This guide can be used as a quick start into `xerus`. It will introduce some basic functionality of the library,
demonstrate the general layout and is enough for very basic applications. It is recommended to also have a look
at the more detailed guides for all classes one wishes to use though - or even have a look at the doxygen class documentation for details on all functions.
It is assumed that you have already obtained and compiled the library itself as well as know how to link against it.
If this is not the case, please refer to the [building xerus](building_xerus) page.
In the following we will solve a FEM equation arising from the heat equation using a QTT decomposition and the ALS algorithm.
To start we create the stiffness matrix as a dense (ie. not sparse or decomposed) tensor.
To define the entries we pass a function to the constructor of the `Tensor` object that will be
called for every entry with a vector of size_t integers defining the indices of the current entry.
As it is simpler to think of the stiffness matrix in its original form rather than the QTT form we will