moothness (measured as the jumps
of the derivative, the first criterion), and the deviation of :math:`g(u_j)`
from the data :math:`x_j` (the second criterion).

Note that the summation in the second criterion is over all data points,
and in the first criterion it is over the internal spline knots (i.e.
those with ``ub < t[i] < ue``). The spline knots are in general a subset
of data, see `generate_knots` for details.

.. versionadded:: 1.15.0

References
----------
.. [1] P. Dierckx, "Algorithms for smoothing data with periodic and
    parametric splines, Computer Graphics and Image Processing",
    20 (1982) 171-184.
.. [2] P. Dierckx, "Curve and surface fitting with splines", Monographs on
    Numerical Analysis, Oxford University Press, 1993.
r