Second derivative of the negative binomial link function.

        Parameters
        ----------
        p : array-like
            Mean parameters

        Returns
        -------
        g''(p) : array
            The second derivative of the negative binomial transform link
            function

        Notes
        -----
        g''(x) = -(1+2*alpha*x)/(x+alpha*x^2)^2

def deriv2(self,p):
        '''
        Second derivative of the negative binomial link function.

        Parameters
        ----------
        p : array-like
            Mean parameters

        Returns
        -------
        g''(p) : array
            The second derivative of the negative binomial transform link
            function

        Notes
        -----
        g''(x) = -(1+2*alpha*x)/(x+alpha*x^2)^2
        '''
        numer = -(1 + 2 * self.alpha * p)
        denom = (p + self.alpha * p**2)**2
        return numer / denom