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Merge branch 'transformation-doc' of github.com:Maison-de-la-Simulati…
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blegouix committed Jul 22, 2024
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Expand Up @@ -25,13 +25,13 @@ There are many use-cases of transformations for simulation or signal processing
finite element methods, filtering (compression), signal analysis (post-process)...

Every basis has its own specificities (ie. orthogonality, n-derivability, etc...) but as they all are
function space basis, they have in common a formalism and a terminology. However, this is currently
function space bases, they have in common a formalism and a terminology. However, this is currently
not very manifest when looking at the API in the DDC implementations (DFT and Splines API are very different).
It must be explained:

- Fourier requires the periodicity of the represented function, thus boundary conditions does not need to be provided.
- Fourier basis functions are indexed with the wave vector \f$ \vec{k} \f$, whose possible values form a set of coordinates which generates
the Fourier space. Mesh of "real" space and mesh of Fourier space are in bijection one-to-the-other.
- Fourier requires the periodicity of the represented function, thus boundary conditions do not need to be provided.
- Fourier basis functions are indexed with the wave vector \f$ \vec{k} \f$, whose possible values form a set of coordinates which generate
the Fourier space. There is a bijection between the mesh of the "real" space and the mesh of the Fourier space.
The situation is more complicated for Splines.
- Fourier and Splines basis use-cases are quite different. Thus - as shown below - the so-called "evaluator" is not
implemented for Fourier (while inverse DFT is) and inverse Spline transform is not implemented (while Spline evaluator is).
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