A set of Orthonormal Functions is termed complete in the Closed Interval if, for
every piecewise Continuous Function in the interval, the minimum square error
See also Bessel's Inequality, Hilbert Space
Arfken, G. ``Completeness of Eigenfunctions.'' §9.4 in Mathematical Methods for Physicists, 3rd ed.
Orlando, FL: Academic Press, pp. 523-538, 1985.