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 automorphisms
DiffFormParal 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 rank
name – (default: None) name given to the tensor field
latex_name – (default: None) LaTeX symbol to denote the tensor field; if none is provided, the LaTeX symbol is set to name
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 arguments
- sym=[(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 for sym

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 method comp(); both return an instance of Components:

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 frame e 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 and antisym. 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 of self.

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 frame
from_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 basis basis

OUTPUT:

components in the vector frame basis, as an instance of the class Components

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 in self 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. If pos1 is not provided, a single contraction on the last index position of self is assumed
other – the tensor field to contract with
pos2 – positions of the indices in other involved in the contraction, with the same conventions as for pos1. If pos2 is not provided, a single contraction on the first index position of other is assumed

OUTPUT:

tensor field resulting from the contraction at the positions pos1 and pos2 of self with other

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

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; if None, then
- if chart is not None, the coordinate frame associated to chart is used
- otherwise, 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; if None, the default chart of the tensor field domain is used
coordinate_labels – (default: True) boolean; if True, coordinate symbols are used by default (instead of integers) as index labels whenever frame is a coordinate frame
only_nonzero – (default: True) boolean; if True, only nonzero components are displayed
only_nonredundant – (default: False) boolean; if True, 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 to False:

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 to vector

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 to vector

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; if None, 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 of self are expanded in power series
order – integer; the order \(n\) of the expansion, defined as the degree of the polynomial representing the truncated power series in symbol

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 of self are expanded in power series
order – integer; the order \(n\) of the expansion, defined as the degree of the polynomial representing the truncated power series in symbol
truncate – (default: False) determines whether the components of self 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 of self are expanded in power series
order – integer; the order \(n\) of the expansion, defined as the degree of the polynomial representing the truncated power series in symbol

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]