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## Derivatives of the DiGamma fonction : Psi

Psi takes as arguments a real a and an integer n (by default n = 0).
Psi returns the value of the n-th derivative of the DiGamma function at x = a, where the DiGamma fonction is the first derivative of ln((x)). This function is used to evaluated sums of rational functions having poles at integers.
Input :
Psi(3,1)
Output :
pi^2/6-5/4

If n=0, you may use Psi(a) instead of Psi(a,0) to compute the value of the DiGamma fonction at x = a.
Input :

Psi(3)
Output :
Psi(1)+3/2
Input :
evalf(Psi(3))
Output :
.922784335098

giac documentation written by Renée De Graeve and Bernard Parisse