scipy.special.gdtrix(a, b, p, out=None) = <ufunc 'gdtrix'>

Inverse of gdtr vs x.

Returns the inverse with respect to the parameter x of p = gdtr(a, b, x), the cumulative distribution function of the gamma distribution. This is also known as the p’th quantile of the distribution.

a : array_like

a parameter values of gdtr(a, b, x). 1/a is the “scale” parameter of the gamma distribution.

b : array_like

b parameter values of gdtr(a, b, x). b is the “shape” parameter of the gamma distribution.

p : array_like

Probability values.

out : ndarray, optional

If a fourth argument is given, it must be a numpy.ndarray whose size matches the broadcast result of a, b and x. out is then the array returned by the function.

x : ndarray

Values of the x parameter such that p = gdtr(a, b, x).

See also

CDF of the gamma distribution.
Inverse with respect to a of gdtr(a, b, x).
Inverse with respect to b of gdtr(a, b, x).


Wrapper for the CDFLIB [1] Fortran routine cdfgam.

The cumulative distribution function p is computed using a routine by DiDinato and Morris [2]. Computation of x involves a search for a value that produces the desired value of p. The search relies on the monotonicity of p with x.


[1](1, 2) Barry Brown, James Lovato, and Kathy Russell, CDFLIB: Library of Fortran Routines for Cumulative Distribution Functions, Inverses, and Other Parameters.
[2](1, 2) DiDinato, A. R. and Morris, A. H., Computation of the incomplete gamma function ratios and their inverse. ACM Trans. Math. Softw. 12 (1986), 377-393.


First evaluate gdtr.

>>> from scipy.special import gdtr, gdtrix
>>> p = gdtr(1.2, 3.4, 5.6)
>>> print(p)

Verify the inverse.

>>> gdtrix(1.2, 3.4, p)

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