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Fast Chebyshev differentiation

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Fast computation of the first derivative of data located along Chebyshev points

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fchd(V) computes the first derivative of the data in V located along the N+1 Chebyshev–Gauss–Lobatto points cos(pi*(0:N)/N).
  
  
  Example 1:
  Use FCHT to differentiate the function f(x) = tan(x) over [-1,1], and
  compare with the exact derivate f'(x) = sec(x)^2.
  
  x = cos(pi*(0:10)/10); % create sparse Chebyshev-spaced grid of 11 points
  xx = linspace(-1,1); % create dense, linearly spaced grid
  plot(xx,sec(xx).^2,x,fchd(tan(x))); % compare Chebyshev derivative to exact
 
  
  Example 2:
  To show the spectral convergence property of the Chebyshev derivative,
  compute the error between the Chebyshev derivative and the exact
  derivative of f(x) = tan(x) for several N.
  
  N = 1:30;
  err = zeros(1,length(N));
  
  for n = N
      x = cos(pi*(0:n)/n)'; % establish grid
      err(n) = max(sec(x).^2 - fchd(tan(x))); % compute error
  end
  
  loglog(N,err); %display

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1.2

made title match usual naming convention

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