function [phi] = sparse_fit(X)
  T = length(X);
  X_bar = mean(X);
  fact = sum((X-X_bar).^2);
  
  for i=[1,2,10,11]
    rho(i) = sum( (X(i+1:T) - X_bar).* (X(1:T-i) - X_bar) ) /  fact;
  end
  phi = [1 , rho(11); rho(1), rho(10)]\ [rho(1);rho(2)];
endfunction