Upon discretization through finite elements, a linear elliptic PDE becomes a discrete linear system 
. The role of Sundance is to assemble the matrix 
and the vector 
. In most Sundance applications, matrices and vectors will never appear at the user level: they are hidden within the solver and their methods are called within the guts of Sundance. However, researchers in numerical algorithms will often want access to matrices and vectors; they are available through the getOperator() and getRHS() method of StaticLinearProblem.
In the course of discretizing a StaticLinearProblem (see Static linear problems) the Sundance engine will configure and fill the matrix through a small set of callback functions in the matrix interface. Sundance can work with any matrix type that implements those callback functions, which means it is possible to add your own matrix type to Sundance (see ExtendingSolvers).
later...
