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Probabilities.trigamma Function
Probabilities | Functions
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Returns trigamma function or psi(1,x)

Pascal
function trigamma(x: double): double;
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Description

Computes the second derivate of the logarithm of the gamma function. The function returns NAN, if the x is less than or equal to zero. 

The algorithm was adapted from: 

The algorithm "AS 121" from "Applied Statistics" (1978) vol 27, no. 1 Available in Fortran at: http://lib.stat.cmu.edu/apstat/121

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