Deve essere

codice:
Public Function poisson(k As Integer, lambda As Byte) As Byte
     If k = 0 Then
          poisson = Exp ((-1) * lambda)
     Else
          poisson = (lambda / k) * poisson(k - 1, lambda)
     End If
End Function
ma sei sicuro che non debba essere


codice:
Public Function poisson(k As Integer, lambda As Double) As Double
?