function f = himmelblau(x1, x2)
  %
  % The eponymous function defined by Himmelblau in his Applied
  % Nonlinear Programming, 1972.
  %
  % It has four identical local minima,
  %
  %   * f(3,2)                                    == 0
  %
  %   * f(-2.805118086952745,  3.131312518250573) == 0
  %
  %   * f(-3.779310253377747, -3.283185991286170) == 0
  %
  %   * f(3.584428340330492, -1.848126526964404)  == 0
  %
  f = (x1^2 + x2 - 11)^2 + (x1 + x2^2 - 7)^2;
end
