Daniel@0: function x = chi2inv(p,v);
Daniel@0: %CHI2INV Inverse of the chi-square cumulative distribution function (cdf).
Daniel@0: %   X = CHI2INV(P,V)  returns the inverse of the chi-square cdf with V  
Daniel@0: %   degrees of freedom at the values in P. The chi-square cdf with V 
Daniel@0: %   degrees of freedom, is the gamma cdf with parameters V/2 and 2.   
Daniel@0: %
Daniel@0: %   The size of X is the common size of P and V. A scalar input
Daniel@0: %   functions as a constant matrix of the same size as the other input.   
Daniel@0: 
Daniel@0: %   References:
Daniel@0: %      [1]  M. Abramowitz and I. A. Stegun, "Handbook of Mathematical
Daniel@0: %      Functions", Government Printing Office, 1964, 26.4.
Daniel@0: %      [2] E. Kreyszig, "Introductory Mathematical Statistics",
Daniel@0: %      John Wiley, 1970, section 10.2 (page 144)
Daniel@0: 
Daniel@0: %   Copyright 1993-2002 The MathWorks, Inc. 
Daniel@0: %   $Revision: 1.1.1.1 $  $Date: 2005/04/26 02:30:30 $
Daniel@0: 
Daniel@0: if nargin < 2, 
Daniel@0:     error('Requires two input arguments.');
Daniel@0: end
Daniel@0: 
Daniel@0: [errorcode p v] = distchck(2,p,v);
Daniel@0: 
Daniel@0: if errorcode > 0
Daniel@0:     error('Requires non-scalar arguments to match in size.');
Daniel@0: end
Daniel@0: 
Daniel@0: % Call the gamma inverse function. 
Daniel@0: x = gaminv(p,v/2,2);
Daniel@0: 
Daniel@0: % Return NaN if the degrees of freedom is not positive.
Daniel@0: k = (v <= 0);
Daniel@0: if any(k(:))
Daniel@0:     x(k) = NaN;
Daniel@0: end