# Concepts

A Green function is simply a function, which has:

• a domain for its variable(s) (e.g. Matsubara/real time/frequencies, Legendre coefficients).

• a target space, i.e. the value of the Green function which can be:

• a scalar (double, complex)
• a matrix,
• another Green function (See below, currying Green functions … REF … ).

In this section, we define the general concepts for these objects.

First, we need to distinguish the domain on which the function is defined from its representation in a computer, which we call a mesh.

Note

“mesh” should be understood here in a general and abstract way, as the representation of the domain in the computer. In most cases, it is indeed a real mesh on a domain (e.g. a Brillouin zone), but the set of Legendre coefficients is also a mesh in our sense.

We will therefore now formally define the concept for domain, for mesh, the notion of pure function on a domain (i.e. a mathematical Green function) and the notion of function on a grid.

## Domain

• Purpose : The domain of definition of a function. It is a mathematical definition of the domain, and does not contain any mesh, or details on its representation in a computer.
• Refines: RegularType.
• Definition:
Elements Comment
point_t Type of element in the domain (int, int, double, k_vector, …) as in the call of a function over this domain.
• Examples :
• Matsubara time
• Matsubara frequencies (boson/fermion): in this case, point_t is matsubara_freq, a simple type containing (n, beta, statistics).
• Real frequencies
• Real time
• Brillouin zone
• Cartesian product of previous domains to build multi-variable functions.

## PureFunctionOnDomain

• Purpose :
A mathematical (pure) function from a domain to a target space.
• it has a domain of definition
• it can be called on any point of the domain, as a pure function, i.e. without any side effect.
• Refines :
• Definition:
Elements Comment
domain_t const & domain() const Returns the domain (deduced as domain_t)
operator (domain_t::point_t) const Calling for all elements of the Domain (including infty if it is in the domain…
• NB: Note that the return type of the function is NOT part of the concept, it has to be deduced by the compiler (using C++11 decltype, std::result_of, eg..).

## Mesh

• Purpose : A mesh over a domain, and more generally the practical representation of the domain in a computer. It does not really need to be a mesh: e.g. if the function is represented on a polynomial basis, it is the parameters of this representation (max number of coordinates, e.g.)
• Refines: RegularType, H5-serializable
• Definition:
Elements Comment
domain_t Type of the Domain represented, modeling the Domain concept
index_t Type of indices of a point on the grid. Typically a tuple of long or a long
long size() const The number of points in the mesh.
domain_t::point_t index_to_point(index_t) const From the index of a mesh point, compute the corresponding point in the domain
long index_to_linear(index_t const &) const Flattening the index of the mesh into a contiguous linear index
mesh_point_t A type modeling MeshPoint concept (see below).
mesh_point_t operator[](index_t const & index ) const From an index, return a mesh_point_t containing this a ref to this mesh and the index.
mesh_pt_generator<mesh_t> const_iterator A generator of all the mesh points.
Free functions Comment
void foreach (mesh_t, F) If F is a function of synopsis auto F( mesh_t::mesh_point_t) it calls F for each point on the mesh, in arbitrary order

## MeshPoint

• Purpose : Abstraction of a point on a mesh. A little more than a ref to the mesh and a index.
• Refines: CopyConstructible.
• Definition:
Elements Comment
mesh_t Type of the mesh
mesh_t const * m A pointer to the mesh to which the point belongs.
mesh_t::index_t index The index of the point
mesh_point_t( mesh_t const &, index_t const &) Constructor: a mesh point at the given index
mesh_point_t( mesh_t const &) Constructor: the first mesh point
mesh_t::index_t [const &,] index() const The index corresponding to the point
size_t linear_index() const The linear index of the point (same as m->index_to_linear(index())
void advance() Advance to the next point on the mesh (used by iterators).
void at_end() Is the point at the end of the grid
void reset() Reset the mesh point to the first point
cast_t operator cast_t() const == mesh_t::domain_t::point_t implicit cast to the corresponding domain point

For one dimensional mesh, we also require that the MeshPoint implement the basic arithmetic operations using the cast.

• Discussion:

A MeshPoint is just an index of a point on the mesh, and containers like gf can easily be overloaded for this type to have a direct access to the grid (Cf [] operator of gf).

However, since the MeshPoint can be implicitely casted into the domain point, simple expression like

g[p] = 1/ (p +2)


make sense and fill the corresponding point wiht the evaluation of 1/ (p+2) in the domain.

As a result, because iterating on a mesh result in a series of object modelling MeshPoint, one can write naturally

// example of g, a Green function in Matsubara frequencies w
for (auto w: g.mesh())
g[w] = 1/(w + 2)
// This runs overs the mesh, and fills the function with 1/(w+2)
// In this expression, w is casted to the domain_t::point_t, here a complex<double>
// which allows to evaluate the function