Operators for vector calculus¶
This module defines the following operators for scalar, vector and tensor
fields on any pseudo-Riemannian manifold (see
pseudo_riemannian
), and in particular
on Euclidean spaces (see euclidean
):
grad()
: gradient of a scalar fielddiv()
: divergence of a vector field, and more generally of a tensor fieldcurl()
: curl of a vector field (3-dimensional case only)laplacian()
: Laplace-Beltrami operator acting on a scalar field, a vector field, or more generally a tensor fielddalembertian()
: d’Alembert operator acting on a scalar field, a vector field, or more generally a tensor field, on a Lorentzian manifold
All these operators are implemented as functions that call the appropriate
method on their argument. The purpose is to allow one to use standard
mathematical notations, e.g. to write curl(v)
instead of v.curl()
.
Note that the norm()
operator is defined in the
module functional
.
See also
Examples 1 and 2 in euclidean
for examples involving these operators in the Euclidean plane and in the
Euclidean 3-space.
AUTHORS:
Eric Gourgoulhon (2018): initial version
- sage.manifolds.operators.curl(vector)¶
Curl operator.
The curl of a vector field \(v\) on an orientable pseudo-Riemannian manifold \((M,g)\) of dimension 3 is the vector field defined by
\[\mathrm{curl}\, v = (*(\mathrm{d} v^\flat))^\sharp\]where \(v^\flat\) is the 1-form associated to \(v\) by the metric \(g\) (see
down()
), \(*(\mathrm{d} v^\flat)\) is the Hodge dual with respect to \(g\) of the 2-form \(\mathrm{d} v^\flat\) (exterior derivative of \(v^\flat\)) (seehodge_dual()
) and \((*(\mathrm{d} v^\flat))^\sharp\) is corresponding vector field by \(g\)-duality (seeup()
).An alternative expression of the curl is
\[(\mathrm{curl}\, v)^i = \epsilon^{ijk} \nabla_j v_k\]where \(\nabla\) is the Levi-Civita connection of \(g\) (cf.
LeviCivitaConnection
) and \(\epsilon\) the volume 3-form (Levi-Civita tensor) of \(g\) (cf.volume_form()
)INPUT:
vector
– vector field on an orientable 3-dimensional pseudo-Riemannian manifold, as an instance ofVectorField
OUTPUT:
instance of
VectorField
representing the curl ofvector
EXAMPLES:
Curl of a vector field in the Euclidean 3-space:
sage: E.<x,y,z> = EuclideanSpace() sage: v = E.vector_field(sin(y), sin(x), 0, name='v') sage: v.display() v = sin(y) e_x + sin(x) e_y sage: from sage.manifolds.operators import curl sage: s = curl(v); s Vector field curl(v) on the Euclidean space E^3 sage: s.display() curl(v) = (cos(x) - cos(y)) e_z sage: s[:] [0, 0, cos(x) - cos(y)]
See the method
curl()
ofVectorField
for more details and examples.
- sage.manifolds.operators.dalembertian(field)¶
d’Alembert operator.
The d’Alembert operator or d’Alembertian on a Lorentzian manifold \((M,g)\) is nothing but the Laplace-Beltrami operator:
\[\Box = \nabla_i \nabla^i = g^{ij} \nabla_i \nabla_j\]where \(\nabla\) is the Levi-Civita connection of the metric \(g\) (cf.
LeviCivitaConnection
) and \(\nabla^i := g^{ij} \nabla_j\)INPUT:
field
– a scalar field \(f\) (instance ofDiffScalarField
) or a tensor field \(f\) (instance ofTensorField
) on a pseudo-Riemannian manifold
OUTPUT:
\(\Box f\), as an instance of
DiffScalarField
or ofTensorField
EXAMPLES:
d’Alembertian of a scalar field in the 2-dimensional Minkowski spacetime:
sage: M = Manifold(2, 'M', structure='Lorentzian') sage: X.<t,x> = M.chart() sage: g = M.metric() sage: g[0,0], g[1,1] = -1, 1 sage: f = M.scalar_field((x-t)^3 + (x+t)^2, name='f') sage: from sage.manifolds.operators import dalembertian sage: Df = dalembertian(f); Df Scalar field Box(f) on the 2-dimensional Lorentzian manifold M sage: Df.display() Box(f): M → ℝ (t, x) ↦ 0
See the method
dalembertian()
ofDiffScalarField
and the methoddalembertian()
ofTensorField
for more details and examples.
- sage.manifolds.operators.div(tensor)¶
Divergence operator.
Let \(t\) be a tensor field of type \((k,0)\) with \(k\geq 1\) on a pseudo-Riemannian manifold \((M, g)\). The divergence of \(t\) is the tensor field of type \((k-1,0)\) defined by
\[(\mathrm{div}\, t)^{a_1\ldots a_{k-1}} = \nabla_i t^{a_1\ldots a_{k-1} i} = (\nabla t)^{a_1\ldots a_{k-1} i}_{\phantom{a_1\ldots a_{k-1} i}\, i}\]where \(\nabla\) is the Levi-Civita connection of \(g\) (cf.
LeviCivitaConnection
).Note that the divergence is taken on the last index of the tensor field. This definition is extended to tensor fields of type \((k,l)\) with \(k\geq 0\) and \(l\geq 1\), by raising the last index with the metric \(g\): \(\mathrm{div}\, t\) is then the tensor field of type \((k,l-1)\) defined by
\[(\mathrm{div}\, t)^{a_1\ldots a_k}_{\phantom{a_1\ldots a_k}\, b_1\ldots b_{l-1}} = \nabla_i (g^{ij} t^{a_1\ldots a_k}_{\phantom{a_1\ldots a_k}\, b_1\ldots b_{l-1} j}) = (\nabla t^\sharp)^{a_1\ldots a_k i}_{\phantom{a_1\ldots a_k i}\, b_1\ldots b_{l-1} i}\]where \(t^\sharp\) is the tensor field deduced from \(t\) by raising the last index with the metric \(g\) (see
up()
).INPUT:
tensor
– tensor field \(t\) on a pseudo-Riemannian manifold \((M,g)\), as an instance ofTensorField
(possibly via one of its derived classes, likeVectorField
)
OUTPUT:
the divergence of
tensor
as an instance of eitherDiffScalarField
if \((k,l)=(1,0)\) (tensor
is a vector field) or \((k,l)=(0,1)\) (tensor
is a 1-form) or ofTensorField
if \(k+l\geq 2\)
EXAMPLES:
Divergence of a vector field in the Euclidean plane:
sage: E.<x,y> = EuclideanSpace() sage: v = E.vector_field(cos(x*y), sin(x*y), name='v') sage: v.display() v = cos(x*y) e_x + sin(x*y) e_y sage: from sage.manifolds.operators import div sage: s = div(v); s Scalar field div(v) on the Euclidean plane E^2 sage: s.display() div(v): E^2 → ℝ (x, y) ↦ x*cos(x*y) - y*sin(x*y) sage: s.expr() x*cos(x*y) - y*sin(x*y)
See the method
divergence()
ofTensorField
for more details and examples.
- sage.manifolds.operators.grad(scalar)¶
Gradient operator.
The gradient of a scalar field \(f\) on a pseudo-Riemannian manifold \((M,g)\) is the vector field \(\mathrm{grad}\, f\) whose components in any coordinate frame are
\[(\mathrm{grad}\, f)^i = g^{ij} \frac{\partial F}{\partial x^j}\]where the \(x^j\)’s are the coordinates with respect to which the frame is defined and \(F\) is the chart function representing \(f\) in these coordinates: \(f(p) = F(x^1(p),\ldots,x^n(p))\) for any point \(p\) in the chart domain. In other words, the gradient of \(f\) is the vector field that is the \(g\)-dual of the differential of \(f\).
INPUT:
scalar
– scalar field \(f\), as an instance ofDiffScalarField
OUTPUT:
instance of
VectorField
representing \(\mathrm{grad}\, f\)
EXAMPLES:
Gradient of a scalar field in the Euclidean plane:
sage: E.<x,y> = EuclideanSpace() sage: f = E.scalar_field(sin(x*y), name='f') sage: from sage.manifolds.operators import grad sage: grad(f) Vector field grad(f) on the Euclidean plane E^2 sage: grad(f).display() grad(f) = y*cos(x*y) e_x + x*cos(x*y) e_y sage: grad(f)[:] [y*cos(x*y), x*cos(x*y)]
See the method
gradient()
ofDiffScalarField
for more details and examples.
- sage.manifolds.operators.laplacian(field)¶
Laplace-Beltrami operator.
The Laplace-Beltrami operator on a pseudo-Riemannian manifold \((M,g)\) is the operator
\[\Delta = \nabla_i \nabla^i = g^{ij} \nabla_i \nabla_j\]where \(\nabla\) is the Levi-Civita connection of the metric \(g\) (cf.
LeviCivitaConnection
) and \(\nabla^i := g^{ij} \nabla_j\)INPUT:
field
– a scalar field \(f\) (instance ofDiffScalarField
) or a tensor field \(f\) (instance ofTensorField
) on a pseudo-Riemannian manifold
OUTPUT:
\(\Delta f\), as an instance of
DiffScalarField
or ofTensorField
EXAMPLES:
Laplacian of a scalar field on the Euclidean plane:
sage: E.<x,y> = EuclideanSpace() sage: f = E.scalar_field(sin(x*y), name='f') sage: from sage.manifolds.operators import laplacian sage: Df = laplacian(f); Df Scalar field Delta(f) on the Euclidean plane E^2 sage: Df.display() Delta(f): E^2 → ℝ (x, y) ↦ -(x^2 + y^2)*sin(x*y) sage: Df.expr() -(x^2 + y^2)*sin(x*y)
The Laplacian of a scalar field is the divergence of its gradient:
sage: from sage.manifolds.operators import div, grad sage: Df == div(grad(f)) True
See the method
laplacian()
ofDiffScalarField
and the methodlaplacian()
ofTensorField
for more details and examples.