STK_TESTFUN_HARTMAN6 computes the "Hartman6" function

    The Hartman6 function is a test function in dimension 6, which is
    part of the famous Dixon & Szego benchmark [1] in global optimization.

    It is usually minimized over [0, 1]^6.

 HISTORICAL REMARKS

    This function belongs to a general class of test functions
    introduced by Hartman [2], hence the name.

    The particular set of coefficients used in the definition of the
    "Hartman6" function, however, seems to have been introduced by [1].

 GLOBAL MINIMUM

    According to [4], the function has one global minimum at

       x = [0.20169 0.150011 0.476874 0.275332 0.311652 0.657300].

    The corresponding function value is:

       f(x) = -3.322368011391339

    A slightly lower value is attained [5] at

       x = [0.20168952 0.15001069 0.47687398 ...
            0.27533243 0.31165162 0.65730054]

    The corresponding function value is:

       f(x) = -3.322368011415512

    The exact global optimum does not appear to be known.

 REFERENCES

  [1] L. C. W. Dixon & G. P. Szego (1978).  Towards Global
      Optimization 2, North-Holland, Amsterdam, The Netherlands

  [2] J. K. Hartman (1973).  Some experiments in global optimization.
      Naval Research Logistics Quarterly, 20(3):569-576.

  [3] V. Picheny, T. Wagner & D. Ginsbourger (2013).  A benchmark
      of kriging-based infill criteria for noisy optimization.
      Structural and Multidisciplinary Optimization, 48:607-626.

  [4] S. Surjanovic & D. Bingham.  Virtual Library of Simulation
      Experiments: Test Functions and Datasets.  Retrieved March 3,
      2022, https://www.sfu.ca/~ssurjano/hart4.html.

  [5] O. Roustant, D. Ginsbourger & Y. Deville (2012).
      DiceKriging package, version 1.6.0 from 2021-02-23
      URL: https://cran.r-project.org/web/packages/DiceKriging/index.html