The idea of variables of type S2Fun is to calculate with spherical functions similarly as Matlab does with vectors and matrices. In order to illustrate this we consider the following two spherical functions

## Basic arithmetic operations

Now the sum of these two spherical functions is again a spherical function

Accordingly, one can use all basic operations like `-`

, `*`

, `^`

, `/`

, `min`

, `max`

, `abs`

, `sqrt`

to calculate with variables of type S2Fun.

## Local Extrema

The obove mentioned functions `min`

and `max`

have very different use cases

- if two spherical functions are passed as arguments a spherical functions defined as the pointwise min/max between these two functions is computed
- if a spherical function and single number are passed as arguments a spherical functions defined as the pointwise min/max between these the function and the value is computed
- if only a single spherical function is provided the global maximum / minimum of the function is returned
- if additionally the option 'numLocal' is provided the certain number of local minima / maxima is computed

## Integration

The surface integral of a spherical function can be computed by either `mean`

or `sum`

. The difference between both commands is that `sum`

normalizes the integral of the identical function on the sphere to \(4 \pi\) the command `mean`

normalizes it to one. Compare

A practical application of integration is the computation of the \(L^2\)-norm which is defined for a spherical function \(f\) as

\[ \lVert f \rVert_2 = \left(\int_{\mathrm{sphere}} f(x)^2 dx\right)^{1/2} \]

accordingly we can compute it by

or more efficiently by the command `norm`

## Differentiation

The differential of a spherical function in a specific point is a gradient, i.e., a three-dimensional vector which can be computed by the command `grad`

The gradients of a spherical function in all points form a spherical vector field and are returned by the function `grad`

as a variable of type S2VectorFieldHarmonic.

We observe long arrows at the positions of big changes in intensity and almost invisible arrows in regions of constant intensity.

## Rotating spherical functions

Rotating a spherical function works with the command `rotate`

A special case of rotation is symmetrysing it with respect to some symmetry. The following example symmetrises our smiley with respect to a two fold axis in \(z\)-direction

The resulting function is of type S2FunHarmonicSym and knows about its symmetry.