Tensor Fields with Values on a Parallelizable Manifold#
The class TensorFieldParal
implements tensor fields along a
differentiable manifolds with values on a parallelizable differentiable
manifold. For non-parallelizable manifolds, see the class
TensorField
.
Various derived classes of TensorFieldParal
are devoted to specific
tensor fields:
VectorFieldParal
for vector fields (rank-1 contravariant tensor fields)AutomorphismFieldParal
for fields of tangent-space automorphismsDiffFormParal
for differential forms (fully antisymmetric covariant tensor fields)MultivectorFieldParal
for multivector fields (fully antisymmetric contravariant tensor fields)
AUTHORS:
Eric Gourgoulhon, Michal Bejger (2013-2015) : initial version
Travis Scrimshaw (2016): review tweaks
Eric Gourgoulhon (2018): method
TensorFieldParal.along()
Florentin Jaffredo (2018) : series expansion with respect to a given parameter
REFERENCES:
EXAMPLES:
A tensor field of type \((1,1)\) on a 2-dimensional differentiable manifold:
sage: M = Manifold(2, 'M', start_index=1)
sage: c_xy.<x,y> = M.chart()
sage: t = M.tensor_field(1, 1, name='T') ; t
Tensor field T of type (1,1) on the 2-dimensional differentiable manifold M
sage: t.tensor_type()
(1, 1)
sage: t.tensor_rank()
2
Components with respect to the manifold’s default frame are created by providing the relevant indices inside square brackets:
sage: t[1,1] = x^2
Unset components are initialized to zero:
sage: t[:] # list of components w.r.t. the manifold's default vector frame
[x^2 0]
[ 0 0]
It is also possible to initialize the components at the tensor field construction:
sage: t = M.tensor_field(1, 1, [[x^2, 0], [0, 0]], name='T')
sage: t[:]
[x^2 0]
[ 0 0]
The full set of components with respect to a given vector frame is
returned by the method
comp()
:
sage: t.comp(c_xy.frame())
2-indices components w.r.t. Coordinate frame (M, (∂/∂x,∂/∂y))
If no vector frame is mentioned in the argument of
comp()
,
it is assumed to be the manifold’s default frame:
sage: M.default_frame()
Coordinate frame (M, (∂/∂x,∂/∂y))
sage: t.comp() is t.comp(c_xy.frame())
True
Individual components with respect to the manifold’s default frame are
accessed by listing their indices inside double square brackets. They
are scalar fields
on the manifold:
sage: t[[1,1]]
Scalar field on the 2-dimensional differentiable manifold M
sage: t[[1,1]].display()
M → ℝ
(x, y) ↦ x^2
sage: t[[1,2]]
Scalar field zero on the 2-dimensional differentiable manifold M
sage: t[[1,2]].display()
zero: M → ℝ
(x, y) ↦ 0
A direct access to the coordinate expression of some component is obtained via the single square brackets:
sage: t[1,1]
x^2
sage: t[1,1] is t[[1,1]].coord_function() # the coordinate function
True
sage: t[1,1] is t[[1,1]].coord_function(c_xy)
True
sage: t[1,1].expr() is t[[1,1]].expr() # the symbolic expression
True
Expressions in a chart different from the manifold’s default one are obtained by specifying the chart as the last argument inside the single square brackets:
sage: c_uv.<u,v> = M.chart()
sage: xy_to_uv = c_xy.transition_map(c_uv, [x+y, x-y])
sage: uv_to_xy = xy_to_uv.inverse()
sage: t[1,1, c_uv]
1/4*u^2 + 1/2*u*v + 1/4*v^2
Note that t[1,1, c_uv]
is the component of the tensor t
with respect
to the coordinate frame associated to the chart \((x,y)\) expressed in terms of
the coordinates \((u,v)\). Indeed, t[1,1, c_uv]
is a shortcut for
t.comp(c_xy.frame())[[1,1]].coord_function(c_uv)
:
sage: t[1,1, c_uv] is t.comp(c_xy.frame())[[1,1]].coord_function(c_uv)
True
Similarly, t[1,1]
is a shortcut for
t.comp(c_xy.frame())[[1,1]].coord_function(c_xy)
:
sage: t[1,1] is t.comp(c_xy.frame())[[1,1]].coord_function(c_xy)
True
sage: t[1,1] is t.comp()[[1,1]].coord_function() # since c_xy.frame() and c_xy are the manifold's default values
True
All the components can be set at once via [:]
:
sage: t[:] = [[1, -x], [x*y, 2]]
sage: t[:]
[ 1 -x]
[x*y 2]
To set the components in a vector frame different from the manifold’s
default one, the method
set_comp()
can be employed:
sage: e = M.vector_frame('e')
sage: t.set_comp(e)[1,1] = x+y
sage: t.set_comp(e)[2,1], t.set_comp(e)[2,2] = y, -3*x
but, as a shortcut, one may simply specify the frame as the first argument of the square brackets:
sage: t[e,1,1] = x+y
sage: t[e,2,1], t[e,2,2] = y, -3*x
sage: t.comp(e)
2-indices components w.r.t. Vector frame (M, (e_1,e_2))
sage: t.comp(e)[:]
[x + y 0]
[ y -3*x]
sage: t[e,:] # a shortcut of the above
[x + y 0]
[ y -3*x]
All the components in some frame can be set at once, via
the operator [:]
:
sage: t[e,:] = [[x+y, 0], [y, -3*x]]
sage: t[e,:] # same as above:
[x + y 0]
[ y -3*x]
Equivalently, one can initialize the components in e
at the tensor field
construction:
sage: t = M.tensor_field(1, 1, [[x+y, 0], [y, -3*x]], frame=e, name='T')
sage: t[e,:] # same as above:
[x + y 0]
[ y -3*x]
To avoid any inconsistency between the various components, the method
set_comp()
clears the components in other frames.
To keep the other components, one must use the method
add_comp()
:
sage: t = M.tensor_field(1, 1, name='T') # Let us restart
sage: t[:] = [[1, -x], [x*y, 2]] # by first setting the components in the frame c_xy.frame()
We now set the components in the frame e with add_comp:
sage: t.add_comp(e)[:] = [[x+y, 0], [y, -3*x]]
The expansion of the tensor field in a given frame is obtained via the method
display
:
sage: t.display() # expansion in the manifold's default frame
T = ∂/∂x⊗dx - x ∂/∂x⊗dy + x*y ∂/∂y⊗dx + 2 ∂/∂y⊗dy
sage: t.display(e)
T = (x + y) e_1⊗e^1 + y e_2⊗e^1 - 3*x e_2⊗e^2
See display()
for more examples.
By definition, a tensor field acts as a multilinear map on 1-forms and vector
fields; in the present case, T
being of type \((1,1)\), it acts on pairs
(1-form, vector field):
sage: a = M.one_form(1, x, name='a')
sage: v = M.vector_field(y, 2, name='V')
sage: t(a,v)
Scalar field T(a,V) on the 2-dimensional differentiable manifold M
sage: t(a,v).display()
T(a,V): M → ℝ
(x, y) ↦ x^2*y^2 + 2*x + y
(u, v) ↦ 1/16*u^4 - 1/8*u^2*v^2 + 1/16*v^4 + 3/2*u + 1/2*v
sage: latex(t(a,v))
T\left(a,V\right)
Check by means of the component expression of t(a,v)
:
sage: t(a,v).expr() - t[1,1]*a[1]*v[1] - t[1,2]*a[1]*v[2] \
....: - t[2,1]*a[2]*v[1] - t[2,2]*a[2]*v[2]
0
A scalar field (rank-0 tensor field):
sage: f = M.scalar_field(x*y + 2, name='f') ; f
Scalar field f on the 2-dimensional differentiable manifold M
sage: f.tensor_type()
(0, 0)
A scalar field acts on points on the manifold:
sage: p = M.point((1,2))
sage: f(p)
4
See DiffScalarField
for
more details on scalar fields.
A vector field (rank-1 contravariant tensor field):
sage: v = M.vector_field(-x, y, name='v') ; v
Vector field v on the 2-dimensional differentiable manifold M
sage: v.tensor_type()
(1, 0)
sage: v.display()
v = -x ∂/∂x + y ∂/∂y
A field of symmetric bilinear forms:
sage: q = M.sym_bilin_form_field(name='Q') ; q
Field of symmetric bilinear forms Q on the 2-dimensional differentiable
manifold M
sage: q.tensor_type()
(0, 2)
The components of a symmetric bilinear form are dealt by the subclass
CompFullySym
of the class
Components
, which takes into
account the symmetry between the two indices:
sage: q[1,1], q[1,2], q[2,2] = (0, -x, y) # no need to set the component (2,1)
sage: type(q.comp())
<class 'sage.tensor.modules.comp.CompFullySym'>
sage: q[:] # note that the component (2,1) is equal to the component (1,2)
[ 0 -x]
[-x y]
sage: q.display()
Q = -x dx⊗dy - x dy⊗dx + y dy⊗dy
More generally, tensor symmetries or antisymmetries can be specified via
the keywords sym
and antisym
. For instance a rank-4 covariant
tensor symmetric with respect to its first two arguments (no. 0 and no. 1) and
antisymmetric with respect to its last two ones (no. 2 and no. 3) is declared
as follows:
sage: t = M.tensor_field(0, 4, name='T', sym=(0,1), antisym=(2,3))
sage: t[1,2,1,2] = 3
sage: t[2,1,1,2] # check of the symmetry with respect to the first 2 indices
3
sage: t[1,2,2,1] # check of the antisymmetry with respect to the last 2 indices
-3
- class sage.manifolds.differentiable.tensorfield_paral.TensorFieldParal(vector_field_module, tensor_type, name=None, latex_name=None, sym=None, antisym=None)#
Bases:
FreeModuleTensor
,TensorField
Tensor field along a differentiable manifold, with values on a parallelizable manifold.
An instance of this class is a tensor field along a differentiable manifold \(U\) with values on a parallelizable manifold \(M\), via a differentiable map \(\Phi: U \rightarrow M\). More precisely, given two non-negative integers \(k\) and \(l\) and a differentiable map
\[\Phi:\ U \longrightarrow M,\]a tensor field of type \((k,l)\) along \(U\) with values on \(M\) is a differentiable map
\[t:\ U \longrightarrow T^{(k,l)}M\](where \(T^{(k,l)}M\) is the tensor bundle of type \((k,l)\) over \(M\)) such that
\[t(p) \in T^{(k,l)}(T_q M)\]for all \(p \in U\), i.e. \(t(p)\) is a tensor of type \((k,l)\) on the tangent space \(T_q M\) at the point \(q=\Phi(p)\). That is to say a multilinear map
\[t(p):\ \underbrace{T_q^*M\times\cdots\times T_q^*M}_{k\ \; \text{times}} \times \underbrace{T_q M\times\cdots\times T_q M}_{l\ \; \text{times}} \longrightarrow K,\]where \(T_q^* M\) is the dual vector space to \(T_q M\) and \(K\) is the topological field over which the manifold \(M\) is defined. The integer \(k+l\) is called the tensor rank.
The standard case of a tensor field on a differentiable 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
TensorField
should be used instead.INPUT:
vector_field_module
– free module \(\mathfrak{X}(U,\Phi)\) of vector fields along \(U\) associated with the map \(\Phi: U \rightarrow M\) (cf.VectorFieldFreeModule
)tensor_type
– pair \((k,l)\) with \(k\) being the contravariant rank and \(l\) the covariant rankname
– (default:None
) name given to the tensor fieldlatex_name
– (default:None
) LaTeX symbol to denote the tensor field; if none is provided, the LaTeX symbol is set toname
sym
– (default:None
) a symmetry or a list of symmetries among the tensor arguments: each symmetry is described by a tuple containing the positions of the involved arguments, with the convention position=0 for the first argument; for instance:sym=(0,1)
for a symmetry between the 1st and 2nd argumentssym=[(0,2),(1,3,4)]
for a symmetry between the 1st and 3rd arguments and a symmetry between the 2nd, 4th and 5th arguments
antisym
– (default:None
) antisymmetry or list of antisymmetries among the arguments, with the same convention as forsym
EXAMPLES:
A tensor field of type \((2,0)\) on a 3-dimensional parallelizable manifold:
sage: M = Manifold(3, 'M') sage: c_xyz.<x,y,z> = M.chart() # makes M parallelizable sage: t = M.tensor_field(2, 0, name='T') ; t Tensor field T of type (2,0) on the 3-dimensional differentiable manifold M
Tensor fields are considered as elements of a module over the ring \(C^k(M)\) of scalar fields on \(M\):
sage: t.parent() Free module T^(2,0)(M) of type-(2,0) tensors fields on the 3-dimensional differentiable manifold M sage: t.parent().base_ring() Algebra of differentiable scalar fields on the 3-dimensional differentiable manifold M
The components with respect to the manifold’s default frame are set or read by means of square brackets:
sage: e = M.vector_frame('e') ; M.set_default_frame(e) sage: for i in M.irange(): ....: for j in M.irange(): ....: t[i,j] = (i+1)**(j+1) sage: [[ t[i,j] for j in M.irange()] for i in M.irange()] [[1, 1, 1], [2, 4, 8], [3, 9, 27]]
A shortcut for the above is using
[:]
:sage: t[:] [ 1 1 1] [ 2 4 8] [ 3 9 27]
The components with respect to another frame are set via the method
set_comp()
and read via the methodcomp()
; both return an instance ofComponents
:sage: f = M.vector_frame('f') # a new frame defined on M, in addition to e sage: t.set_comp(f)[0,0] = -3 sage: t.comp(f) 2-indices components w.r.t. Vector frame (M, (f_0,f_1,f_2)) sage: t.comp(f)[0,0] -3 sage: t.comp(f)[:] # the full list of components [-3 0 0] [ 0 0 0] [ 0 0 0]
To avoid any inconsistency between the various components, the method
set_comp()
deletes the components in other frames. Accordingly, the components in the framee
have been deleted:sage: t._components {Vector frame (M, (f_0,f_1,f_2)): 2-indices components w.r.t. Vector frame (M, (f_0,f_1,f_2))}
To keep the other components, one must use the method
add_comp()
:sage: t = M.tensor_field(2, 0, name='T') # let us restart sage: t[0,0] = 2 # sets the components in the frame e
We now set the components in the frame f with add_comp:
sage: t.add_comp(f)[0,0] = -3
The components w.r.t. frame e have been kept:
sage: t._components # random (dictionary output) {Vector frame (M, (e_0,e_1,e_2)): 2-indices components w.r.t. Vector frame (M, (e_0,e_1,e_2)), Vector frame (M, (f_0,f_1,f_2)): 2-indices components w.r.t. Vector frame (M, (f_0,f_1,f_2))}
The basic properties of a tensor field are:
sage: t.domain() 3-dimensional differentiable manifold M sage: t.tensor_type() (2, 0)
Symmetries and antisymmetries are declared via the keywords
sym
andantisym
. For instance, a rank-6 covariant tensor that is symmetric with respect to its 1st and 3rd arguments and antisymmetric with respect to the 2nd, 5th and 6th arguments is set up as follows:sage: a = M.tensor_field(0, 6, name='T', sym=(0,2), antisym=(1,4,5)) sage: a[0,0,1,0,1,2] = 3 sage: a[1,0,0,0,1,2] # check of the symmetry 3 sage: a[0,1,1,0,0,2], a[0,1,1,0,2,0] # check of the antisymmetry (-3, 3)
Multiple symmetries or antisymmetries are allowed; they must then be declared as a list. For instance, a rank-4 covariant tensor that is antisymmetric with respect to its 1st and 2nd arguments and with respect to its 3rd and 4th argument must be declared as:
sage: r = M.tensor_field(0, 4, name='T', antisym=[(0,1), (2,3)]) sage: r[0,1,2,0] = 3 sage: r[1,0,2,0] # first antisymmetry -3 sage: r[0,1,0,2] # second antisymmetry -3 sage: r[1,0,0,2] # both antisymmetries acting 3
Tensor fields of the same type can be added and subtracted:
sage: a = M.tensor_field(2, 0) sage: a[0,0], a[0,1], a[0,2] = (1,2,3) sage: b = M.tensor_field(2, 0) sage: b[0,0], b[1,1], b[2,2], b[0,2] = (4,5,6,7) sage: s = a + 2*b ; s Tensor field of type (2,0) on the 3-dimensional differentiable manifold M sage: a[:], (2*b)[:], s[:] ( [1 2 3] [ 8 0 14] [ 9 2 17] [0 0 0] [ 0 10 0] [ 0 10 0] [0 0 0], [ 0 0 12], [ 0 0 12] ) sage: s = a - b ; s Tensor field of type (2,0) on the 3-dimensional differentiable manifold M sage: a[:], b[:], s[:] ( [1 2 3] [4 0 7] [-3 2 -4] [0 0 0] [0 5 0] [ 0 -5 0] [0 0 0], [0 0 6], [ 0 0 -6] )
Symmetries are preserved by the addition whenever it is possible:
sage: a = M.tensor_field(2, 0, sym=(0,1)) sage: a[0,0], a[0,1], a[0,2] = (1,2,3) sage: s = a + b sage: a[:], b[:], s[:] ( [1 2 3] [4 0 7] [ 5 2 10] [2 0 0] [0 5 0] [ 2 5 0] [3 0 0], [0 0 6], [ 3 0 6] ) sage: a.symmetries() symmetry: (0, 1); no antisymmetry sage: b.symmetries() no symmetry; no antisymmetry sage: s.symmetries() no symmetry; no antisymmetry
Let us now make b symmetric:
sage: b = M.tensor_field(2, 0, sym=(0,1)) sage: b[0,0], b[1,1], b[2,2], b[0,2] = (4,5,6,7) sage: s = a + b sage: a[:], b[:], s[:] ( [1 2 3] [4 0 7] [ 5 2 10] [2 0 0] [0 5 0] [ 2 5 0] [3 0 0], [7 0 6], [10 0 6] ) sage: s.symmetries() # s is symmetric because both a and b are symmetry: (0, 1); no antisymmetry
The tensor product is taken with the operator
*
:sage: c = a*b ; c Tensor field of type (4,0) on the 3-dimensional differentiable manifold M sage: c.symmetries() # since a and b are both symmetric, a*b has two symmetries: symmetries: [(0, 1), (2, 3)]; no antisymmetry
The tensor product of two fully contravariant tensors is not symmetric in general:
sage: a*b == b*a False
The tensor product of a fully contravariant tensor by a fully covariant one is symmetric:
sage: d = M.diff_form(2) # a fully covariant tensor field sage: d[0,1], d[0,2], d[1,2] = (3, 2, 1) sage: s = a*d ; s Tensor field of type (2,2) on the 3-dimensional differentiable manifold M sage: s.symmetries() symmetry: (0, 1); antisymmetry: (2, 3) sage: s1 = d*a ; s1 Tensor field of type (2,2) on the 3-dimensional differentiable manifold M sage: s1.symmetries() symmetry: (0, 1); antisymmetry: (2, 3) sage: d*a == a*d True
Example of tensor field associated with a non-trivial differentiable map \(\Phi\): tensor field along a curve in \(M\):
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, [cos(t), sin(t), t], name='Phi') ; Phi Differentiable map Phi from the 1-dimensional differentiable manifold R to the 3-dimensional differentiable manifold M sage: h = R.tensor_field(2, 0, name='h', dest_map=Phi) ; h Tensor field h of type (2,0) along the 1-dimensional differentiable manifold R with values on the 3-dimensional differentiable manifold M sage: h.parent() Free module T^(2,0)(R,Phi) of type-(2,0) tensors fields along the 1-dimensional differentiable manifold R mapped into the 3-dimensional differentiable manifold M sage: h[0,0], h[0,1], h[2,0] = 1+t, t^2, sin(t) sage: h.display() h = (t + 1) ∂/∂x⊗∂/∂x + t^2 ∂/∂x⊗∂/∂y + sin(t) ∂/∂z⊗∂/∂x
- add_comp(basis=None)#
Return the components of the tensor field in a given vector frame for assignment.
The components with respect to other frames on the same domain are kept. To delete them, use the method
set_comp()
instead.INPUT:
basis
– (default:None
) vector frame in which the components are defined; if none is provided, the components are assumed to refer to the tensor field domain’s default frame
OUTPUT:
components in the given frame, as an instance of the class
Components
; if such components did not exist previously, they are created
EXAMPLES:
sage: M = Manifold(2, 'M') sage: X.<x,y> = M.chart() sage: e_xy = X.frame() sage: t = M.tensor_field(1,1, name='t') sage: t.add_comp(e_xy) 2-indices components w.r.t. Coordinate frame (M, (∂/∂x,∂/∂y)) sage: t.add_comp(e_xy)[1,0] = 2 sage: t.display(e_xy) t = 2 ∂/∂y⊗dx
Adding components with respect to a new frame (
e
):sage: e = M.vector_frame('e') sage: t.add_comp(e) 2-indices components w.r.t. Vector frame (M, (e_0,e_1)) sage: t.add_comp(e)[0,1] = x sage: t.display(e) t = x e_0⊗e^1
The components with respect to the frame
e_xy
are kept:sage: t.display(e_xy) t = 2 ∂/∂y⊗dx
Adding components in a frame defined on a subdomain:
sage: U = M.open_subset('U', coord_def={X: x>0}) sage: f = U.vector_frame('f') sage: t.add_comp(f) 2-indices components w.r.t. Vector frame (U, (f_0,f_1)) sage: t.add_comp(f)[0,1] = 1+y sage: t.display(f) t = (y + 1) f_0⊗f^1
The components previously defined are kept:
sage: t.display(e_xy) t = 2 ∂/∂y⊗dx sage: t.display(e) t = x e_0⊗e^1
- along(mapping)#
Return the tensor field deduced from
self
via a differentiable map, the codomain of which is included in the domain ofself
.More precisely, if
self
is a tensor field \(t\) on \(M\) and if \(\Phi: U \rightarrow M\) is a differentiable map from some differentiable manifold \(U\) to \(M\), the returned object is a tensor field \(\tilde t\) along \(U\) with values on \(M\) such that\[\forall p \in U,\ \tilde t(p) = t(\Phi(p)).\]INPUT:
mapping
– differentiable map \(\Phi: U \rightarrow M\)
OUTPUT:
tensor field \(\tilde t\) along \(U\) defined above.
EXAMPLES:
Let us consider the map \(\Phi\) between the interval \(U=(0,2\pi)\) and the Euclidean plane \(M=\RR^2\) defining the lemniscate of Gerono:
sage: M = Manifold(2, 'M') sage: X.<x,y> = M.chart() sage: t = var('t', domain='real') sage: Phi = M.curve({X: [sin(t), sin(2*t)/2]}, (t, 0, 2*pi), ....: name='Phi') sage: U = Phi.domain(); U Real interval (0, 2*pi)
and a vector field on \(M\):
sage: v = M.vector_field(-y , x, name='v')
We have then:
sage: vU = v.along(Phi); vU Vector field v along the Real interval (0, 2*pi) with values on the 2-dimensional differentiable manifold M sage: vU.display() v = -cos(t)*sin(t) ∂/∂x + sin(t) ∂/∂y sage: vU.parent() Free module X((0, 2*pi),Phi) of vector fields along the Real interval (0, 2*pi) mapped into the 2-dimensional differentiable manifold M sage: vU.parent() is Phi.tangent_vector_field().parent() True
We check that the defining relation \(\tilde t(p) = t(\Phi(p))\) holds:
sage: p = U(t) # a generic point of U sage: vU.at(p) == v.at(Phi(p)) True
Case of a tensor field of type
(0,2)
:sage: a = M.tensor_field(0, 2) sage: a[0,0], a[0,1], a[1,1] = x+y, x*y, x^2-y^2 sage: aU = a.along(Phi); aU Tensor field of type (0,2) along the Real interval (0, 2*pi) with values on the 2-dimensional differentiable manifold M sage: aU.display() (cos(t) + 1)*sin(t) dx⊗dx + cos(t)*sin(t)^2 dx⊗dy + sin(t)^4 dy⊗dy sage: aU.parent() Free module T^(0,2)((0, 2*pi),Phi) of type-(0,2) tensors fields along the Real interval (0, 2*pi) mapped into the 2-dimensional differentiable manifold M sage: aU.at(p) == a.at(Phi(p)) True
- at(point)#
Value of
self
at a point of its domain.If the current tensor field is
\[t:\ U \longrightarrow T^{(k,l)} M\]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 \(p\in U\), \(t(p)\) is a tensor on the tangent space to \(M\) at the point \(\Phi(p)\).
INPUT:
point
–ManifoldPoint
point \(p\) in the domain of the tensor field \(U\)
OUTPUT:
FreeModuleTensor
representing the tensor \(t(p)\) on the tangent vector space \(T_{\Phi(p)} M\)
EXAMPLES:
Vector in a tangent space of a 2-dimensional manifold:
sage: M = Manifold(2, 'M') sage: c_xy.<x,y> = M.chart() sage: p = M.point((-2,3), name='p') sage: v = M.vector_field(y, x^2, name='v') sage: v.display() v = y ∂/∂x + x^2 ∂/∂y sage: vp = v.at(p) ; vp Tangent vector v at Point p on the 2-dimensional differentiable manifold M sage: vp.parent() Tangent space at Point p on the 2-dimensional differentiable manifold M sage: vp.display() v = 3 ∂/∂x + 4 ∂/∂y
A 1-form gives birth to a linear form in the tangent space:
sage: w = M.one_form(-x, 1+y, name='w') sage: w.display() w = -x dx + (y + 1) dy sage: wp = w.at(p) ; wp Linear form w on the Tangent space at Point p on the 2-dimensional differentiable manifold M sage: wp.parent() Dual of the Tangent space at Point p on the 2-dimensional differentiable manifold M sage: wp.display() w = 2 dx + 4 dy
A tensor field of type \((1,1)\) yields a tensor of type \((1,1)\) in the tangent space:
sage: t = M.tensor_field(1, 1, name='t') sage: t[0,0], t[0,1], t[1,1] = 1+x, x*y, 1-y sage: t.display() t = (x + 1) ∂/∂x⊗dx + x*y ∂/∂x⊗dy + (-y + 1) ∂/∂y⊗dy sage: tp = t.at(p) ; tp Type-(1,1) tensor t on the Tangent space at Point p on the 2-dimensional differentiable manifold M sage: tp.parent() Free module of type-(1,1) tensors on the Tangent space at Point p on the 2-dimensional differentiable manifold M sage: tp.display() t = -∂/∂x⊗dx - 6 ∂/∂x⊗dy - 2 ∂/∂y⊗dy
A 2-form yields an alternating form of degree 2 in the tangent space:
sage: a = M.diff_form(2, name='a') sage: a[0,1] = x*y sage: a.display() a = x*y dx∧dy sage: ap = a.at(p) ; ap Alternating form a of degree 2 on the Tangent space at Point p on the 2-dimensional differentiable manifold M sage: ap.parent() 2nd exterior power of the dual of the Tangent space at Point p on the 2-dimensional differentiable manifold M sage: ap.display() a = -6 dx∧dy
Example with a non trivial map \(\Phi\):
sage: U = Manifold(1, 'U') # (0,2*pi) as a 1-dimensional manifold sage: T.<t> = U.chart(r't:(0,2*pi)') # canonical chart on U sage: Phi = U.diff_map(M, [cos(t), sin(t)], name='Phi', ....: latex_name=r'\Phi') sage: v = U.vector_field(1+t, t^2, name='v', dest_map=Phi) ; v Vector field v along the 1-dimensional differentiable manifold U with values on the 2-dimensional differentiable manifold M sage: v.display() v = (t + 1) ∂/∂x + t^2 ∂/∂y sage: p = U((pi/6,)) sage: vp = v.at(p) ; vp Tangent vector v at Point on the 2-dimensional differentiable manifold M sage: vp.parent() is M.tangent_space(Phi(p)) True sage: vp.display() v = (1/6*pi + 1) ∂/∂x + 1/36*pi^2 ∂/∂y
- comp(basis=None, from_basis=None)#
Return the components in a given vector frame.
If the components are not known already, they are computed by the tensor change-of-basis formula from components in another vector frame.
INPUT:
basis
– (default:None
) vector frame in which the components are required; if none is provided, the components are assumed to refer to the tensor field domain’s default framefrom_basis
– (default:None
) vector frame from which the required components are computed, via the tensor change-of-basis formula, if they are not known already in the basisbasis
OUTPUT:
components in the vector frame
basis
, as an instance of the classComponents
EXAMPLES:
sage: M = Manifold(2, 'M', start_index=1) sage: X.<x,y> = M.chart() sage: t = M.tensor_field(1,2, name='t') sage: t[1,2,1] = x*y sage: t.comp(X.frame()) 3-indices components w.r.t. Coordinate frame (M, (∂/∂x,∂/∂y)) sage: t.comp() # the default frame is X.frame() 3-indices components w.r.t. Coordinate frame (M, (∂/∂x,∂/∂y)) sage: t.comp()[:] [[[0, 0], [x*y, 0]], [[0, 0], [0, 0]]] sage: e = M.vector_frame('e') sage: t[e, 2,1,1] = x-3 sage: t.comp(e) 3-indices components w.r.t. Vector frame (M, (e_1,e_2)) sage: t.comp(e)[:] [[[0, 0], [0, 0]], [[x - 3, 0], [0, 0]]]
- contract(*args)#
Contraction with another tensor field, on one or more indices.
INPUT:
pos1
– positions of the indices inself
involved in the contraction;pos1
must be a sequence of integers, with 0 standing for the first index position, 1 for the second one, etc. Ifpos1
is not provided, a single contraction on the last index position ofself
is assumedother
– the tensor field to contract withpos2
– positions of the indices inother
involved in the contraction, with the same conventions as forpos1
. Ifpos2
is not provided, a single contraction on the first index position ofother
is assumed
OUTPUT:
tensor field resulting from the contraction at the positions
pos1
andpos2
ofself
withother
EXAMPLES:
Contraction of a tensor field of type \((2,0)\) with a tensor field of type \((1,1)\):
sage: M = Manifold(2, 'M') sage: X.<x,y> = M.chart() sage: a = M.tensor_field(2,0, [[1+x, 2], [y, -x^2]], name='a') sage: b = M.tensor_field(1,1, [[-y, 1], [x, x+y]], name='b') sage: s = a.contract(0, b, 1); s Tensor field of type (2,0) on the 2-dimensional differentiable manifold M sage: s.display() -x*y ∂/∂x⊗∂/∂x + (x^2 + x*y + y^2 + x) ∂/∂x⊗∂/∂y + (-x^2 - 2*y) ∂/∂y⊗∂/∂x + (-x^3 - x^2*y + 2*x) ∂/∂y⊗∂/∂y
Check:
sage: all(s[ind] == sum(a[k, ind[0]]*b[ind[1], k] for k in [0..1]) ....: for ind in M.index_generator(2)) True
The same contraction with repeated index notation:
sage: s == a['^ki']*b['^j_k'] True
Contraction on the second index of
a
:sage: s = a.contract(1, b, 1); s Tensor field of type (2,0) on the 2-dimensional differentiable manifold M sage: s.display() (-(x + 1)*y + 2) ∂/∂x⊗∂/∂x + (x^2 + 3*x + 2*y) ∂/∂x⊗∂/∂y + (-x^2 - y^2) ∂/∂y⊗∂/∂x + (-x^3 - (x^2 - x)*y) ∂/∂y⊗∂/∂y
Check:
sage: all(s[ind] == sum(a[ind[0], k]*b[ind[1], k] for k in [0..1]) ....: for ind in M.index_generator(2)) True
The same contraction with repeated index notation:
sage: s == a['^ik']*b['^j_k'] True
See also
sage.manifolds.differentiable.tensorfield.TensorField.contract()
for more examples.
- display_comp(frame=None, chart=None, coordinate_labels=True, only_nonzero=True, only_nonredundant=False)#
Display the tensor components with respect to a given frame, one per line.
The output is either text-formatted (console mode) or LaTeX-formatted (notebook mode).
INPUT:
frame
– (default:None
) vector frame with respect to which the tensor field components are defined; ifNone
, thenif
chart
is notNone
, the coordinate frame associated tochart
is usedotherwise, the default basis of the vector field module on which the tensor field is defined is used
chart
– (default:None
) chart specifying the coordinate expression of the components; ifNone
, the default chart of the tensor field domain is usedcoordinate_labels
– (default:True
) boolean; ifTrue
, coordinate symbols are used by default (instead of integers) as index labels wheneverframe
is a coordinate frameonly_nonzero
– (default:True
) boolean; ifTrue
, only nonzero components are displayedonly_nonredundant
– (default:False
) boolean; ifTrue
, only nonredundant components are displayed in case of symmetries
EXAMPLES:
Display of the components of a type-\((2,1)\) tensor field on a 2-dimensional manifold:
sage: M = Manifold(2, 'M') sage: X.<x,y> = M.chart() sage: t = M.tensor_field(2, 1, name='t', sym=(0,1)) sage: t[0,0,0], t[0,1,0], t[1,1,1] = x+y, x*y, -3 sage: t.display_comp() t^xx_x = x + y t^xy_x = x*y t^yx_x = x*y t^yy_y = -3
By default, only the non-vanishing components are displayed; to see all the components, the argument
only_nonzero
must be set toFalse
:sage: t.display_comp(only_nonzero=False) t^xx_x = x + y t^xx_y = 0 t^xy_x = x*y t^xy_y = 0 t^yx_x = x*y t^yx_y = 0 t^yy_x = 0 t^yy_y = -3
t
being symmetric with respect to its first two indices, one may ask to skip the components that can be deduced by symmetry:sage: t.display_comp(only_nonredundant=True) t^xx_x = x + y t^xy_x = x*y t^yy_y = -3
Instead of coordinate labels, one may ask for integers:
sage: t.display_comp(coordinate_labels=False) t^00_0 = x + y t^01_0 = x*y t^10_0 = x*y t^11_1 = -3
Display in a frame different from the default one (note that since
f
is not a coordinate frame, integer are used to label the indices):sage: a = M.automorphism_field() sage: a[:] = [[1+y^2, 0], [0, 2+x^2]] sage: f = X.frame().new_frame(a, 'f') sage: t.display_comp(frame=f) t^00_0 = (x + y)/(y^2 + 1) t^01_0 = x*y/(x^2 + 2) t^10_0 = x*y/(x^2 + 2) t^11_1 = -3/(x^2 + 2)
Display with respect to a chart different from the default one:
sage: Y.<u,v> = M.chart() sage: X_to_Y = X.transition_map(Y, [x+y, x-y]) sage: Y_to_X = X_to_Y.inverse() sage: t.display_comp(chart=Y) t^uu_u = 1/4*u^2 - 1/4*v^2 + 1/2*u - 3/2 t^uu_v = 1/4*u^2 - 1/4*v^2 + 1/2*u + 3/2 t^uv_u = 1/2*u + 3/2 t^uv_v = 1/2*u - 3/2 t^vu_u = 1/2*u + 3/2 t^vu_v = 1/2*u - 3/2 t^vv_u = -1/4*u^2 + 1/4*v^2 + 1/2*u - 3/2 t^vv_v = -1/4*u^2 + 1/4*v^2 + 1/2*u + 3/2
Note that the frame defining the components is the coordinate frame associated with chart
Y
, i.e. we have:sage: str(t.display_comp(chart=Y)) == str(t.display_comp(frame=Y.frame(), chart=Y)) True
Display of the components with respect to a specific frame, expressed in terms of a specific chart:
sage: t.display_comp(frame=f, chart=Y) t^00_0 = 4*u/(u^2 - 2*u*v + v^2 + 4) t^01_0 = (u^2 - v^2)/(u^2 + 2*u*v + v^2 + 8) t^10_0 = (u^2 - v^2)/(u^2 + 2*u*v + v^2 + 8) t^11_1 = -12/(u^2 + 2*u*v + v^2 + 8)
- lie_der(vector)#
Compute the Lie derivative with respect to a vector field.
INPUT:
vector
– vector field with respect to which the Lie derivative is to be taken
OUTPUT:
the tensor field that is the Lie derivative of
self
with respect tovector
EXAMPLES:
Lie derivative of a vector:
sage: M = Manifold(2, 'M', start_index=1) sage: c_xy.<x,y> = M.chart() sage: v = M.vector_field(-y, x, name='v') sage: w = M.vector_field(2*x+y, x*y) sage: w.lie_derivative(v) Vector field on the 2-dimensional differentiable manifold M sage: w.lie_derivative(v).display() ((x - 2)*y + x) ∂/∂x + (x^2 - y^2 - 2*x - y) ∂/∂y
The result is cached:
sage: w.lie_derivative(v) is w.lie_derivative(v) True
An alias is
lie_der
:sage: w.lie_der(v) is w.lie_derivative(v) True
The Lie derivative is antisymmetric:
sage: w.lie_der(v) == -v.lie_der(w) True
For vectors, it coincides with the commutator:
sage: f = M.scalar_field(x^3 + x*y^2) sage: w.lie_der(v)(f).display() M → ℝ (x, y) ↦ -(x + 2)*y^3 + 3*x^3 - x*y^2 + 5*(x^3 - 2*x^2)*y sage: w.lie_der(v)(f) == v(w(f)) - w(v(f)) # rhs = commutator [v,w] acting on f True
Lie derivative of a 1-form:
sage: om = M.one_form(y^2*sin(x), x^3*cos(y)) sage: om.lie_der(v) 1-form on the 2-dimensional differentiable manifold M sage: om.lie_der(v).display() (-y^3*cos(x) + x^3*cos(y) + 2*x*y*sin(x)) dx + (-x^4*sin(y) - 3*x^2*y*cos(y) - y^2*sin(x)) dy
Parallel computation:
sage: Parallelism().set('tensor', nproc=2) sage: om.lie_der(v) 1-form on the 2-dimensional differentiable manifold M sage: om.lie_der(v).display() (-y^3*cos(x) + x^3*cos(y) + 2*x*y*sin(x)) dx + (-x^4*sin(y) - 3*x^2*y*cos(y) - y^2*sin(x)) dy sage: Parallelism().set('tensor', nproc=1) # switch off parallelization
Check of Cartan identity:
sage: om.lie_der(v) == (v.contract(0, om.exterior_derivative(), 0) ....: + om(v).exterior_derivative()) True
- lie_derivative(vector)#
Compute the Lie derivative with respect to a vector field.
INPUT:
vector
– vector field with respect to which the Lie derivative is to be taken
OUTPUT:
the tensor field that is the Lie derivative of
self
with respect tovector
EXAMPLES:
Lie derivative of a vector:
sage: M = Manifold(2, 'M', start_index=1) sage: c_xy.<x,y> = M.chart() sage: v = M.vector_field(-y, x, name='v') sage: w = M.vector_field(2*x+y, x*y) sage: w.lie_derivative(v) Vector field on the 2-dimensional differentiable manifold M sage: w.lie_derivative(v).display() ((x - 2)*y + x) ∂/∂x + (x^2 - y^2 - 2*x - y) ∂/∂y
The result is cached:
sage: w.lie_derivative(v) is w.lie_derivative(v) True
An alias is
lie_der
:sage: w.lie_der(v) is w.lie_derivative(v) True
The Lie derivative is antisymmetric:
sage: w.lie_der(v) == -v.lie_der(w) True
For vectors, it coincides with the commutator:
sage: f = M.scalar_field(x^3 + x*y^2) sage: w.lie_der(v)(f).display() M → ℝ (x, y) ↦ -(x + 2)*y^3 + 3*x^3 - x*y^2 + 5*(x^3 - 2*x^2)*y sage: w.lie_der(v)(f) == v(w(f)) - w(v(f)) # rhs = commutator [v,w] acting on f True
Lie derivative of a 1-form:
sage: om = M.one_form(y^2*sin(x), x^3*cos(y)) sage: om.lie_der(v) 1-form on the 2-dimensional differentiable manifold M sage: om.lie_der(v).display() (-y^3*cos(x) + x^3*cos(y) + 2*x*y*sin(x)) dx + (-x^4*sin(y) - 3*x^2*y*cos(y) - y^2*sin(x)) dy
Parallel computation:
sage: Parallelism().set('tensor', nproc=2) sage: om.lie_der(v) 1-form on the 2-dimensional differentiable manifold M sage: om.lie_der(v).display() (-y^3*cos(x) + x^3*cos(y) + 2*x*y*sin(x)) dx + (-x^4*sin(y) - 3*x^2*y*cos(y) - y^2*sin(x)) dy sage: Parallelism().set('tensor', nproc=1) # switch off parallelization
Check of Cartan identity:
sage: om.lie_der(v) == (v.contract(0, om.exterior_derivative(), 0) ....: + om(v).exterior_derivative()) True
- restrict(subdomain, dest_map=None)#
Return the restriction of
self
to some subdomain.If the restriction has not been defined yet, it is constructed here.
INPUT:
subdomain
–DifferentiableManifold
; open subset \(U\) of the tensor field domain \(S\)dest_map
–DiffMap
(default:None
); destination map \(\Psi:\ U \rightarrow V\), where \(V\) is an open subset of the manifold \(M\) where the tensor field takes it values; ifNone
, the restriction of \(\Phi\) to \(U\) is used, \(\Phi\) being the differentiable map \(S \rightarrow M\) associated with the tensor field
OUTPUT:
instance of
TensorFieldParal
representing the restriction
EXAMPLES:
Restriction of a vector 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: v = M.vector_field(x+y, -1+x^2, name='v') sage: D = M.open_subset('D') # the unit open disc sage: c_cart_D = c_cart.restrict(D, x^2+y^2<1) sage: v_D = v.restrict(D) ; v_D Vector field v on the Open subset D of the 2-dimensional differentiable manifold R^2 sage: v_D.display() v = (x + y) ∂/∂x + (x^2 - 1) ∂/∂y
The symbolic expressions of the components with respect to Cartesian coordinates are equal:
sage: bool( v_D[1].expr() == v[1].expr() ) True
but neither the chart functions representing the components (they are defined on different charts):
sage: v_D[1] == v[1] False
nor the scalar fields representing the components (they are defined on different open subsets):
sage: v_D[[1]] == v[[1]] False
The restriction of the vector field to its own domain is of course itself:
sage: v.restrict(M) is v True
- series_expansion(symbol, order)#
Expand the tensor field in power series with respect to a small parameter.
If the small parameter is \(\epsilon\) and \(T\) is
self
, the power series expansion to order \(n\) is\[T = T_0 + \epsilon T_1 + \epsilon^2 T_2 + \cdots + \epsilon^n T_n + O(\epsilon^{n+1}),\]where \(T_0, T_1, \ldots, T_n\) are \(n+1\) tensor fields of the same tensor type as
self
and do not depend upon \(\epsilon\).INPUT:
symbol
– symbolic variable (the “small parameter” \(\epsilon\)) with respect to which the components ofself
are expanded in power seriesorder
– integer; the order \(n\) of the expansion, defined as the degree of the polynomial representing the truncated power series insymbol
OUTPUT:
list of the tensor fields \(T_i\) (size
order+1
)
EXAMPLES:
sage: M = Manifold(4, 'M', structure='Lorentzian') sage: C.<t,x,y,z> = M.chart() sage: e = var('e') sage: g = M.metric() sage: h1 = M.tensor_field(0,2,sym=(0,1)) sage: h2 = M.tensor_field(0,2,sym=(0,1)) sage: g[0, 0], g[1, 1], g[2, 2], g[3, 3] = -1, 1, 1, 1 sage: h1[0, 1], h1[1, 2], h1[2, 3] = 1, 1, 1 sage: h2[0, 2], h2[1, 3] = 1, 1 sage: g.set(g + e*h1 + e^2*h2) sage: g_ser = g.series_expansion(e, 2); g_ser [Field of symmetric bilinear forms on the 4-dimensional Lorentzian manifold M, Field of symmetric bilinear forms on the 4-dimensional Lorentzian manifold M, Field of symmetric bilinear forms on the 4-dimensional Lorentzian manifold M] sage: g_ser[0][:] [-1 0 0 0] [ 0 1 0 0] [ 0 0 1 0] [ 0 0 0 1] sage: g_ser[1][:] [0 1 0 0] [1 0 1 0] [0 1 0 1] [0 0 1 0] sage: g_ser[2][:] [0 0 1 0] [0 0 0 1] [1 0 0 0] [0 1 0 0] sage: all([g_ser[1] == h1, g_ser[2] == h2]) True
- set_calc_order(symbol, order, truncate=False)#
Trigger a power series expansion with respect to a small parameter in computations involving the tensor field.
This property is propagated by usual operations. The internal representation must be
SR
for this to take effect.If the small parameter is \(\epsilon\) and \(T\) is
self
, the power series expansion to order \(n\) is\[T = T_0 + \epsilon T_1 + \epsilon^2 T_2 + \cdots + \epsilon^n T_n + O(\epsilon^{n+1}),\]where \(T_0, T_1, \ldots, T_n\) are \(n+1\) tensor fields of the same tensor type as
self
and do not depend upon \(\epsilon\).INPUT:
symbol
– symbolic variable (the “small parameter” \(\epsilon\)) with respect to which the components ofself
are expanded in power seriesorder
– integer; the order \(n\) of the expansion, defined as the degree of the polynomial representing the truncated power series insymbol
truncate
– (default:False
) determines whether the components ofself
are replaced by their expansions to the given order
EXAMPLES:
sage: M = Manifold(4, 'M', structure='Lorentzian') sage: C.<t,x,y,z> = M.chart() sage: e = var('e') sage: g = M.metric() sage: h1 = M.tensor_field(0, 2, sym=(0,1)) sage: h2 = M.tensor_field(0, 2, sym=(0,1)) sage: g[0, 0], g[1, 1], g[2, 2], g[3, 3] = -1, 1, 1, 1 sage: h1[0, 1], h1[1, 2], h1[2, 3] = 1, 1, 1 sage: h2[0, 2], h2[1, 3] = 1, 1 sage: g.set(g + e*h1 + e^2*h2) sage: g.set_calc_order(e, 1) sage: g[:] [ -1 e e^2 0] [ e 1 e e^2] [e^2 e 1 e] [ 0 e^2 e 1] sage: g.set_calc_order(e, 1, truncate=True) sage: g[:] [-1 e 0 0] [ e 1 e 0] [ 0 e 1 e] [ 0 0 e 1]
- set_comp(basis=None)#
Return the components of the tensor field in a given vector frame for assignment.
The components with respect to other frames on the same domain are deleted, in order to avoid any inconsistency. To keep them, use the method
add_comp()
instead.INPUT:
basis
– (default:None
) vector frame in which the components are defined; if none is provided, the components are assumed to refer to the tensor field domain’s default frame
OUTPUT:
components in the given frame, as an instance of the class
Components
; if such components did not exist previously, they are created
EXAMPLES:
sage: M = Manifold(2, 'M') sage: X.<x,y> = M.chart() sage: e_xy = X.frame() sage: t = M.tensor_field(1,1, name='t') sage: t.set_comp(e_xy) 2-indices components w.r.t. Coordinate frame (M, (∂/∂x,∂/∂y)) sage: t.set_comp(e_xy)[1,0] = 2 sage: t.display(e_xy) t = 2 ∂/∂y⊗dx
Setting components in a new frame (
e
):sage: e = M.vector_frame('e') sage: t.set_comp(e) 2-indices components w.r.t. Vector frame (M, (e_0,e_1)) sage: t.set_comp(e)[0,1] = x sage: t.display(e) t = x e_0⊗e^1
The components with respect to the frame
e_xy
have be erased:sage: t.display(e_xy) Traceback (most recent call last): ... ValueError: no basis could be found for computing the components in the Coordinate frame (M, (∂/∂x,∂/∂y))
Setting components in a frame defined on a subdomain deletes previously defined components as well:
sage: U = M.open_subset('U', coord_def={X: x>0}) sage: f = U.vector_frame('f') sage: t.set_comp(f) 2-indices components w.r.t. Vector frame (U, (f_0,f_1)) sage: t.set_comp(f)[0,1] = 1+y sage: t.display(f) t = (y + 1) f_0⊗f^1 sage: t.display(e) Traceback (most recent call last): ... ValueError: no basis could be found for computing the components in the Vector frame (M, (e_0,e_1))
- truncate(symbol, order)#
Return the tensor field truncated at a given order in the power series expansion with respect to some small parameter.
If the small parameter is \(\epsilon\) and \(T\) is
self
, the power series expansion to order \(n\) is\[T = T_0 + \epsilon T_1 + \epsilon^2 T_2 + \cdots + \epsilon^n T_n + O(\epsilon^{n+1}),\]where \(T_0, T_1, \ldots, T_n\) are \(n+1\) tensor fields of the same tensor type as
self
and do not depend upon \(\epsilon\).INPUT:
symbol
– symbolic variable (the “small parameter” \(\epsilon\)) with respect to which the components ofself
are expanded in power seriesorder
– integer; the order \(n\) of the expansion, defined as the degree of the polynomial representing the truncated power series insymbol
OUTPUT:
the tensor field \(T_0 + \epsilon T_1 + \epsilon^2 T_2 + \cdots + \epsilon^n T_n\)
EXAMPLES:
sage: M = Manifold(4, 'M', structure='Lorentzian') sage: C.<t,x,y,z> = M.chart() sage: e = var('e') sage: g = M.metric() sage: h1 = M.tensor_field(0,2,sym=(0,1)) sage: h2 = M.tensor_field(0,2,sym=(0,1)) sage: g[0, 0], g[1, 1], g[2, 2], g[3, 3] = -1, 1, 1, 1 sage: h1[0, 1], h1[1, 2], h1[2, 3] = 1, 1, 1 sage: h2[0, 2], h2[1, 3] = 1, 1 sage: g.set(g + e*h1 + e^2*h2) sage: g[:] [ -1 e e^2 0] [ e 1 e e^2] [e^2 e 1 e] [ 0 e^2 e 1] sage: g.truncate(e, 1)[:] [-1 e 0 0] [ e 1 e 0] [ 0 e 1 e] [ 0 0 e 1]