function x=chi2rv(df);
% chi2rv(df) generates a random number from a chi-square distribution 
% with df degrees of freedom; 
% See G.S. Fishman, 1973, Concepts and Methods in Discrete Event
% Digital Simulation, Wiley, New York, p. 213.
% REQUIRES THE FILES betarv.m AND gammarv.m

if mod(df,2)==0
    x=-2*log(prod(rand(1,df/2)));
else 
    k=(df/2)-0.5;
    v=sum(-log(rand(1,k)));
    y=betarv(0.5,0.125,1);
    z=-log(rand);
    x=2*(v+y*z);
end;