` below.
callback : callable, optional
    Called after each iteration. The signature is::

        callback(xk)

    where ``xk`` is the current parameter vector.

Returns
-------
res : OptimizeResult
    The optimization result represented as a ``OptimizeResult`` object.
    Important attributes are: ``x`` the solution array, ``success`` a
    Boolean flag indicating if the optimizer exited successfully and
    ``message`` which describes the cause of the termination. See
    `OptimizeResult` for a description of other attributes.

Options
-------
disp : bool
    Set to True to print convergence messages.
xtol : float
    Relative error in solution `xopt` acceptable for convergence.
ftol : float
    Relative error in ``fun(xopt)`` acceptable for convergence.
maxiter, maxfev : int
    Maximum allowed number of iterations and function evaluations.
    Will default to ``N*1000``, where ``N`` is the number of
    variables, if neither `maxiter` or `maxfev` is set. If both
    `maxiter` and `maxfev` are set, minimization will stop at the
    first reached.
direc : ndarray
    Initial set of direction vectors for the Powell method.
return_all : bool, optional
    Set to True to return a list of the best solution at each of the
    iterations.
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