een 1 and 5.

Returns
-------
best_m : int
    Related to bounds for error control.
best_s : int
    Amount of scaling.

Notes
-----
This is code fragment (3.1) in Al-Mohy and Higham (2011).
The discussion of default values for m_max and ell
is given between the definitions of equation (3.11)
and the definition of equation (3.12).

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