function f = jagged(x,n,a,s)
%JAGGED	Computes an approximation of a continuous nowhere differentiable
%	function.
%
% jagged(x,n,a,s) is given by
%                         __
%                        \  | n     (s-2)m      m
%                         >        a       sin(a  x)
%                        /__| m=0
%
% In the limit as n->infinity this gives a continuous nowhere differentiable
% function if a>1 and 1<s<2.  The case a=4, s=3/2 is (I think) the
% classical example of Weierstrass.  n=10 is a reasonable value for graphical
% purposes.
%
% A nice example for classroom purposes is
%
%  x = 0:2.e-5:1 ; y = jagged(x,10,4,1.2) ; plot(x,y)

f = zeros(size(x));
% loop backward to minimize round-off effects
for m = n:-1:0
  k = a^(n-m);
  f = f + a^((s-2)*(n-m)) * sin(k*x);
end