// retard amximum majore par T/4
function [rho] = autocorel(X)
  X_bar = mean(X);
  T = length(X);
  fact = sum((X-X_bar).^2)
  
  for i=1:T/4
    rho(i) = sum( (X(i+1:T) - X_bar).* (X(1:T-i) - X_bar) ) /  fact;
  end
endfunction