Tangent measure

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In measure theory, tangent measures are used to study the local behavior of Radon measures, in much the same way as tangent spaces are used to study the local behavior of differentiable manifolds. Tangent measures are a useful tool in geometric measure theory. For example, they are used in proving Marstrand’s theorem.

[edit] Definition

Consider a Radon measure μ defined on an open subset Ω of n-dimensional Euclidean space Rn and let a be an arbitrary point in Ω. We can “zoom in” on a small open ball of radius r around a, Br(a), via the transformation

T_{a,r}(x)=\frac{x-a}{r},

which enlarges the ball of radius r about a to a ball of radius 1 centered at 0. With this, we may now zoom in on how μ behaves on Br(a) by looking at the push-forward measure defined by

T_{a,r \#}\mu(A)=\mu(a+rA)

where

a+rA=\{a+rx:x\in A\}.

As r gets smaller, this transformation on the measure μ spreads out and enlarges the portion of μ supported around the point a. We can get information about our measure around a by looking at what these measures tend to look like in the limit as r approaches zero.

Definition. A tangent measure of a Radon measure μ at the point a is a second Radon measure ν such that there exist sequences of positive numbers ci > 0 and decreasing radii ri → 0 such that
\lim_{i\rightarrow\infty} c_{i}T_{a,r_{i}\#}\mu =\nu
where the limit is taken in the weak-∗ topology, i.e., for any continuous function φ with compact support in Ω,
\lim_{i\rightarrow\infty}\int_{\Omega} \varphi \, \mathrm{d} (c_{i}T_{a,r_{i}\#}\mu)=\int_{\Omega} \varphi \, \mathrm{d} \nu.
We denote the set of tangent measures of μ at a by T(μ, a).

There is an associated notion of the tangent space of a measure. A k-dimensional subspace P of Rn is called the k-dimensional tangent space of μ at a ∈ Ω if — after appropriate rescaling — μ “looks like” k-dimensional Hausdorff measure Hk on P. More precisely:

Definition. P is the k-dimensional tangent space of μ at a if there is a θ > 0 such that
\mu_{a, r} \xrightarrow[r \to 0]{*} \theta H^{k} \lfloor_{P},
where μa,r is the translated and rescaled measure given by
\mu_{a, r} (A) = \frac1{r^{n - 1}} \mu(a + r A).
The number θ is called the multiplicity of μ at a, and the tangent space of μ at a is denoted Ta(μ).

Further study of tangent measures and tangent spaces leads to the notion of a varifold.[1]

[edit] Examples

  • Suppose we have a circle in R2 with uniform measure on that circle. Then, for any point a in the circle, the set of tangent measures will just be positive constants times 1-dimensional Hausdorff measure supported on the line tangent to the circle at that point.
  • In 1995, Toby O'Neil produced an example of a Radon measure μ on Rd such that, for almost every point a ∈ Rd, T(μ, a) consists of all nonzero Radon measures.[2]

[edit] References

  1. ^ Röger, Matthias (2004). "Solutions for the Stefan problem with Gibbs-Thomson law by a local minimisation". Interfaces Free Bound. 6 (1): 105–133. ISSN 1463-9963.  MR2047075
  2. ^ O'Neil, Toby (1995). "A measure with a large set of tangent measures". Proc. of the AMS 123: 2217–2220. doi:10.2307/2160960.