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Quantile function
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See also Wikipedia :cite:p:`WikipediaDef09`.




Calculating the quantile function from the cdf
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.. method:: ctx.qtf_from cdf(x, cf)

    where ``ctx`` is ``dec``, ``mpm``, or ``gmp``.


    This requires a starting point for a Newton iteration. If a specific starting point is not available, then a Cornish-Fisher expansion can be tried first.



Approximating the quantile function from the cumulant generating function
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See inverse Jension saddlepoint approximation for examples



Approximating the quantile function from the cumulants 
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See Cornish-Fisher expansion for example.