class_docstr: HolderTable objective function.
  
  This class defines the HolderTable [1]_ global optimization problem. This
  is a multimodal minimization problem defined as follows:
  
  .. math::
  
      f_{\text{HolderTable}}({x}) = - \left|{e^{\left|{1
      - \frac{\sqrt{x_{1}^{2} + x_{2}^{2}}}{\pi} }\right|}
      \sin\left(x_{1}\right) \cos\left(x_{2}\right)}\right|
  
  
  with :math:`x_i \in [-10, 10]` for :math:`i = 1, 2`.
  
  *Global optimum*: :math:`f(x) = -19.20850256788675` for
  :math:`x_i = \pm 9.664590028909654` for :math:`i = 1, 2`
  
  .. [1] Gavana, A. Global Optimization Benchmarks and AMPGO retrieved 2015
  
  TODO: Jamil #146 equation is wrong - should be squaring the x1 and x2
  terms, but isn't. Gavana does.
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code: def fun(self, x, *args):
      self.nfev += 1
  
      return -abs(
          sin(x[0])
          * cos(x[1])
          * exp(abs(1 - sqrt(x[0] ** 2 + x[1] ** 2) / pi))
      )
  
imports: import numpy
  from numpy import abs, arctan2, asarray, cos, exp, arange, pi, sin, sqrt, sum
  from .go_benchmark import Benchmark
label_desc: Use image node_id 2 for calling the HolderTable obj's underlying member method code with example usage: obj.fun(x) and returns: unknown