% -*- texinfo -*-
% @deftypefn {Function File} {@var{d} =} del2 (@var{m})
% @deftypefnx {Function File} {@var{d} =} del2 (@var{m}, @var{h})
% @deftypefnx {Function File} {@var{d} =} del2 (@var{m}, @var{dx}, @var{dy}, @dots{})
%
% Calculate the discrete Laplace
% @tex
% operator $( \nabla^2 )$.
% @end tex
% @ifnottex
% operator.
% @end ifnottex
% For a 2-dimensional matrix @var{m} this is defined as
%
% @tex
% $$d = {1 \over 4} \left( {d^2 \over dx^2} M(x,y) + {d^2 \over dy^2} M(x,y) \right)$$
% @end tex
% @ifnottex
% @example
% @group
% 1 / d^2 d^2 ...
% D = --- * | --- M(x,y) + --- M(x,y) |
% 4 \ dx^2 dy^2 /
% @end group
% @end example
% @end ifnottex
%
% For N-dimensional arrays the sum in parentheses is expanded to include second derivatives
% over the additional higher dimensions.
%
% The spacing between evaluation points may be defined by @var{h}, which is a
% scalar defining the equidistant spacing in all dimensions. Alternatively,
% the spacing in each dimension may be defined separately by @var{dx}, @var{dy},
% etc. A scalar spacing argument defines equidistant spacing, whereas a vector
% argument can be used to specify variable spacing. The length of the spacing vectors
% must match the respective dimension of @var{m}. The default spacing value
% is 1.
%
% At least 3 data points are needed for each dimension. Boundary points are
% calculated from the linear extrapolation of interior points.
%
% @seealso{gradient, diff}
% @end deftypefn