# MWFunctions¶

Everything that is discussed in the following chapter is available to the application program by including:

#include "MRCPP/MWFunctions"


Multiwavelet (MW) representations of real-valued scalar functions are in MRCPP called FunctionTrees. These are in principle available in any dimension using the template parameter D (in practice D=1,2,3). There are several different ways of constructing MW functions (computing the expansion coefficients in the MW basis):

• Projection of analytic function
• Arithmetic operations
• Mapping of function values
• Application of MW operator

The first three will be discribed in this chapter, while the last one regarding operators will be the topic of the next chapter.

The interface for constructing MW representations in MRCPP has a dual focus: on the one hand we want a simple, intuitive way of producing adaptive numerical approximations with guaranteed precision that does not require detailed knowledge of the internals of the MW code and with a minimal number of parameters that have to be set. On the other hand we want the possibility for more detailed control of the construction and refinement of the numerical grid where such control is possible and even necessary. In the latter case it is important to be able to reuse the existing grids in e.g. iterative algorithms without excessive allocation/deallocation of memory.

## MultiResolution Analysis (MRA)¶

In order to combine different functions and operators in mathematical operations, they need to be compatible. That is, they must be defined on the same computational domain and constructed using the same polynomial basis (order and type). This information constitutes an MRA, which needs to be defined and passed as argument to all function and operator constructors, and only functions and operators with compatible MRAs can be combined in subsequent calculations. An MRA is defined in two steps, first the computational domain is given by a BoundingBox (D is the dimension)

int n;                                          // Root scale defines box size 2^{-n}
std::array<int, D> l;                           // Translation of first box
std::array<int, D> nb;                          // Number of boxes
BoundingBox<D> world(n, l, nb);


which is combined with a ScalingBasis to give an MRA

int N;                                          // Maximum refinement scale 2^{-N}
int k;                                          // Polynomial order
ScalingBasis basis(k);                          // Legendre or Interpolating basis
MultiResolutionAnalysis<D> MRA(world, basis, N);


Two types of ScalingBasis are supported (LegendreBasis and InterpolatingBasis), and they are both available at orders $$k=1,2,\dots,40$$ (note that some operators are constructed using intermediates of order $$2k$$, so in that case the maximum order available is $$k=20$$).

## FunctionTree¶

Constructing a full grown FunctionTree involves a number of steps, including setting up a memory allocator, constructing root nodes according to the given MRA, building an adaptive tree structure and computing MW coefficients. The FunctionTree constructor does only half of these steps:

FunctionTree<D> tree(MRA);


It takes an MRA argument, which defines the computational domain and scaling basis (these are fixed parameters that cannot be changed after construction). The tree is initialized with a memory allocator and a set of root nodes, but it does not compute any coefficients and the function is initially undefined. An undefined FunctionTree will have a well defined tree structure (at the very least the root nodes of the given MRA, but possibly with additional refinement, as discussed below) and its MW coefficient will be allocated but uninitialized, and its square norm will be negative (minus one).

### Creating defined FunctionTrees¶

The following functions will define MW coefficients where there are none, and thus require that the output FunctionTree is in an undefined state. All functions marked with ‘adaptive grid’ will use the same building algorithm:

2. Compute the MW coefficients for the output function on the current grid
3. Refine the grid where necessary based on the local wavelet norm
4. Iterate points 2 and 3 until the grid is converged

With a negative precision argument, the grid will be fixed, e.i. it will not be refined beyond the initial grid. There is also an argument to limit the number of extra refinement levels beyond the initial grid, in which the adaptive refinement will stop, even if the local precision requirement is not met.

setZero
Set the MW coefficients to zero, fixed grid.
copy_func
Copy existing function into a new tree, fixed grid.
project
Project an analytic function onto the MW basis, adaptively grid.
multiply
square
Multiply an existing function with itself, adaptive grid.
power
Raise an existing function to a given power, adaptive grid.

### Creating undefined FunctionTrees¶

The grid of a FunctionTree can also be constructed without computing any MW coefficients:

build_grid
Build an empty grid based on information from an analytic function, e.g. position and exponent of Gaussian, or based on the structure of another grid.
copy_grid
Build an empty grid that is identical to that of an existing function.
clear_grid
Clear MW coefficients of an existing function. Keeps grid refinement.
clear
Clear MW coefficients and remove all grid refinement.

### Changing FunctionTrees¶

There are also a number of in-place operations that change the MW coefficients of a given defined FunctionTree:

rescale
Multiply the function with a scalar, fixed grid.
normalize
Rescale the function by its norm, fixed grid.
Add an existing function, fixed grid.
multiply
Multiply an existing function, fixed grid.
square
Multiply an existing function with itself, fixed grid.
power
Raise an existing function to a given power, fixed grid.
crop
Truncate the wavelet expansion accoring to a new precision threshold.
refine_grid
Refine grid and interpolate the existing function to the new (larger) grid. Three versions: (1) refine globally a given number of levels, (2) refine locally based on precision and wavelet norm, (3) refine locally based on the structure of another grid.

All changing operations require that the FunctionTree is in a defined state.

### File I/O¶

saveTree
Write function to file.
Read function from file. Requires the MRA of the target tree is identical to the MRA of the saved tree.

### Extracting data¶

Given a FunctionTree that is a well defined function representation, the following data can be extracted:

getSquareNorm
Returns the squared L2 norm of the function.
integrate
Returns the integral of the function over the entire computational domain.
evalf
Returns the function value in a given point. Possibly inaccurate, see below.
dot
Returns the dot product of two functions over the entire computational domain.

Note

When evaluating FunctionTrees, only the scaling part of the leaf nodes will be evaluated, which means that the function values will not be fully accurate. This is done to allow a fast and const function evaluation that can be done in OMP parallel. If you want to include also the final wavelet corrections to your function values, you’ll have to manually extend the MW grid by one level before evaluating using mrcpp::refine_grid(tree, 1).

## FunctionTreeVector¶

The FunctionTreeVector is simply an alias for a std::vector of tuples containing a numerical coefficient and a FunctionTree pointer. Elements can be appended to the vector using the std::make_tuple, elements are obtained with the get_func and get_coef functions:

FunctionTreeVector<D> tree_vec;
tree_vec.push_back(std::make_tuple(2.0, &tree_a)); // Push back pointer to FunctionTree
double coef = get_coef(tree_vec, 0);               // Get coefficient of first entry
FunctionTree<3> &tree = get_func(tree_vec, 0);     // Get function of first entry
clear(tree_vec, false);                            // Bool argument for tree destruction


Clearing the vector means removing all its elements, and the bool argument tells if the elements should be properly deallocated (default false).

## Examples¶

### Building empty grids¶

Sometimes it is useful to construct an empty grid based on some available information of the function that is about to be represented. This can be e.g. that you want to copy the grid of an existing FunctionTree or that an analytic function has more or less known grid requirements (like Gaussians). Sometimes it is even necessary to force the grid refinement beyond the coarsest scales in order for the adaptive refining algorithm to detect a wavelet “signal” that allows it to do its job properly (this happens for narrow Gaussians where none of the initial quadrature points hits a function value significantly different from zero).

The simplest way to build an empty grid is to copy the grid from an existing tree (assume that f_tree has been properly built so that it contains more than just root nodes)

mrcpp::FunctionTree<D> f_tree(MRA);                     // Input tree
mrcpp::FunctionTree<D> g_tree(MRA);                     // Output tree

mrcpp::project(prec, f_tree, f_func);                   // Build adaptive grid for f_tree
mrcpp::copy_grid(g_tree, f_tree);                       // Copy grid from f_tree to g_tree


Passing an analytic function as argument to the generator will build a grid based on some predefined information of the function (if there is any, otherwise it will do nothing)

mrcpp::RepresentableFunction<D> func;                   // Analytic function
mrcpp::FunctionTree<D> tree(MRA);                       // Output tree
mrcpp::build_grid(tree, func);                          // Build grid based on f_func


The lambda analytic functions do not provide such information, this must be explicitly implemented as a RepresentableFunction sub-class (see MRCPP programmer’s guide for details).

Actually, the effect of the build_grid is to extend the existing grid with any missing nodes relative to the input. There is also a version of build_grid taking a FunctionTree argument. Its effect is very similar to the copy_grid above, with the only difference that now the output grid is extended with the missing nodes (e.i. the nodes that are already there are not removed first). This means that we can build the union of two grids by successive applications of build_grid

mrcpp::FunctionTree<D> f_tree(MRA);             // Construct empty grid of root nodes
mrcpp::build_grid(f_tree, g_tree);              // Extend f with missing nodes relative to g
mrcpp::build_grid(f_tree, h_tree);              // Extend f with missing nodes relative to h


In contrast, doing the same with copy_grid would clear the f_tree grid in between, and you would only get a (identical) copy of the last h_tree grid, with no memory of the g_tree grid that was once there. One can also make the grids of two functions equal to their union

mrcpp::build_grid(f_tree, g_tree);              // Extend f with missing nodes relative to g
mrcpp::build_grid(g_tree, f_tree);              // Extend g with missing nodes relative to f


The union grid of several trees can be constructed in one go using a FunctionTreeVector

mrcpp::FunctionTreeVector<D> inp_vec;
inp_vec.push_back(std::make_tuple(1.0, tree_1));
inp_vec.push_back(std::make_tuple(1.0, tree_2));
inp_vec.push_back(std::make_tuple(1.0, tree_3));

mrcpp::FunctionTree<D> f_tree(MRA);
mrcpp::build_grid(f_tree, inp_vec);


### Projection¶

The project function takes an analytic D-dimensional scalar function (which can be defined as a lambda function or one of the explicitly implemented sub-classes of the RepresentableFunction base class in MRCPP) and projects it with the given precision onto the MRA defined by the FunctionTree. E.g. a unit charge Gaussian is projected in the following way (the MRA must be initialized as above)

// Defining an analytic function
double beta = 10.0;
double alpha = std::pow(beta/pi, 3.0/2.0);
auto func = [alpha, beta] (const mrcpp::Coord<3> &r) -> double {
double R = std::sqrt(r[0]*r[0] + r[1]*r[1] + r[2]*r[2]);
return alpha*std::exp(-beta*R*R);
};

double prec = 1.0e-5;
mrcpp::FunctionTree<3> tree(MRA);
mrcpp::project(prec, tree, func);


This projection will start at the default initial grid (only the root nodes of the given MRA), and adaptively build the full grid. Alternatively, the grid can be estimated a priori if the analytical function has some known features, such as for Gaussians:

double prec;                                            // Precision of the projection
int max_iter;                                           // Maximum levels of refinement

mrcpp::GaussFunc<D> func;                               // Analytic Gaussian function
mrcpp::FunctionTree<D> tree(MRA);                       // Output tree

mrcpp::build_grid(tree, func);                          // Empty grid from analytic function
mrcpp::project(prec, tree, func, max_iter);             // Starts projecting from given grid


This will first produce an empty grid suited for representing the analytic function func (this is meant as a way to make sure that the projection starts on a grid where the function is actually visible, as for very narrow Gaussians, it’s not meant to be a good approximation of the final grid) and then perform the projection on the given numerical grid. With a negative prec (or max_iter = 0) the projection will be performed strictly on the given initial grid, with no further refinements.

Arithmetic operations in the MW representation are performed using the FunctionTreeVector, and the general sum $$f = \sum_i c_i f_i(x)$$ is done in the following way

double a, b, c;                                         // Addition parameters
mrcpp::FunctionTree<D> a_tree(MRA);                     // Input function
mrcpp::FunctionTree<D> b_tree(MRA);                     // Input function
mrcpp::FunctionTree<D> c_tree(MRA);                     // Input function

mrcpp::FunctionTreeVector<D> inp_vec;                   // Vector to hold input functions
inp_vec.push_back(std::make_tuple(a, &a_tree));         // Append to vector
inp_vec.push_back(std::make_tuple(b, &b_tree));         // Append to vector
inp_vec.push_back(std::make_tuple(c, &c_tree));         // Append to vector

mrcpp::FunctionTree<D> f_tree(MRA);                     // Output function


The default initial grid is again only the root nodes, and a positive prec is required to build an adaptive tree structure for the result. The special case of adding two functions can be done directly without initializing a FunctionTreeVector

mrcpp::FunctionTree<D> f_tree(MRA);
mrcpp::add(prec, f_tree, a, a_tree, b, b_tree);


Addition of two functions is usually done on their (fixed) union grid

mrcpp::FunctionTree<D> f_tree(MRA);                     // Construct empty root grid
mrcpp::build_grid(f_tree, a_tree);                      // Copy grid of g
mrcpp::build_grid(f_tree, b_tree);                      // Copy grid of h
mrcpp::add(-1.0, f_tree, a, a_tree, b, b_tree);         // Add functions on fixed grid


Note that in the case of addition there is no extra information to be gained by going beyond the finest refinement levels of the input functions, so the union grid summation is simply the best you can do, and adding a positive prec will not make a difference. There are situations where you want to use a smaller grid, though, e.g. when performing a unitary transformation among a set of FunctionTrees. In this case you usually don’t want to construct all the output functions on the union grid of all the input functions, and this can be done by adding the functions adaptively starting from root nodes.

If you have a summation over several functions but want to perform the addition on the grid given by the first input function, you first copy the wanted grid and then perform the operation on that grid

mrcpp::FunctionTreeVector<D> inp_vec;
inp_vec.push_back(std::make_tuple(a, a_tree));
inp_vec.push_back(std::make_tuple(b, b_tree));
inp_vec.push_back(std::make_tuple(c, c_tree));

mrcpp::FunctionTree<D> f_tree(MRA);                     // Construct empty root grid
mrcpp::copy_grid(f_tree, get_func(inp_vec, 0));         // Copy grid of first input function


Here you can of course also add a positive prec to the addition and the resulting function will be built adaptively starting from the given initial grid.

### Multiplication¶

The multiplication follows the exact same syntax as the addition, where the product $$f = \prod_i c_i f_i(x)$$ is done in the following way

double a, b, c;                                         // Multiplication parameters
mrcpp::FunctionTree<D> a_tree(MRA);                     // Input function
mrcpp::FunctionTree<D> b_tree(MRA);                     // Input function
mrcpp::FunctionTree<D> c_tree(MRA);                     // Input function

mrcpp::FunctionTreeVector<D> inp_vec;                   // Vector to hold input functions
inp_vec.push_back(std::make_tuple(a, &a_tree));         // Append to vector
inp_vec.push_back(std::make_tuple(b, &b_tree));         // Append to vector
inp_vec.push_back(std::make_tuple(c, &c_tree));         // Append to vector

mrcpp::FunctionTree<D> f_tree(MRA);                     // Output function
mrcpp::multipy(prec, f_tree, inp_vec);                  // Adaptive multiplication


In the special case of multiplying two functions the coefficients are collected into one argument

mrcpp::FunctionTree<D> f_tree(MRA);
mrcpp::multiply(prec, f_tree, a*b, a_tree, b_tree);


For multiplications, there might be a loss of accuracy if the product is restricted to the union grid. The reason for this is that the product will contain signals of higher frequency than each of the input functions, which require a higher grid refinement for accurate representation. By specifying a positive prec you will allow the grid to adapt to the higher frequencies, but it is usually a good idea to restrict to one extra refinement level beyond the union grid (by setting max_iter=1) as the grids are not guaranteed to converge for such local operations (like arithmetics, derivatives and function mappings)

mrcpp::FunctionTree<D> f_tree(MRA);                     // Construct empty root grid
mrcpp::build_grid(f_tree, a_tree);                      // Copy grid of a
mrcpp::build_grid(f_tree, b_tree);                      // Copy grid of b
mrcpp::multiply(prec, f_tree, a*b, a_tree, b_tree, 1);  // Allow 1 extra refinement


### Re-using grids¶

Given a FunctionTree that is a valid function representation, we can clear its MW expansion coefficients as well as its grid refinement

mrcpp::FunctionTree<D> tree(MRA);                       // tree is an undefined function
mrcpp::project(prec, tree, f_func);                     // tree represents analytic function f
tree.clear();                                           // tree is an undefined function
mrcpp::project(prec, tree, f_func);                     // tree represents analytic function g


This action will leave the FunctionTree in the same state as after construction (undefined function, only root nodes), and its coefficients can now be re-computed.

In certain situations it might be desireable to separate the actions of computing MW coefficients and refining the grid. For this we can use the refine_grid, which will adaptively refine the grid one level (based on the wavelet norm and the given precision) and project the existing function representation onto the new finer grid

mrcpp::refine_grid(tree, prec);


E.i., this will not change the function that is represented in tree, but it might increase its grid size. The same effect can be made using another FunctionTree argument instead of the precision parameter

mrcpp::refine_grid(tree_out, tree_in);


which will extend the grid of tree_out in the same way as build_grid as shown above, but it will keep the function representation in tree_out.

This functionality can be combined with clear_grid to make a “manual” adaptive building algorithm. One example where this might be useful is in iterative algorithms where you want to fix the grid size for all calculations within one cycle and then relax the grid in the end in preparation for the next iteration. The following is equivalent to the adaptive projection above (refine_grid returns the number of new nodes that were created in the process)

int n_nodes = 1;
while (n_nodes > 0) {
mrcpp::project(-1.0, tree, func);                   // Project f on fixed grid
n_nodes = mrcpp::refine_grid(tree, prec);           // Refine grid based on prec
if (n_nodes > 0) mrcpp::clear_grid(tree);           // Clear grid for next iteration
}