 d/dy
They are actually the restrictions of the original frame vectors:
sage: e_U[1] is e[1].restrict(U) True sage: e_U[2] is e[2].restrict(U) True
-
structure_coeff
()¶ Evaluate the structure coefficients associated to
self
.being the manifold’s dimension, the structure coefficients of the vector frame
are the
scalar fields
defined by
OUTPUT:
- the structure coefficients
, as an instance of
CompWithSym
with 3 indices ordered as.
EXAMPLES:
Structure coefficients of the orthonormal frame associated to spherical coordinates in the Euclidean space
:
sage: M = Manifold(3, 'R^3', '\RR^3', start_index=1) # Part of R^3 covered by spherical coordinates sage: c_spher.<r,th,ph> = M.chart(r'r:(0,+oo) th:(0,pi):\theta ph:(0,2*pi):\phi') sage: ch_frame = M.automorphism_field() sage: ch_frame[1,1], ch_frame[2,2], ch_frame[3,3] = 1, 1/r, 1/(r*sin(th)) sage: M.frames() [Coordinate frame (R^3, (d/dr,d/dth,d/dph))] sage: e = c_spher.frame().new_frame(ch_frame, 'e') sage: e[1][:] # components of e_1 in the manifold's default frame (d/dr, d/dth, d/dth) [1, 0, 0] sage: e[2][:] [0, 1/r, 0] sage: e[3][:] [0, 0, 1/(r*sin(th))] sage: c = e.structure_coeff() ; c 3-indices components w.r.t. Vector frame (R^3, (e_1,e_2,e_3)), with antisymmetry on the index positions (1, 2) sage: c[:] [[[0, 0, 0], [0, 0, 0], [0, 0, 0]], [[0, -1/r, 0], [1/r, 0, 0], [0, 0, 0]], [[0, 0, -1/r], [0, 0, -cos(th)/(r*sin(th))], [1/r, cos(th)/(r*sin(th)), 0]]] sage: c[2,1,2] # C^2_{12} -1/r sage: c[3,1,3] # C^3_{13} -1/r sage: c[3,2,3] # C^3_{23} -cos(th)/(r*sin(th))
- the structure coefficients