This question came up when I was doing some reading into convolution squares of singular measures. Recall a function f on the torus T=[−1/2,1/2] is said to be alpha-Hölder (for 0<alpha<1) if suptinmathbbTsuphneq0|h|−alpha|f(t+h)−f(t)|<infty. In this case, define this value, omegaalpha(f)=suptinmathbbTsuphneq0|h|−alpha|f(t+h)−f(t)|. This behaves much like a metric, except functions differing by a constant will differ in omegaalpha value. My primary question is this:
1) Is it true that the smooth functions are "dense" in the space of continuous alpha-Hölder functions, i.e., for a given continuous alpha-Hölder f and varepsilon>0, does there exists a smooth function g with omegaalpha(f−g)<varepsilon?
To be precise, where this came up was worded somewhat differently. Suppose Kn are positive, smooth functions supported on [−1/n,1/n] with intKn=1.
2) Given a fixed continuous function f which is alpha-Hölder and varepsilon>0, does there exist N such that ngeqN ensures omegaalpha(f−f∗Kn)<varepsilon?
This second formulation is stronger than the first, but is not needed for the final result, I believe.
To generalize, fix 0<alpha<1 and suppose psi is a function defined on [0,1/2] that is strictly increasing, psi(0)=0, and psi(t)geqtalpha. Say that a function f is psi-Hölder if suptinmathbbTsuphneq0psi(|h|)−1|f(t+h)−f(t)|<infty. In this case, define this value, omegapsi(f)=suptinmathbbTsuphneq0psi(|h|)−1|f(t+h)−f(t)|. Then we can ask 1) and 2) again with alpha replaced by psi.
I suppose the motivation would be that the smooth functions are dense in the space of continuous functions under the usual metrics on function spaces, and this "Hölder metric" seems to be a natural way of defining a metric of the equivalence classes of functions (where f and g are equivalent if f=g+c for a constant c). Any insight would be appreciated.
No comments:
Post a Comment