function y = quadratic(x, sigma, type)
%QUADRATIC   Quadratic "robust" function.
%   QUADRATIC(X, SIGMA, TYPE) evaluates the Quadratic "robust" function
%   with sigma SIGMA at point(s) X.  
%   TYPE selects the evaluation type:
%    - 0: function value
%    - 1: first derivative
%    - 2: second derivative
%  
%   This is a private member function of the class 'robust_function'. 
%
%   Author:  Stefan Roth, Department of Computer Science, TU Darmstadt
%   Contact: sroth@cs.tu-darmstadt.de
%   $Date:  $
%   $Revision: $

% Copyright 2004-2007, Brown University, Providence, RI. USA
% Copyright 2007-2010 TU Darmstadt, Darmstadt, Germany.
% 
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  switch (type)
   case 0
    y = x.^2 / sigma^2;
   case 1
    y = 2 * x / sigma^2;
   case 2
    y = repmat(2 / sigma^2, size(x));
  end