given direction.

Parameters
----------
v : `numpy.ndarray`, shape (n,)
    Direction along which the curvature of the quadratic models of the
    nonlinear equality functions is evaluated.
mask : `numpy.ndarray`, shape (m_nonlinear_eq,), optional
    Mask of the quadratic models to consider.

Returns
-------
`numpy.ndarray`
    Curvature of the quadratic models of the nonlinear equality
    functions along `v`.
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