### 2.9.9 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(Γ(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`