Equations (3) and (4) compute the same thing by two different methods, and .When it is viewed as fitting goals minimizing and used along with suitable constraints, (3) leads to finding filters and spectra, while (4) leads to finding missing data.
A new concept embedded in (3) and (4)
is that one filter can be applicable for different stretchings
of the filter's time axis.
``Of all classes of filters,
what subset remains appropriate for stretchings of the axes?''