function [a] = autocorel_part(X)
  rho = autocorel(X);
  T = length(X);
  r=zeros(T,T);
  for i=1:T/4-1
    r = r+diag(rho(i)*ones(1,T-i),i);
  end
  
  r = r+r'+eye(T,T);

  for i=1:T/4
  x = r(1:i,1:i)\rho(1:i);
  a(i)=x(i);
  end
endfunction