H + (dx - H df) df^\dagger / ( df^\dagger df)

corresponding to Broyden's second method.

References
----------
.. [1] B.A. van der Rotten, PhD thesis,
   "A limited memory Broyden method to solve high-dimensional
   systems of nonlinear equations". Mathematisch Instituut,
   Universiteit Leiden, The Netherlands (2003).

   https://web.archive.org/web/20161022015821/http://www.math.leidenuniv.nl/scripties/Rotten.pdf

Examples
--------
The following functions define a system of nonlinear equations

>>> def fun(x):
...     return [x[0]  + 0.5 * (x[0] - x[1])**3 - 1.0,
...             0.5 * (x[1] - x[0])**3 + x[1]]

A solution can be obtained as follows.

>>> from scipy import optimize
>>> sol = optimize.broyden2(fun, [0, 0])
>>> sol
array([0.84116365, 0.15883529])

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