function r=ator(a,b)
%ATOR	Inverse Levinson-Durbin recursion.
%----
%USAGE: r=ator(a,b)
%
%	Finds the autocorrelation sequence r(k) of an
%       autoregressive process that is generated by 
%	filtering unit variance white noise with the filter
%		H(z)=b(0)/A(z)
%       With r = ator(a) the autocorrelation sequence is 
%	normalized so that r(0)=1.
%
%  see also ATOG, GTOA, GTOR, RTOA, RTOG
%
%---------------------------------------------------------------
% copyright 1996, by M.H. Hayes.  For use with the book 
% "Statistical Digital Signal Processing and Modeling"
% (John Wiley & Sons, 1996).
%---------------------------------------------------------------

   p=length(a)-1;
   gamma=atog(a);
   r=gtor(gamma);
   if nargin == 2,
   r = r*b^2/prod(1-abs(gamma).^2);
   end;