Method on @infsup: psi (X)

Compute the digamma function, also known as the psi function.

           ∞
          /  exp (-t)         exp (-xt)
psi (x) = |----------  -  -------------- dt
          /     t           1 - exp (-t)
         0

Accuracy: The result is a tight enclosure.

psi (infsup (1))
  ⇒ ans ⊂ [-0.57722, -0.57721]

Package: interval