As mentioned in §E.2, a measure of ``flatness'' is the number of leading zero terms in a function's Taylor expansion (not counting the first (constant) term). Thus, by this measure, the bell curve is ``infinitely flat'' at infinity, or, equivalently, is infinitely flat at .
Another property of is that it has an infinite number of ``zeros'' at . The fact that a function has an infinite number of zeros at can be verified by showing
The reciprocal of a function containing an infinite-order zero at has what is called an essential singularity at [15, p. 157], also called a non-removable singularity. Thus, has an essential singularity at , and has one at .
In summary, a Taylor series expansion about the point will always yield a constant approximation when the function being approximated is infinitely flat at . For this reason, polynomial approximations are often applied over a restricted range of , with constraints added to provide transitions from one interval to the next. This leads to the general subject of splines . In particular, cubic spline approximations are composed of successive segments which are each third-order polynomials. In each segment, four degrees of freedom are available (the four polynomial coefficients). Two of these are usually devoted to matching the amplitude and slope of the polynomial to one side, while the other two are used to maximize some measure of fit across the segment. The points at which adjacent polynomial segments connect are called ``knots'', and finding optimal knot locations is usually a relatively expensive, iterative computation.
Differentiability of Audio Signals
Weierstrass Approximation Theorem