def logpdf_link(self, link_f, y, Y_metadata=None):
log_objective = np.log(link_f) - y*link_f<EOL>return log_objective<EOL>
Log Likelihood Function given link(f)

.. math::
    \\ln p(y_{i}|\lambda(f_{i})) = \\ln \\lambda(f_{i}) - y_{i}\\lambda(f_{i})

:param link_f: latent variables (link(f))
:type link_f: Nx1 array
:param y: data
:type y: Nx1 array
:param Y_metadata: Y_metadata which is not used in exponential distribution
:returns: likelihood evaluated for this point
:rtype: float