# Introduction¶

## What is h2tools?¶

It is an open-source software, designed to work with $$\mathcal{H}^2$$ -matrices. Matrices, close to $$\mathcal{H}^2$$-matrices, often appear in different physical problems (i.e. described as integral equations or particle-to-particle interactions). Special structure of such matrices enables representation with relatively small number of parameters and sparse-like arithmetics. You can get more information on it in the book [H2matrix-book].

h2tools is distributed as a Python module, which works with both major revisions of Python (i.e. 2.7.10 and 3.5 versions are succesfully tested for compatibility). It has h2tools.collections submodule with predefined classes for different types of problems.

h2tools supports vector/matrix/tensor kernels and MPI parallel factorization and matrix-vector operations.

 [H2matrix-book] Hackbusch W., Khoromskij B., Sauter S.A. On $$\mathcal{H}^2$$-matrices.—Springer, 2000.

## Why h2tools and not any other library?¶

Main feature of h2tools is an algebraic method of constructing $$\mathcal{H}^2$$-approximation using only matrix entries [MCBH]. This method does not require preliminary approximation by $$\mathcal{H}$$-matrix, is linear in means of problem size and has lower complexity in terms of memory and operations, than any method of $$\mathcal{H}$$-approximation. It also uses less matrix entries, than any $$\mathcal{H}$$-approximation method.

 [MCBH] Mikhalev A.Yu., Oseledets I.V. Iterative representing set selection for nested cross approximation // Numer. Linear Algebra Appl. (available online here)

## Requirements¶

h2tools extensively uses following Python modules:

• numpy for matrix operations,
• maxvolpy for selection of representing sets,
• numba to run examples (which are optimized with help of numba),
• cython, required by maxvolpy.

If there is no predefined class in h2tools.collections for your problem, consider using numba and/or cython to accelerate Python code. In example, such an optimization reduces time of matrix entry computation by a factor of about 100, which is usually a bottleneck of approximation procedure.

## Installation¶

Easiest way to install h2tools is to use pip command:

pip install h2tools


To enable approximation error measurement, install pypropack. If you want to run examples or use h2tools.collections submodule, additionally install numba (i.e. via pip install numba).

If you want to use latest version of h2tools, you can install it from git:

git clone https://bitbucket.org/muxas/h2tools
cd h2tools
python setup.py install


## Examples¶

Examples in ipython notebook format can be found in examples folder of git repository.

## Documentation¶

Latest documentation is available at http://pythonhosted.org/h2tools