# 5 Applications to support measures

Im Dokument Hessian measures of semi-convex functions and applications to support measures of convex bodies∗ (Seite 21-32)

In this section we shall explain the basic relationship between Hessian measures of semi-convex functions and support measures of sets of positive reach. In , the epigraph of a semi-convex function (which is a set of positive reach) was used in a non-trivial way to construct the Hessian measures of the given function. Now we shall see that support measures of a set with positive reach can be expressed in a simple way as Hessian measures of some particular functions related to the given set, such as the distance function, support function and convex characteristic function.

Before we go into the details of the results, we recall some basic notions and introduce some notations regarding sets of positive reach and convex bodies in Rd. For a subset X of Rd and for r ≥0 we denote by Xr the set of points whose distance from X is r at the most. The reachof X, reach(X), is the supremum over all r≥0 such that for every point x ∈ Xr there exists a unique nearest point in X to x. We say that X ⊂ Rd is a set of positive reach (has positive reach) if reach(X)>0.

A convex body K in Rd, i.e. a convex compact subset ofRd with non-empty interior, is clearly a set of positive reach and reach(K) = ∞.

LetX ⊂Rd have positive reach; forr <reach(X) and x∈Xr\X we writep(X, x) for the nearest point in X to x, and we set u(X, x) := dX(x)−1(x−p(X, x)), where dX(x) is the distance of x to X. Furthermore for every x∈ X, we denote by N(X, x) the normal cone of X at x (see ); the definition given there is consistent with the one adopted in convex geometry. Also recall that if X is a convex body andu∈Sd−1, then

F(X, u) :={x∈X : u∈N(X, x)}

is the support set of X in direction u.

The support measuresΘj(X,·),j = 0,1, . . . , d−1, of a setX ⊂Rdof positive reach are signed measures defined for all bounded Borel subsets of Rd×Sd−1. They can be obtained as coefficients of the local Steiner formula

Hd(Mr(X, η)) = 1 d

d−1

X

j=1

rd−j d

r

Θj(X, η), where 0< r <reach(X), η is a bounded Borel subset of Rd×Sd−1 and

Mr(X, η) := {x∈Xr\X : (p(X, x), u(X, x))∈η}.

By specializing η we obtain the Federer curvature measures (briefly: curvature measures) of X as follows

Cj(X, α) := Θj(X, α×Sd−1), j ∈ {0,1, . . . , d−1},

for every bounded Borel setα⊂Rd. Moreover in the special case whereX =K is a convex body we can also introduce the surface area measuresof K by setting

Sj(K, β) := Θj(K,Rd×β), j ∈ {0,1, . . . , d−1},

for every Borel subset β of Sd−1.

Now we are in the position to show some applications of the results proved in the previuos sections to support measures of sets of positive reach. We start with a preliminary lemma which shows that we could have chosen a seemingly weaker definition of semi-convexity in the first instance. We include this simple lemma, because the method of proof will also be useful in the following.

Lemma 5.1 Let Ω⊂Rd be open, and let u: Ω→R be a function. Thenu is semi-convex if and only if for each x ∈ Ω there exists a neighbourhood U ⊂⊂ Ω of x and a constant C ≥0 such that

k(x) :=u(x) + C

2kxk2, x∈U , defines a convex function.

Proof. We prove the “if part”. Let Ω0 ⊂⊂ Ω be given. Then there are neighbourhoods U1, . . . , Up ⊂Ω and constants C1, . . . , Cp ≥0 such that

ki(x) :=u(x) + Ci

2 kxk2, x∈Ui, for i= 1, . . . , p, defines a convex function. We set

C := max{Ci : 1≤i≤p}

and

k(x) :=u(x) + C

2kxk2, x∈Ω0.

Obviously, k|Ui is a convex function for each i ∈ {1, . . . , p}, and therefore the epigraph of each of these functions is an open convex set. But then the epigraph of k|0 fulfills the local support property which is required for an application of Tietze’s convexity criterion;

see , Theorem 4.10. Thus k|0 is a convex function.2

In the more general context of Riemannian geometry, a less explicit version of the following proposition has been proved by Kleinjohann , Satz 2.8; see also  and , Corollary 3.4, for a converse.

Proposition 5.2 Let X ⊂ Rd satisfy reach(X) > r > 0 and let 0 < δ < r. Then dX is semi-convex on int Xr−δ; in particular, sc(dX,Ω0)≤δ−1 for all Ω0 ⊂⊂int Xr−δ.

Proof. We show that

k(x) := dX(x) + 1

2δkxk2, x∈Rd,

defines a convex function on convex subsets of int Xr−δ. In view of the proof of Lemma 5.1 we have to show that for each x0 ∈int Xr−δ there is some r0 > 0 such that the epigraph of k|B(x0,r0) is supported by a hyperplane at (x0, k(x0)).

If x0 ∈ X, then the support property is satisfied since dX ≥ 0 and x 7→ (2δ)−1kxk2 is convex. Now let x0 ∈ (int Xr−δ)\X. We define p0 := p(X, x0), u0 := u(X, x0) and r0 := 2−1min{dX(x0), r−δ−dX(x0)}, and hence we obtain

B(x0, r0)⊂B(p0+ru0, r)\B(p0+ru0, δ).

Furthermore, we define C := Rd\int B(p0 +ru0, r); a straightforward calculation then shows that

x7→dC(x) + 1 2δkxk2

defines a convex function on B(x0, r0). Therefore we find some (uniquely determined) vector v ∈Rd such that

dC(x0 +h) + 1

2δkx0+hk2−dC(x0)− 1

2δkx0k2 ≥ hv, hi

for all h ∈ Rd with khk < r0. Since dC(x0) = dX(x0) and dC(x0 +h) ≤ dX(x0 +h) for h∈Rd with khk< r0, we obtain

dX(x0+h) + 1

2δkx0+hk2−dX(x0)− 1

2δkx0k2 ≥ hv, hi,

for khk < r0. Thus the epigraph of x 7→ dX(x) + (2δ)−1kxk2 locally has support planes, which was to be shown.2

Remark 5.1 Instead of sets of positive reach one can consider the more general sets having the unique footpoint property (UFP-sets); see  and . These are closed sets X ⊂Rd for which reach(X, x) > 0 for all x ∈ X. Recall from  that reach(X,·) is a continous function on X, and therefore compact UFP-sets are sets of positive reach. The method of proof for Proposition 5.2 can be used to show that the distance function of a closed set having the unique footpoint property is semi-convex inU(X), where

U(X) := [

x∈X

int B(x,reach(X, x))

is an open neighbourhood of X. In  this was proved in Riemannian spaces by using H-Jacobi fields. We wish to point out that Proposition 5.3 and Theorems 5.4 and 5.5 could also be stated for UPF-sets, since all relevant notions are locally defined.

Our next purpose is to describe the subdifferential of the distance function of a set X, satisfying reach(X)> r, in terms of geometric quantities. The result can be paraphrased by saying that sets of positive reach are regular in the sense of nonsmooth analysis.

Proposition 5.3 Let X ⊂Rd satisfy reach(X)> r >0. Then

∂dX(x) =

N(X, x)∩B(0,1), x∈X , {u(X, x)}, x /∈Xr\X .

Proof. The case x /∈X is covered by Theorem 4.8 (3) and (5) in .

Now let x ∈ X. Consider sequences xi ∈ X, vi ∈ Rd\ {0}, i ∈ N, such that xi → x, vi → 0 and vi/kvik → v ∈ Rd as i → ∞ and such that p(X, xi +vi) = xi for all i ∈ N. Then vi ∈ N(X, xi) and therefore xi = p(X, xi+rvi/kvik); see , Theorem 4.8. Since xi +rvi/kvik → x+rv as i → ∞ and since p(X,·) is continuous on Xr, we deduce that x = p(X, x+rv), and hence v ∈ N(X, x). But then , Theorem 2.5.6, implies that

∂dX(x)⊂N(X, x)∩B(0,1), since N(X, x)∩B(0,1) is convex.

Conversely, let v ∈ N(X, x)∩B(0,1) for some x ∈ X. Since 0 ∈ ∂dX(x) (note that dX(x) = 0 and dX ≥0), we can assume that v 6= 0. Then we have to show that

lim sup

y→x t↓0

dX(y+tw)−dX(y)

t ≥ hv, wi, (29)

for all w∈Rd. We prove that lim sup

t↓0

dX(x+tw)−dX(x)

t ≥ hv, wi,

for allw∈Rd, from which (29) follows. Ifhv, wi ≤0, then there is nothing to prove; hence we assume that hv, wi>0. But thenx+tw ∈B(x+rkvk−1v, r)⊂(Rd\X)∪ {x}, if t >0 is sufficiently small, which implies

dX(x+tw)

t ≥ r− k −rkvk−1v+twk

t .

Passing to the limit, we obtain lim sup

t↓0

dX(x+tw)

t ≥

v kvk, w

≥ hv, wi,

since kvk ∈(0,1].2

The following theorem describes the surprisingly simple connection between the support measures of a set of positive reach and the Hessian measures of the distance function dX

of X.

Theorem 5.4 Let X ⊂Rd have positive reach, let α ⊂X be relatively compact and Borel measurable, let β ⊂Sd−1 be Borel measurable, and set βˆ:= conv(β∪ {0}). Then

Θj(X, α×β) = dΘj(dX, α×β)ˆ , j ∈ {0,1, . . . , d−1}.

Proof. Let reach(X)> r >0 and letU ⊂⊂int Xr/2 be arbitrarily fixed. By Proposition 5.2 we obtain sc(dX, U)≤2/r, and hence

Hd(Pρ(dX, γ×β)) =ˆ

d

X

j=0

ρd−j d

j

Θj(dX, γ×β)ˆ (30)

holds for ρ∈ [0, r/2) and Borel sets γ ⊂ U; see Theorem 5.2 in . For γ ⊂ U ∩X and ρ >0 we also have

Pρ(dX, γ×β) =ˆ {x+ρp∈Rd:x∈γ, p∈βˆ∩∂dX(x)}

= (γ∩int X)∪ {x+ρp∈Rd:x∈γ∩∂X, p∈βˆ∩N(X, x)}

= (γ∩X)∪Mρ(X, γ×β),

where Theorem 4.8 (12) in  was used. Hence the Steiner formula for sets with positive reach yields, for a Borel set γ ⊂U ∩X and 0< ρ < r/2, that we also have

Hd(Pρ(dX, γ×β)) =ˆ Hd(γ∩X) + 1 d

d−1

X

j=0

ρd−j d

j

Θj(X, γ×β)ˆ . (31)

A comparison of coefficients in (30) and (31) yields the assertion, first for Borel setsγ ⊂U, and then in the general case by additivity.2

The preceding theorem can be used to deduce results about support measures of a set with positive reach from corresponding results about Hessian measures of the distance function of the given set. As a particular example we establish a generalization of Theorem 3.2 in .

Theorem 5.5 Let X ⊂ Rd be a set with positive reach, and let j ∈ {0,1, . . . , d −1}.

Further, let α ⊂ Rd be a Borel set having σ-finite j-dimensional Hausdorff measure, and let η⊂α×Sd−1 be Borel measurable. Then

d−1 j

Θj(X, η) = Z

Rd

Hd−1−j(N(X, x)∩ηx)dHj(x),

where ηx :={u∈Sd−1 : (x, u)∈η}.

Proof. It is sufficient to prove the asserted equality forη =γ×β, where γ ⊂α and β ⊂ Sd−1 are Borel sets and γ is bounded. Recall that Θj(X,·) is concentrated on ∂X×Sd−1, and by Proposition 5.3 we have∂dX(x)∩βˆ=N(X, x)∩βˆif x∈∂X. Using Theorems 5.4

and 4.2 together with Remark 4.1, we can conclude d−1

j

Θj(X, γ×β) =

d−1 j

Θj(X,(γ∩∂X)×β)

=d

d−1 j

Θj(dX,(γ∩∂X)×β)ˆ

= (d−j) Z

γ∩∂X

Hd−j(∂dX(x)∩β)dHˆ j(x)

= (d−j) Z

γ∩∂X

Hd−j(N(X, x)∩β)dHˆ j(x)

= Z

γ∩∂X

Hd−1−j(N(X, x)∩β)dHj(x)

= Z

Rd

Hd−1−j(N(X, x)∩(γ×β)x)dHj(x). In order to justify the fifth equality it is sufficient to check that

(d−j)Hd−j(N(X, x)∩β) =ˆ Hd−1−j(N(X, x)∩β) for all x∈∂X for which dim N(X, x) =d−j.2

Subsequently, we study convex functions which may take infinite values and whose epigraph is a closed set. We can assume that such a function is defined in Rd, since we may extend a function u : Ω → R¯, Ω ⊂ Rd non-empty, open and convex, by setting u|Rd\Ω :≡ ∞.

The theory developed in  extends to (proper) closed convex functions u: Rd→ R¯. The only change concerns the definition of the normal bundle of u in Rd+1×Rd+1, which is now given by

Nor(u) :={(X, V)∈Nor(epi(u)) :hV, En+1i 6= 0};

see  for the notation and further details. We should also emphasize that if p∈ ∂u(x), then necessarilyu(x)<∞.

For a closed convex setK ⊂Rd, letIK denote the convex characteristic function ofK, IK(x) :=

0, if x∈K ,

∞, if x /∈K . The following lemma is easy to check from the definitions.

Lemma 5.6 Let K ⊂Rd be a closed convex set. Then

∂IK(x) =

N(K, x), x∈K ,

∅, x /∈K .

From Lemma 5.6 we can now derive a representation for the support measures of a con-vex set in terms of the Hessian measures of the concon-vex characteristic function which is associated with the set.

Theorem 5.7 Let K ⊂ Rd be closed and convex. Let α ⊂ Rd and β ⊂ Sd−1 be Borel measurable. Then

Θj(K, α×β) = dΘj(IK, α×β)ˆ , j ∈ {0,1, . . . , d−1}. Proof. For any ρ >0 we obtain

Pρ(IK, α×β) =ˆ {x+ρp∈Rd :x∈α, p∈∂IK(x)∩β}ˆ

= (α∩int K)∪ {x+ρp∈Rd :x∈α∩∂K, p ∈N(K, x)∩β}ˆ

= (α∩K)∪Mρ(K, α×β).

By an application of the Steiner formulae for convex functions and convex sets, we conclude

d

X

j=0

ρd−j d

j

Θj(IK, α×β) =ˆ 1 d

d−1

X

m=0

ρd−m d

m

Θm(K, α×β) +Hd(α∩K). A comparison of coefficients yields the desired result.2

For a closed convex function u:Rd→R¯ the conjugate function is defined by u(y) := sup{hy, xi −u(x) :x∈Rd}, y∈Rd.

The conjugate function u is again closed and convex and u∗∗ = u. In a certain (but very vague) sense, the formation of the polar body of a given convex set can be seen in analogy to the conjugation of convex functions. For the following theorem, however, a corresponding result for convex sets is not true. On the other hand, in spherical space an analogous theorem has been proved for spherical support measures and pairs of polar (spherically) convex sets; see Glasauer .

Theorem 5.8 Letu:Rd →R¯ be closed and convex, and letα, β ⊂Rdbe Borel measurable.

Then

Θj(u, α×β) = Θd−j(u, β×α), j ∈ {0,1, . . . , d}. Proof. By , Theorem 23.5, we obtain for any ρ >0 that

Pρ(u, α×β) = {x+ρp∈Rd:x∈α, p∈β, p∈∂u(x)}

=ρ{p+ρ−1x∈Rd:p∈β, x∈α, x∈∂u(p)}

=ρPρ−1(u, β×α).

The Steiner formula for convex functions, Theorem 3.1 in , now yields

d

X

j=0

ρd−j d

j

Θj(u, α×β) = ρdHd(Pρ−1(u, β×α))

d

d

X

j=0

ρj−d d

j

Θj(u, β×α)

=

d

X

j=0

ρj d

j

Θj(u, β×α). A comparison of coefficients completes the proof.2

For a compact convex set K ⊂Rdwe denote by hK the support function ofK; see .

Corollary 5.9 Let K ⊂Rd be a compact convex set, and let α ⊂Rd, β ⊂ Sd−1 be Borel measurable. Then

Θj(K, α×β) =dΘd−j(hK,βˆ×α), j ∈ {0,1, . . . , d−1}. Proof. Use (hK) =IK and Theorems 5.7 and 5.8.2

In particular, the following special cases of Theorem 5.4 and Corollary 5.9 deserve to be emphasized.

Corollary 5.10 Let X ⊂ Rd be a set of positive reach, let K ⊂ Rd be a compact convex set, let α ⊂Rd, β ⊂Sd−1 be Borel sets, and assume that α is bounded. Then

Cj(X, α) =dFj(dX, α∩∂X), j ∈ {0,1, . . . , d−1}, and

Sj(K, β) =dFd−j(hK,β)ˆ , j ∈ {0,1, . . . , d−1}.

Our final result improves Theorem 4.3 in  in the same way as Theorem 5.5 generalizes Theorem 3.2 in .

Theorem 5.11 Let K ⊂ Rd be a convex body, and let j ∈ {0,1, . . . , d−1}. Further, let ω ⊂ Sd−1 be a Borel set having σ-finite (d−1−j)-dimensional Hausdorff measure, and let η⊂Rd×ω be Borel measurable. Then

d−1 j

Θj(K, η) = Z

Sd−1

Hj(F(K, u)∩ηu)dHd−1−j(u), where ηu :={x∈Rd : (x, u)∈η}.

Proof. It is sufficient to consider the case where η = γ ×β with Borel sets γ ⊂ Rd and β ⊂ ω. We can assume that γ ⊂ K. First, we show that ˆβ has σ-finite (d− j)-dimensional Hausdorff measure. To see this, we assume that Hd−1−j(β)<∞ and deduce thatHd−j( ˆβ)<∞. By Theorem 2.10.45 in , we obtainHd−j([0,1]×β)<∞. The map f : [0,1]×Sd−1 →Rd, (λ, u)7→λu, is Lipschitz and hence

Hd−j( ˆβ) = Hd−j(f([0,1]×β))≤(lip(f))d−jHd−j([0,1]×β)<∞. Using Corollary 5.9 and Theorem 4.2 together with Remark 4.1, we obtain

d−1 j

Θj(K, γ×β) = d

d−1 j

Θd−j(hK,βˆ×γ)

= (d−j) Z

βˆ

Hj(∂hK(x)∩γ)dHd−j(x)

= (d−j) Z

(β∩Σd−j−1(K))

Hj(∂hK(x)∩γ)dHd−j(x)

= Z

β∩Σd−j−1(K)

Hj(F(K, u)∩γ)dHd−1−j(u)

= Z

Sd−1

Hj(F(K, u)∩(γ ×β)u)dHd−1−j(u), where

Σr(K) = {u∈Sd−1 : dim F(K, u)≥d−1−r}

is an r-rectifiable Borel set, for r ∈ {0,1, . . . , d−1}; therefore the fourth equality can be justified by the area/coarea theorem .2

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Andrea Colesanti

Dipartimento di Matematica “U. Dini”

Universit`a degli Studi di Firenze Viale Morgagni 67/A

50134 Firenze Italy

andrea.colesanti@bb.math.unifi.it

Daniel Hug

Mathematisches Institut Albert-Ludwigs-Universit¨at Eckerstraße 1

D-79104 Freiburg Germany

hug@sun2.mathematik.uni-freiburg.de

Im Dokument Hessian measures of semi-convex functions and applications to support measures of convex bodies∗ (Seite 21-32)