class_docstr: Hawkes process model exponential kernels with fixed and given decay.
  It is modeled with (opposite) log likelihood loss:
  
  .. math::
      \sum_{i=1}^{D} \left(
          \int_0^T \lambda_i(t) dt
          - \int_0^T \log \lambda_i(t) dN_i(t)
      \right)
  
  where :math:`\lambda_i` is the intensity:
  
  .. math::
      \forall i \in [1 \dots D], \quad
      \lambda_i(t) = \mu_i + \sum_{j=1}^D
      \sum_{t_k^j < t} \phi_{ij}(t - t_k^j)
  
  where
  
  * :math:`D` is the number of nodes
  * :math:`\mu_i` are the baseline intensities
  * :math:`\phi_{ij}` are the kernels
  * :math:`t_k^j` are the timestamps of all events of node :math:`j`
  
  and with an exponential parametrisation of the kernels
  
  .. math::
      \phi_{ij}(t) = \alpha^{ij} \beta^{ij}
                     \exp (- \beta^{ij} t) 1_{t > 0}
  
  In our implementation we denote:
  
  * Integer :math:`D` by the attribute `n_nodes`
  * Matrix :math:`B = (\beta_{ij})_{ij} \in \mathbb{R}^{D \times D}` by the
    parameter `decays`. This parameter is given to the model
  
  Parameters
  ----------
  decay : `float`
      The decay coefficient of the exponential kernels.
      All kernels share this decay.
  
  n_threads : `int`, default=1
      Number of threads used for parallel computation.
  
      * if ``int <= 0``: the number of threads available on
        the CPU
      * otherwise the desired number of threads
  
  Attributes
  ----------
  n_nodes : `int` (read-only)
      Number of components, or dimension of the Hawkes model
  
  data : `list` of `numpy.array` (read-only)
      The events given to the model through `fit` method.
      Note that data given through `incremental_fit` is not stored
code: def decays(self):
      return self.decay
  
file_path: tick/tick/hawkes/model/model_hawkes_expkern_loglik.py
imports: import numpy
  from tick.base_model import ModelSecondOrder, ModelSelfConcordant, LOSS_AND_GRAD
  from tick.hawkes.model.build.hawkes_model import ModelHawkesExpKernLogLik
  from .base import ModelHawkes
label_desc: Use image node_id 6 for calling the ModelHawkesExpKernLogLik obj's underlying member method code with example usage: obj.decays() and returns: self