# How to change coordinates#

This tutorial introduces some vector calculus capabilities of SageMath within the 3-dimensional Euclidean space. The corresponding tools have been developed via the SageManifolds project.

The tutorial is also available as a Jupyter notebook, either
passive (`nbviewer`

)
or interactive (`binder`

).

## Starting from Cartesian coordinates#

In this tutorial, we choose to start from the Cartesian coordinates \((x,y,z)\). Hence, we declare the 3-dimensional Euclidean space \(\mathbb{E}^3\) as:

```
sage: E.<x,y,z> = EuclideanSpace()
sage: E
Euclidean space E^3
```

By default, i.e. without the optional argument `coordinates`

in
`EuclideanSpace`

, \(\mathbb{E}^3\) is initialized with the chart of
Cartesian coordinates:

```
sage: E.atlas()
[Chart (E^3, (x, y, z))]
```

See the tutorial How to perform vector calculus in curvilinear coordinates for examples of initialization of the Euclidean space with spherical coordinates or cylindrical coordinates instead of the Cartesian ones.

Let us denote by `cartesian`

the chart of Cartesian coordinates:

```
sage: cartesian = E.cartesian_coordinates()
sage: cartesian
Chart (E^3, (x, y, z))
```

The access to the individual coordinates is performed via the square bracket operator:

```
sage: cartesian[1]
x
sage: cartesian[:]
(x, y, z)
```

Thanks to use of `<x,y,z>`

when declaring `E`

, the Python variables `x`

,
`y`

and `z`

have been created to store the coordinates \((x,y,z)\) as
symbolic expressions. There is no need to declare them via `var()`

, i.e. to
type `x, y, z = var('x y z')`

; they are immediately available:

```
sage: y is cartesian[2]
True
sage: type(y)
<class 'sage.symbolic.expression.Expression'>
```

Each of the Cartesian coordinates spans the entire real line:

```
sage: cartesian.coord_range()
x: (-oo, +oo); y: (-oo, +oo); z: (-oo, +oo)
```

Being the only coordinate chart created so far, `cartesian`

is the default
chart on `E`

:

```
sage: cartesian is E.default_chart()
True
```

\(\mathbb{E}^3\) is endowed with the orthonormal vector frame \((e_x, e_y, e_z)\) associated with Cartesian coordinates:

```
sage: E.frames()
[Coordinate frame (E^3, (e_x,e_y,e_z))]
```

Let us denote it by `cartesian_frame`

:

```
sage: cartesian_frame = E.cartesian_frame()
sage: cartesian_frame
Coordinate frame (E^3, (e_x,e_y,e_z))
sage: cartesian_frame is E.default_frame()
True
```

Each element of this frame is a unit vector field; for instance, we have \(e_x\cdot e_x = 1\):

```
sage: e_x = cartesian_frame[1]
sage: e_x
Vector field e_x on the Euclidean space E^3
sage: e_x.dot(e_x).expr()
1
```

as well as \(e_x\cdot e_y = 0\):

```
sage: e_y = cartesian_frame[2]
sage: e_x.dot(e_y).expr()
0
```

## Introducing spherical coordinates#

Spherical coordinates are introduced by:

```
sage: spherical.<r,th,ph> = E.spherical_coordinates()
sage: spherical
Chart (E^3, (r, th, ph))
```

We have:

```
sage: spherical[:]
(r, th, ph)
sage: spherical.coord_range()
r: (0, +oo); th: (0, pi); ph: [0, 2*pi] (periodic)
```

\(\mathbb{E}^3\) is now endowed with two coordinate charts:

```
sage: E.atlas()
[Chart (E^3, (x, y, z)), Chart (E^3, (r, th, ph))]
```

The change-of-coordinate formulas have been automatically implemented during
the above call `E.spherical_coordinates()`

:

```
sage: E.coord_change(spherical, cartesian).display()
x = r*cos(ph)*sin(th)
y = r*sin(ph)*sin(th)
z = r*cos(th)
sage: E.coord_change(cartesian, spherical).display()
r = sqrt(x^2 + y^2 + z^2)
th = arctan2(sqrt(x^2 + y^2), z)
ph = arctan2(y, x)
```

These formulas are automatically used if we ask to plot the grid of spherical coordinates in terms of Cartesian coordinates:

```
sage: spherical.plot(cartesian, color={r:'red', th:'green', ph:'orange'})
Graphics3d Object
```

Note that

the red lines are those along which \(r\) varies, while \((\theta,\phi)\) are kept fixed;

the grid lines are those along which \(\theta\) varies, while \((r,\phi)\) are kept fixed;

the orange lines are those along which \(\phi\) varies, while \((r,\theta)\) are kept fixed.

For customizing the plot, see the list of options in the documentation of
`plot()`

. For instance, we may draw the
spherical coordinates in the plane \(\theta=\pi/2\) in terms of the coordinates
\((x, y)\):

```
sage: spherical.plot(cartesian, fixed_coords={th: pi/2}, ambient_coords=(x,y),
....: color={r:'red', th:'green', ph:'orange'})
Graphics object consisting of 18 graphics primitives
```

Similarly the grid of spherical coordinates in the half-plane \(\phi=0\) drawn in terms of the coordinates \((x, z)\) is obtained via:

```
sage: spherical.plot(cartesian, fixed_coords={ph: 0}, ambient_coords=(x,z),
....: color={r:'red', th:'green', ph:'orange'})
Graphics object consisting of 18 graphics primitives
```

## Relations between the Cartesian and spherical vector frames#

At this stage, \(\mathbb{E}^3\) is endowed with three vector frames:

```
sage: E.frames()
[Coordinate frame (E^3, (e_x,e_y,e_z)),
Coordinate frame (E^3, (∂/∂r,∂/∂th,∂/∂ph)),
Vector frame (E^3, (e_r,e_th,e_ph))]
```

The second one is the *coordinate* frame \(\left(\frac{\partial}{\partial r},
\frac{\partial}{\partial\theta}, \frac{\partial}{\partial \phi}\right)\) of
spherical coordinates, while the third one is the standard *orthonormal* frame
\((e_r,e_\theta,e_\phi)\) associated with spherical coordinates. For Cartesian
coordinates, the coordinate frame and the orthonormal frame coincide: it is
\((e_x,e_y,e_z)\). For spherical coordinates, the orthonormal frame is returned
by the method
`spherical_frame()`

:

```
sage: spherical_frame = E.spherical_frame()
sage: spherical_frame
Vector frame (E^3, (e_r,e_th,e_ph))
```

We may check that it is an orthonormal frame, i.e. that it obeys \(e_i\cdot e_j = \delta_{ij}\):

```
sage: es = spherical_frame
sage: [[es[i].dot(es[j]).expr() for j in E.irange()] for i in E.irange()]
[[1, 0, 0], [0, 1, 0], [0, 0, 1]]
```

Via the method `display`

, we may express the orthonormal spherical frame in
terms of the Cartesian one:

```
sage: for vec in spherical_frame:
....: vec.display(cartesian_frame, spherical)
e_r = cos(ph)*sin(th) e_x + sin(ph)*sin(th) e_y + cos(th) e_z
e_th = cos(ph)*cos(th) e_x + cos(th)*sin(ph) e_y - sin(th) e_z
e_ph = -sin(ph) e_x + cos(ph) e_y
```

The reverse is:

```
sage: for vec in cartesian_frame:
....: vec.display(spherical_frame, spherical)
e_x = cos(ph)*sin(th) e_r + cos(ph)*cos(th) e_th - sin(ph) e_ph
e_y = sin(ph)*sin(th) e_r + cos(th)*sin(ph) e_th + cos(ph) e_ph
e_z = cos(th) e_r - sin(th) e_th
```

We may also express the orthonormal frame \((e_r,e_\theta,e_\phi)\) in terms on
the coordinate frame \(\left(\frac{\partial}{\partial r},
\frac{\partial}{\partial\theta}, \frac{\partial}{\partial \phi}\right)\) (the
latter being returned by the method
`frame()`

acting on the
chart `spherical`

):

```
sage: for vec in spherical_frame:
....: vec.display(spherical.frame(), spherical)
e_r = ∂/∂r
e_th = 1/r ∂/∂th
e_ph = 1/(r*sin(th)) ∂/∂ph
```

## Introducing cylindrical coordinates#

Cylindrical coordinates are introduced in a way similar to spherical coordinates:

```
sage: cylindrical.<rh,ph,z> = E.cylindrical_coordinates()
sage: cylindrical
Chart (E^3, (rh, ph, z))
```

We have:

```
sage: cylindrical[:]
(rh, ph, z)
sage: rh is cylindrical[1]
True
sage: cylindrical.coord_range()
rh: (0, +oo); ph: [0, 2*pi] (periodic); z: (-oo, +oo)
```

\(\mathbb{E}^3\) is now endowed with three coordinate charts:

```
sage: E.atlas()
[Chart (E^3, (x, y, z)), Chart (E^3, (r, th, ph)), Chart (E^3, (rh, ph, z))]
```

The transformations linking the cylindrical coordinates to the Cartesian ones are:

```
sage: E.coord_change(cylindrical, cartesian).display()
x = rh*cos(ph)
y = rh*sin(ph)
z = z
sage: E.coord_change(cartesian, cylindrical).display()
rh = sqrt(x^2 + y^2)
ph = arctan2(y, x)
z = z
```

There are now five vector frames defined on \(\mathbb{E}^3\):

```
sage: E.frames()
[Coordinate frame (E^3, (e_x,e_y,e_z)),
Coordinate frame (E^3, (∂/∂r,∂/∂th,∂/∂ph)),
Vector frame (E^3, (e_r,e_th,e_ph)),
Coordinate frame (E^3, (∂/∂rh,∂/∂ph,∂/∂z)),
Vector frame (E^3, (e_rh,e_ph,e_z))]
```

The orthonormal frame associated with cylindrical coordinates is \((e_\rho, e_\phi, e_z)\):

```
sage: cylindrical_frame = E.cylindrical_frame()
sage: cylindrical_frame
Vector frame (E^3, (e_rh,e_ph,e_z))
```

We may check that it is an orthonormal frame:

```
sage: ec = cylindrical_frame
sage: [[ec[i].dot(ec[j]).expr() for j in E.irange()] for i in E.irange()]
[[1, 0, 0], [0, 1, 0], [0, 0, 1]]
```

and express it in terms of the Cartesian frame:

```
sage: for vec in cylindrical_frame:
....: vec.display(cartesian_frame, cylindrical)
e_rh = cos(ph) e_x + sin(ph) e_y
e_ph = -sin(ph) e_x + cos(ph) e_y
e_z = e_z
```

The reverse is:

```
sage: for vec in cartesian_frame:
....: vec.display(cylindrical_frame, cylindrical)
e_x = cos(ph) e_rh - sin(ph) e_ph
e_y = sin(ph) e_rh + cos(ph) e_ph
e_z = e_z
```

Of course, we may express the orthonormal cylindrical frame in terms of the spherical one:

```
sage: for vec in cylindrical_frame:
....: vec.display(spherical_frame, spherical)
e_rh = sin(th) e_r + cos(th) e_th
e_ph = e_ph
e_z = cos(th) e_r - sin(th) e_th
```

along with the reverse transformation:

```
sage: for vec in spherical_frame:
....: vec.display(cylindrical_frame, spherical)
e_r = sin(th) e_rh + cos(th) e_z
e_th = cos(th) e_rh - sin(th) e_z
e_ph = e_ph
```

The orthonormal frame \((e_\rho,e_\phi,e_z)\) can be expressed in terms on the
coordinate frame \(\left(\frac{\partial}{\partial\rho},
\frac{\partial}{\partial\phi}, \frac{\partial}{\partial z}\right)\) (the latter
being returned by the method
`frame()`

acting on the
chart `cylindrical`

):

```
sage: for vec in cylindrical_frame:
....: vec.display(cylindrical.frame(), cylindrical)
e_rh = ∂/∂rh
e_ph = 1/rh ∂/∂ph
e_z = ∂/∂z
```

## How to evaluate the coordinates of a point in various systems#

Let us introduce a point \(p\in \mathbb{E}^3\) via the generic SageMath
syntax for creating an element from its parent (here
\(\mathbb{E}^3\)), i.e. the call operator `()`

, with the
coordinates of the point as the first argument:

```
sage: p = E((-1, 1,0), chart=cartesian, name='p')
sage: p
Point p on the Euclidean space E^3
```

Actually, since the Cartesian coordinates are the default ones, the argument
`chart=cartesian`

can be omitted:

```
sage: p = E((-1, 1,0), name='p')
sage: p
Point p on the Euclidean space E^3
```

The coordinates of \(p\) in a given coordinate chart are obtained by letting the corresponding chart act on \(p\):

```
sage: cartesian(p)
(-1, 1, 0)
sage: spherical(p)
(sqrt(2), 1/2*pi, 3/4*pi)
sage: cylindrical(p)
(sqrt(2), 3/4*pi, 0)
```

Here some example of a point defined from its spherical coordinates:

```
sage: q = E((4,pi/3,pi), chart=spherical, name='q')
sage: q
Point q on the Euclidean space E^3
```

We have then:

```
sage: spherical(q)
(4, 1/3*pi, pi)
sage: cartesian(q)
(-2*sqrt(3), 0, 2)
sage: cylindrical(q)
(2*sqrt(3), pi, 2)
```

## How to express a scalar field in various coordinate systems#

Let us define a scalar field on \(\mathbb{E}^3\) from its expression in Cartesian coordinates:

```
sage: f = E.scalar_field(x^2+y^2 - z^2, name='f')
```

Note that since the Cartesian coordinates are the default ones, we have not specified them in the above definition. Thanks to the known coordinate transformations, the expression of \(f\) in terms of other coordinates is automatically computed:

```
sage: f.display()
f: E^3 → ℝ
(x, y, z) ↦ x^2 + y^2 - z^2
(r, th, ph) ↦ -2*r^2*cos(th)^2 + r^2
(rh, ph, z) ↦ rh^2 - z^2
```

We can limit the output to a single coordinate system:

```
sage: f.display(cartesian)
f: E^3 → ℝ
(x, y, z) ↦ x^2 + y^2 - z^2
sage: f.display(cylindrical)
f: E^3 → ℝ
(rh, ph, z) ↦ rh^2 - z^2
```

The coordinate expression in a given coordinate system is obtained via the
method `expr()`

:

```
sage: f.expr() # expression in the default chart (Cartesian coordinates)
x^2 + y^2 - z^2
sage: f.expr(spherical)
-2*r^2*cos(th)^2 + r^2
sage: f.expr(cylindrical)
rh^2 - z^2
```

The values of \(f\) at points \(p\) and \(q\) are:

```
sage: f(p)
2
sage: f(q)
8
```

Of course, we may define a scalar field from its coordinate expression in a chart that is not the default one:

```
sage: g = E.scalar_field(r^2, chart=spherical, name='g')
```

Instead of using the keyword argument `chart`

, one can pass a dictionary as
the first argument, with the chart as key:

```
sage: g = E.scalar_field({spherical: r^2}, name='g')
```

The computation of the expressions of \(g\) in the other coordinate systems is
triggered by the method `display()`

:

```
sage: g.display()
g: E^3 → ℝ
(x, y, z) ↦ x^2 + y^2 + z^2
(r, th, ph) ↦ r^2
(rh, ph, z) ↦ rh^2 + z^2
```

## How to express a vector field in various frames#

Let us introduce a vector field on \(\mathbb{E}^3\) by its components in the Cartesian frame. Since the latter is the default vector frame on \(\mathbb{E}^3\), it suffices to write:

```
sage: v = E.vector_field(-y, x, z^2, name='v')
sage: v.display()
v = -y e_x + x e_y + z^2 e_z
```

Equivalently, a vector field can be defined directly from its expansion on the Cartesian frame:

```
sage: ex, ey, ez = cartesian_frame[:]
sage: v = -y*ex + x*ey + z^2*ez
sage: v.display()
-y e_x + x e_y + z^2 e_z
```

Let us provide `v`

with some name, as above:

```
sage: v.set_name('v')
sage: v.display()
v = -y e_x + x e_y + z^2 e_z
```

The components of \(v\) are returned by the square bracket operator:

```
sage: v[1]
-y
sage: v[:]
[-y, x, z^2]
```

The computation of the expression of \(v\) in terms of the orthonormal
spherical frame is triggered by the method `display()`

:

```
sage: v.display(spherical_frame)
v = z^3/sqrt(x^2 + y^2 + z^2) e_r
- sqrt(x^2 + y^2)*z^2/sqrt(x^2 + y^2 + z^2) e_th + sqrt(x^2 + y^2) e_ph
```

We note that the components are still expressed in the default chart
(Cartesian coordinates). To have them expressed in the spherical chart, it
suffices to pass the latter as a second argument to `display()`

:

```
sage: v.display(spherical_frame, spherical)
v = r^2*cos(th)^3 e_r - r^2*cos(th)^2*sin(th) e_th + r*sin(th) e_ph
```

Again, the components of \(v\) are obtained by means of the square bracket operator, by specifying the vector frame as first argument and the coordinate chart as the last one:

```
sage: v[spherical_frame, 1]
z^3/sqrt(x^2 + y^2 + z^2)
sage: v[spherical_frame, 1, spherical]
r^2*cos(th)^3
sage: v[spherical_frame, :, spherical]
[r^2*cos(th)^3, -r^2*cos(th)^2*sin(th), r*sin(th)]
```

Similarly, the expression of \(v\) in terms of the cylindrical frame is:

```
sage: v.display(cylindrical_frame, cylindrical)
v = rh e_ph + z^2 e_z
sage: v[cylindrical_frame, :, cylindrical]
[0, rh, z^2]
```

The value of the vector field \(v\) at point \(p\) is:

```
sage: vp = v.at(p)
sage: vp
Vector v at Point p on the Euclidean space E^3
sage: vp.display()
v = -e_x - e_y
sage: vp.display(spherical_frame.at(p))
v = sqrt(2) e_ph
sage: vp.display(cylindrical_frame.at(p))
v = sqrt(2) e_ph
```

The value of the vector field \(v\) at point \(q\) is:

```
sage: vq = v.at(q)
sage: vq
Vector v at Point q on the Euclidean space E^3
sage: vq.display()
v = -2*sqrt(3) e_y + 4 e_z
sage: vq.display(spherical_frame.at(q))
v = 2 e_r - 2*sqrt(3) e_th + 2*sqrt(3) e_ph
sage: vq.display(cylindrical_frame.at(q))
v = 2*sqrt(3) e_ph + 4 e_z
```

## How to change the default coordinates and default vector frame#

At any time, one may change the default coordinates by the method
`set_default_chart()`

:

```
sage: E.set_default_chart(spherical)
```

Then:

```
sage: f.expr()
-2*r^2*cos(th)^2 + r^2
sage: v.display()
v = -r*sin(ph)*sin(th) e_x + r*cos(ph)*sin(th) e_y + r^2*cos(th)^2 e_z
```

Note that the default vector frame is still the Cartesian one; to change to
the orthonormal spherical frame, use
`set_default_frame()`

:

```
sage: E.set_default_frame(spherical_frame)
```

Then:

```
sage: v.display()
v = r^2*cos(th)^3 e_r - r^2*cos(th)^2*sin(th) e_th + r*sin(th) e_ph
sage: v.display(cartesian_frame, cartesian)
v = -y e_x + x e_y + z^2 e_z
```