Volterra's function
In mathematics, Volterra's function, named for Vito Volterra, is a real-valued function V(x) defined on the real line R with the following curious combination of properties:
- V(x) is differentiable everywhere
- The derivative V ′(x) is bounded everywhere
- The derivative is not Riemann-integrable.
Definition and construction
The function is defined by making use of the Smith–Volterra–Cantor set and "copies" of the function defined by f(x) = x2 sin(1/x) for x ≠ 0 and f(x) = 0 for x = 0. The construction of V(x) begins by determining the largest value of x in the interval [0, 1/8] for which f ′(x) = 0. Once this value (say x0) is determined, extend the function to the right with a constant value of f(x0) up to and including the point 1/8. Once this is done, a mirror image of the function can be created starting at the point 1/4 and extending downward towards 0. This function, which we call f1(x), will be defined to be 0 outside of the interval [0, 1/4]. We then translate this function to the interval [3/8, 5/8] so that the function is nonzero only on the middle interval as removed by the SVC. To construct f2(x), f ′(x) is then considered on the smaller interval 1/16 and two translated copies of the resulting function are added to f1(x). Volterra's function then results by repeating this procedure for every interval removed in the construction of the SVC.
Further properties
Volterra's function is differentiable everywhere just as f(x) (defined above) is. One can show that f′(x) = 2x sin(1/x) - sin(1/x) for x ≠ 0, which means that in any neighborhood of zero, there are points where f′(x) takes 1 and −1. Thus there are points where V ′(x) takes values 1 and −1 in every neighborhood of each of the endpoints of intervals removed in the construction of the Smith–Volterra–Cantor set CSV. In fact, V ′ is discontinuous at every point of CSV, even though V itself is differentiable at every point of CSV, with derivative 0. The set of points where V ′ is the set CSV.
Since the Smith–Volterra–Cantor set CSV has positive Lebesgue measure, this means that V ′ is discontinuous on a set of positive measure. By Lebesgue's criterion for Riemann integrability, V ′ is not integrable. If one were to repeat the construction of Volterra's function with the ordinary measure-0 Cantor set C in place of the "fat" (positive-measure) Cantor set CSV, one would obtain a function with many similar properties, but the derivative would then be discontinuous on the measure-0 set C instead of the positive-measure set CSV, and so the resulting function would have an integrable derivative.