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Arnaud:\r\nheasarc.gsfc.nasa.gov/docs/xanadu/xspec/wstat.ps\r\nOn page 5, it says something to the effect: \r\n\"...to give a quadratic in b_i whose solution is:.....\"\r\nI have to look at this more closely but it looks like he is using the grade school formula \r\n(-b +/- sqrt(b^2-4ac))/2a for one of the roots (see equation a_i) of quadratic equation which is not numerically stable.  "}}