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The ith p-geominimal surface area
Journal of Inequalities and Applications volume 2014, Article number: 356 (2014)
Abstract
In this paper, we introduce the concept of i th p-geominimal surface area, which extends the notion of p-geominimal surface area by Lutwak. Further, we prove some of its properties and related inequalities for this new notion.
MSC:52A30, 52A40.
1 Introduction
During the past three decades, the investigations of the classical affine surface area have received great attention from the articles [1–16] or books [17, 18]. Based on the classical affine surface area, Lutwak [19] introduced the notion of p-affine surface area and obtained some isoperimetric inequalities for p-affine surface area.
Geominimal surface area was introduced by Petty [11] more than three decades ago. As Petty stated, this concept serves as a bridge connecting affine differential geometry, relative differential geometry, and Minkowskian geometry. Based on the classical geominimal surface area, Lutwak [19] introduced the notion of p-geominimal surface area and obtained some inequalities for it. Regarding the studies of p-affine surface area and p-geominimal surface area as well as its dual object, also see [13, 20–27].
Let denote the set of compact convex subsets of the Euclidean n-space . The subset of consisting of convex bodies (compact, convex sets with non-empty interiors) will be denoted by . For the set of convex bodies containing the origin in their interiors, write , and let denote the set of convex bodies whose centroid lies at the origin. As usual, denotes the unit sphere with unit ball , the volume of .
Petty [11] defined the geominimal surface area of a body by
For and , Lutwak [19] defined the p-geominimal surface area of K by
Moreover, Lutwak proved the following inequalities for the p-geominimal surface area.
Theorem 1.1 Let and , then
with equality if and only if K is an ellipsoid.
Let denote the subset of which has a positive continuous function, and let denote the p-affine surface area of K.
Theorem 1.2 Let and , then
with equality if and only if K is of p-elliptic type.
Theorem 1.3 If and , then
with equality if and only if K is p-selfminimal.
The purpose of this paper is to further extend Lutwak’s p-geominimal surface area to the i th p-geominimal surface area. The technique we will use is that of the method designed by Lutwak [19]. Now, we define the notion of i th p-geominimal surface area as follows:
where .
The main results are stated as follows. First, we establish the extended versions of Theorems 1.1, 1.2 and 1.3 given by Theorems 1.4, 1.5 and 1.6.
Theorem 1.4 If and , and , then
with equality for if and only if K is an ellipsoid, for if and only if all -dimensional convex bodies which are contained in K are balls.
If , (1.7) is just inequality (1.3).
Let denote the -type p-affine surface area of K (see Section 2.3).
Theorem 1.5 If , , and , then
with equality if and only if . For the precise definition of , see Section 2.3.
Theorem 1.6 If and , then for ,
with equality if and only if K is ith p-selfminimal.
The proofs of Theorems 1.4-1.6 will be given in Section 4 of this paper. Moreover, in Section 3 we also establish some properties of the i th p-geominimal surface area which may be required in the proofs of main results.
2 Background material for Brunn-Minkowski-Firey theory
2.1 Support function, radial function and polar of a convex body
For , let , , and denote the transpose, inverse, and inverse of the transpose of ϕ. For , let denote the support function of , i.e., for ,
where denotes the standard inner product of u and x. For , then obviously . For the sake of convenience, we write rather than for the support function of K. Apparently, for , if and only if . The set will be viewed as equipped with the Hausdorff metric d defined by , where is the sup (or max) norm on the space of continuous functions on the unit sphere .
For a compact subset L of , which is star-shaped with respect to the origin, we shall use to denote its radial function; i.e., for ,
If is continuous and positive, L will be called a star body, and will be used to denote the class of star bodies in containing the origin in their interiors. Apparently, for , if and only if . Two star bodies K and L are said to be dilates (of one another) if is independent of . Let denote the radial Hausdorff metric as follows: if , then .
For , the polar body of K is defined by
Obviously, we have . For , the support and radial functions of the polar body of K are defined respectively by (see [18, 28])
for all .
Define the Santaló product of by . The Blaschke-Santaló inequality (see [18, 28]) is one of the fundamental affine isoperimetric inequalities. It states that if , then
with equality if and only if K is an ellipsoid.
2.2 The mixed p-quermassintegrals and dual mixed p-quermassintegrals
For and , the quermassintegrals of K are defined by (see [29])
From (2.2), we easily see that .
In the literature they have two representations, the quermassintegrals and the intrinsic volumes , and we shall use both throughout. They are defined by
For real , , and (not both zero), the Firey linear combination is defined by (see [30])
Note that ‘⋅’ rather than ‘’ is written for Firey scalar multiplication.
For , , and real , the mixed p-quermassintegrals of K and L, , are defined by (see [29])
Obviously, for , is just the classical mixed quermassintegrals . For , the mixed p-quermassintegrals are just the p-mixed volume .
For , and each , there exists a positive Borel measure on such that the mixed p-quermassintegrals have the following integral representation (see [29]):
for all . It turns out that the measure , , on is absolutely continuous with respect to , and has the Radon-Nikodym derivative
Together with (2.2) and (2.4), for , , we have .
An immediate consequence of the definition of Firey linear combination, and the integral representation (2.4), is that for , the mixed p-quermassintegrals are Firey linear.
Proposition 2.1 Suppose and . If , , then
It will be helpful to introduce the following notation. Define the inner radius and the outer radius of by
Recall that is the unit ball centered at the origin. Thus,
Obviously, the body K is contained in the closure of the annulus . Note that the notions of inner and outer radii as defined here are not translation invariant.
The next proposition shows that the functional is Lipschitzian. This observation will be needed in Sections 3 and 4.
Proposition 2.2 If and , , then
Proof The Minkowski integral inequality, together with (2.4) and (2.5), gives
□
For and any real i, the i th dual quermassintegrals of K are defined by (see [18, 28])
Obviously, .
An immediate consequence of the definition of i th dual quermassintegrals is as follows.
Proposition 2.3 If and , then the functional is continuous.
For , and (not both zero), the p-harmonic radial combination is defined by (see [19])
If (rather than being in ), then
For , , , and real , the dual mixed p-quermassintegrals of K and L are defined by (see [31])
If , we easily see that (2.9) is just the definition of dual p-mixed volume, i.e., .
From (2.9), the integral representation of the dual mixed p-quermassintegrals is given by Wang and Leng [31]: If , , and real , , then
Together with (2.8) and (2.10), for , , and , it follows that .
Further, Wang and Leng [31] proved the following analog of the Minkowski inequality for the dual mixed p-quermassintegrals.
Lemma 2.4 If , , then for or ,
with equality in every inequality if and only if K and L are dilates of each other. For , inequality (2.11) is reverse.
Another consequence of Lemma 2.4 will be needed.
Lemma 2.5 ([32])
Suppose , and . If real or , then
with equality in every inequality if and only if K and L are dilates of each other. For , inequality (2.12) is reverse.
2.3 The i th p-curvature function and i th p-curvature image
A convex body is said to have a continuous i th curvature function if its mixed surface area measure is absolutely continuous with respect to the spherical Lebesgue measure S and has the Radon-Nikodym derivative (see [29])
Let , , denote the sets of all bodies in , , , respectively, that have an i th positive continuous curvature function. In particular, , , .
If ∂K is a regular -hypersurface with (everywhere) positive principal curvatures, then for all i, and the curvature functions of K are proportional to the elementary symmetric functions of the principal radii of curvature (viewed as functions of the outer normals) of K. Thus, is the reciprocal Gauss curvature of ∂K at the point of ∂K whose outer normal is u, while is proportional to the arithmetic mean of the radii of curvature of ∂K at the point whose outer normal is u.
A convex body is said to have a p-curvature function if its p-surface area measure is absolutely continuous with respect to the spherical Lebesgue measure S and has the Radon-Nikodym derivative (see [19])
Lutwak [19] showed the notion of p-curvature image as follows: For each and , define , the p-curvature image of K, by
Note that for , this definition is different from the classical curvature image (see [19]).
Recently, Liu et al. [33], Lu and Wang [34] as well as Ma and Liu [35, 36] independently introduced the concept of i th p-curvature function of as follows: Let and , a convex body is said to have an i th p-curvature function if its i th p-surface area measure is absolutely continuous with respect to the spherical Lebesgue measure S and has the Radon-Nikodym derivative
If the i th surface area measure is absolutely continuous with respect to the spherical Lebesgue measure S, we have
According to the concept of i th p-curvature function of a convex body, Lu and Wang [34] and Ma [26] introduced independently the concept of i th p-curvature image of a convex body as follows: For each , and real , define , the i th p-curvature image of K, by
The unusual normalization of definition (2.18) is chosen so that, for the unit ball , it follows that . From definitions (2.15), (2.18) and formula (2.17), if , then .
An immediate consequence of the definition of i th p-curvature image and the integral representations of and is the following proposition.
Proposition 2.6 If , and , then
for all .
Recently, Ma [26] introduced the concept of -type p-affine surface area as follows: Let and , the -type p-affine surface area of is defined by
An immediate consequence of the definition of i th p-curvature image and the integral representations of and is the following proposition.
Proposition 2.7 If , , and , then
A body is of i th p-elliptic type if the function is the support function of a convex body in , i.e., K is of i th p-elliptic type if there exists a body such that
Define
An immediate consequence of the definition of and the definition of is the following.
Proposition 2.8 If , , and , then
3 The i th p-geominimal surface area
Let denote an orthogonal transformation group in . We will give the following lemmas and propositions.
Lemma 3.1 ([29])
Suppose , and , then for any ,
Lemma 3.2 ([37])
Suppose , and real as well as , , then, for any ,
Specifically,
An immediate consequence of the definition of and Lemma 3.1 and Lemma 3.2 is the following.
Proposition 3.3 Suppose . If , and , then
Lemma 3.4 If , and is a sequence of bodies in such that , then for , weakly.
Proof Suppose . Since , by the definition of support function, uniformly on . Since the continuous function is positive, are uniformly bounded away from 0. It follows that uniformly on , and thus
But also implies that
follows from the weak continuity of surface area measures (see, for example, Schneider [12, 19]). Hence,
or equivalently,
□
Lemma 3.5 Suppose and . If and , then .
Proof Since uniformly on , and is continuous, the are uniformly bounded on . Hence,
By Lemma 3.4, implies that
Hence,
□
By the definition of dual mixed p-quermassintegrals and the continuity of the radial function, we have the following.
Lemma 3.6 Suppose and . If , , and , , then .
An immediate consequence of the definition of and Lemma 3.5 and Lemma 3.6 is the following.
Proposition 3.7 For and , the function is upper semicontinuous.
Suppose and . For , define by
Since for , it follows from the integral representation (2.4) that, if L happens to belong to (rather than just to ), the new definition of agrees with the old definition.
An immediate consequence of Lemma 3.5 is as follows.
Lemma 3.8 If and , then is continuous.
The following simple fact will be needed.
Proposition 3.9 Suppose , and . If the sequence is bounded, then .
Proof Note that for and , , it follows that with equality if and only if A is an n-ball centered at the origin (see [38]). We chose two non-negative real numbers , such that for all j. Since L is compact, there exists a real such that , and since , the number may be chosen so that for all i, as well. (Recall that is the unit ball centered at the origin.)
For each j, let
where is any point where this minimum is attained. Since , it follows that contains the point . Since , it follows that .
Thus, contains the right cone whose apex is and whose base is an -dimensional ball of radius that lies in the subspace orthogonal to . Thus, for ,
and hence . Hence the ball, centered at the origin, of radius is contained in each , and thus this ball is contained in L as well. □
For , let denote the compact convex set whose support function, for , is given by
The fact that the function is convex and hence is the support function of a compact convex set is a direct consequence of the Minkowski integral inequality. Obviously, on . That on follows from the fact that the surface area measure of a convex body cannot be concentrated on a closed hemisphere of . If it were the case that , then , and thus the i th surface area measure would be concentrated on a closed hemisphere bounded by the great sphere of that is orthogonal to . Since is positive, .
For and , let denote the -dimensional quermassintegrals of , the image of the orthogonal projection of K onto the -dimensional subspace of that is orthogonal to u. The i th projection body of , , is the body whose support function is given by
for . (See the survey of Lutwak [39].) Since for ,
it follows from (2.5) that
As noted previously, on . However a slightly stronger statement will be needed in this section.
Lemma 3.10 For , , and , then
for all .
Proof Since from (2.5),
and
it follows from Jensen’s inequality that for all ,
To complete the proof, recall that . □
Proposition 3.11 If , , and , there exists a unique body such that
Proof Choose such that on . From the definition of , there exists a sequence such that with for all j, and
To see that are uniformly bounded, let
where is any of the points in at which this maximum is attained.
Since the support function of dominates that of the convex set , and since the measure is positive, it follows that . Hence,
Since are uniformly bounded, the Blaschke selection theorem guarantees the existence of a subsequence of , which will also be denoted by , and a compact convex , such that . Since , Proposition 3.9 gives . Now, implies that , and since , it follows that . Lemma 3.5 can now be used to conclude that L will serve as the desired body .
The uniqueness of the minimizing body is easily demonstrated as follows. Suppose , such that , and
Define by
Proposition 2.1 shows that
Since, obviously,
and , it follows from Lemma 2.5 that
with equality if and only if . Thus,
is the contradiction that would arise if it were the case that . □
The unique body whose existence is guaranteed by Proposition 3.11 will be denoted by and will be called the i th p-Petty body of K. The polar body of will be denoted by rather than . When , the subscript will often be suppressed. Thus, for , , and , the body is defined by
The next proposition shows that the mapping is an orthogonal transformation invariant mapping.
Proposition 3.12 If , , and , then for ,
Proof From the definition of and Proposition 3.3,
By Lemma 3.1,
The uniqueness part of Proposition 3.11 shows that , which is the desired result. □
In order to prove Lemma 3.14, the following fact will be needed.
Lemma 3.13 ([40])
If and , then
with equality if and only if K is an n-ball.
The following crude bound on the size of will be helpful.
Lemma 3.14 Suppose , , and . If are such that
then
for all .
Proof From the integral representation (2.4) and formulas (2.2) and (2.5), the trivial estimate follows:
From the minimality property of , it follows that
Let be any point in such that
Since the support function of dominates that of the convex set , it follows that
But from Lemma 3.10, Lemma 3.13 and formula (2.3), it follows that
The desired result is obtained by combining all these inequalities. □
If the outer radii of a sequence of bodies are uniformly bounded from above and the inner radii of the sequence are bounded away from 0, then the same is true for the radii of the i th p-Petty bodies of the sequence. This is contained in the following lemma.
Lemma 3.15 Suppose , . If is a family of bodies for which there exist such that
then there exist such that
Proof The existence of , and thus the fact that are uniformly bounded, is contained in Lemma 3.14. Let denote the inner radius of . Thus,
where is any point where this minimum is attained. Suppose that the infimum of is 0. Thus, there exists a subsequence of , which will not be relabeled, such that
The Blaschke selection theorem, in conjunction with Proposition 3.9, demonstrates the existence of such that for a subsequence of , which will also not be relabeled,
But and imply that , which is impossible since the continuous function is positive. □
The case of the following proposition is due to Lutwak [19]. The proof of this proposition is based on the one given by Petty and Lutwak.
Proposition 3.16 If and , then the functional is continuous.
Proof That is upper semicontinuous follows immediately from Lemma 3.8: The i th p-geominimal surface area
is defined as the infimum of continuous functions
as Q ranges over .
To see that is lower semicontinuous at , let be a sequence of bodies such that with . It will be shown that , and thus
By Lemma 3.15 the are uniformly bounded. The Blaschke selection theorem, in conjunction with Proposition 3.9, yields the existence of a body and a subsequence of , which will not be relabeled, such that and . Lemma 3.5 and the facts that and may be used to conclude that . Now , since . But the definition of shows that
and completes the argument. □
The case of the following result is due to Lutwak [19].
Proposition 3.17 If and , then the map is continuous.
Proof Suppose such that . Let denote a subsequence of . Since , Lemma 3.15 shows that are uniformly bounded. The Blaschke selection theorem, in conjunction with Proposition 3.9, yields the existence of a body and a subsequence of , which will not be relabeled, such that and . Lemma 3.5 and the facts that and may be used to conclude that . But by Proposition 3.16, . Hence, , and the uniqueness part of Proposition 3.11 shows that .
Hence, every subsequence of the sequence has a subsequence converging to . □
4 The i th p-geominimal surface area ratio
In [19], Lutwak defined the p-geominimal area ratio of K by
For , we define the i th p-geominimal area ratio of K as
and define the i th Santaló product of by .
The i th p-geominimal area ratio does not exceed the i th Santaló product divided by . To see this, just take in the definition of i th p-geominimal surface area
and get the following.
Proposition 4.1 If and , and , then
An immediate consequence of Proposition 4.1 is as follows.
Theorem 4.2 If and , and , then
Lemma 4.3 ([38])
If and , then
with equality if and only if K is an n-ball (centered at the origin).
Proof of Theorem 1.4 Inequality (4.1), together with (4.2), (2.3) and (2.1), yields
According to the conditions of equality in inequalities (4.2) and (2.1), we know that for equality of inequality (1.7) holds if and only if K is an ellipsoid for if and only if all -dimensional convex bodies contained in K are balls. □
Theorem 4.4 If , , and , then
with equality if and only if .
Proof Since and , Proposition 2.6 gives
Apply inequality (2.11) and get
with equality if and only if and are dilates of each other. Since , equality implies that , and by Proposition 2.8 this means that .
To see that if , there is equality in the inequality of the theorem, combine Proposition 2.6 with the definition of to get
Since , by Proposition 2.8, . Thus, gives and demonstrates the desired equality in the inequality. □
An immediate consequence of Theorem 4.4 and Proposition 2.7 is Theorem 1.5.
Suppose and with . Since
the Hölder inequality yields the following.
Proposition 4.5 Suppose , and . If , then
with equality if and only if there exists such that almost everywhere with respect to .
Suppose , and with . From the integral representation of , the easy estimate follows:
This gives the following proposition.
Proposition 4.6 Suppose , and . Then the function defined on by
is continuous.
The definition of of , by
together with Proposition 4.5, shows that the i th p-geominimal area ratios are monotone non-decreasing in p.
Proposition 4.7 If and , then for ,
The equality conditions for the inequality of Proposition 4.7 are given in Theorem 1.6.
Proposition 4.7 provides a key step in showing the following.
Proposition 4.8 If and , then the function defined on by
is continuous.
Proof Proposition 4.7 shows that the function defined by
is monotone. The continuity of will be demonstrated by establishing the continuity of ψ.
Suppose . By Proposition 3.11, there exist such that and
First assume that for all j. From the definition of i th p-geominimal surface area and Proposition 4.5, it follows that
The continuity of the function shows that
and hence
Now assume that for all j. That will be proven by showing that every subsequence of has a subsequence converging to . Let denote a subsequence of .
Lemma 3.14 shows that are uniformly bounded. Thus, the Blaschke selection theorem and Proposition 3.9 can now be used to deduce the existence of a subsequence of , which will also be denoted by , and a body , with , such that . Obviously,
By Proposition 4.6 the second term of this sum tends to 0. By Proposition 2.2 the first term in this sum is bounded by
and since , this also tends to 0. Hence,
where the inequality is justified by the definition of i th p-geominimal surface area. But by Proposition 4.8, ψ is monotone non-decreasing, and hence . □
For , let denote the compact set that is the support of the i th surface area measure of K; i.e., is the largest open subset of for which . Let denote the set of extreme normal directions of ∂K.
Lemma 4.9 Suppose and . If almost everywhere with respect to , then everywhere.
Proof Since and are continuous, and almost everywhere with respect to , it follows that on . But , and hence
But
shows that . Since , it follows that with equality if and only if .
We now show that indeed there is equality in this inequality, and hence . First note that since almost everywhere with respect to , it follows that almost everywhere with respect to . Hence, from the integral representation (2.4) it follows that
From this and the definition of i th p-geominimal surface area, it follows that
Hence . □
Proposition 4.1 states that the i th p-geominimal ratio is always dominated by the Santaló product (divided by ); i.e., for , and ,
The main result of this section is that in the limit (as ) these two quantities are equal.
Theorem 4.10 If , , and , then
Proof Since the first equality is an immediate consequence of Proposition 4.7, only the second equality needs to be demonstrated.
Proposition 3.11 guarantees the existence of such that and for all p.
By Lemma 3.14 it follows that there exists such that for all p. The Blaschke selection theorem and Proposition 3.9 may be used to deduce the existence of a subsequence of , which will also be denoted by , and a body with such that as .
Define . Now,
Proposition 2.2 shows that the first term in this sum is dominated by
and hence tends to 0 as . To see that the second term of this sum tends to 0, note that from (2.4) and (2.5) it follows that
where the maximum is taken over the support of the measure . But
is easily verified: First abbreviate . From the definition of , it follows that on and hence . From the definition of , it follows that on . Since , the set of extreme normal directions of ∂K is a subset of (see, e.g., Schneider [41]), on . Thus,
But implies that , and thus .
From the definition of it follows that and hence
Thus,
But from Proposition 4.1 it follows that
and this completes the proof. □
A body in will be called i th p-selfminimal if and K are dilates of each other. From this definition and Proposition 3.12 it follows that the class of i th p-selfminimal bodies is an orthogonal transformation invariant class of bodies.
The next proposition characterizes the i th p-selfminimal bodies as those bodies whose i th p-geominimal ratio is their i th Santaló product.
Proposition 4.11 If and , and , then
with equality if and only if K is ith p-selfminimal.
Proof The inequality is just Proposition 4.1. To obtain the equality conditions, first assume that K is i th p-selfminimal. Now, implies that
But implies that , and since , it follows that . This shows that there is equality in the inequality.
Suppose now that there is equality in the inequality of the proposition; i.e.,
Since and , rewrite the last displayed identity as follows:
Since the i th dual quermassintegrals of the polar body of are , it follows from the uniqueness of that
and thus that K is i th p-selfminimal. □
An immediate consequence of Proposition 4.11 and Proposition 4.7 is the fact that an i th p-selfminimal body is i th q-selfminimal for all .
An immediate consequence of Propositions 4.11 and 4.7 is that if is i th p-selfminimal, then
for all .
As the next proposition shows, this property characterizes the i th p-selfminimal bodies among bodies in ; i.e., if for some the i th q-geominimal area ratio of a body in is equal to its i th p-geominimal area ratio, then the body must be an i th p-selfminimal body (and thus in denote the set of convex bodies having their Santaló point at the origin).
Proof of Theorem 1.6 In light of Proposition 4.7, only the equality conditions need to be established. Assume that there is equality in this inequality. From Propositions 3.11 and 4.5, and the definition of i th p-geominimal surface area, it follows that
The hypothesis forces:
The middle equality and the equality conditions of Proposition 4.5 yield such that almost everywhere with respect to . The right equality and the uniqueness of show . The fact that now follows from Lemma 4.9. □
Theorem 1.6 contains a new characterization of selfminimal bodies as follows.
Proposition 4.12 If and , then for ,
with equality if and only if K is ith selfminimal.
When Proposition 4.7 is combined with Propositions 4.11 and 4.12, the result is that for and ,
Finally, we propose the following two open questions.
Conjecture 4.13 Suppose , , and . Does it follow that
With equality in inequality for if and only if K is an ellipsoid; for if and only if K is a ball.
Obviously, the case of Conjecture 4.13 is just the inequalities for the p-geominimal surface area by Lutwak (see [19]).
Conjecture 4.14 Suppose and . Does it follow that
With equality in inequality for if and only if K is an ellipsoid; for if and only if K is a ball.
Obviously, the case of Conjecture 4.14 is just the Blaschke-Santaló inequality (see [18, 28]).
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Acknowledgements
The author is indebted to the referee for valuable suggestions and a very careful reading of the original manuscript. This work is supported by the National Natural Science Foundation of China (Grant No. 11161019) and is supported by the Science and Technology Plan of the Gansu province (Grant No. 145RJZG227), and is partly supported by the National Natural Science Foundation of China (Grant No. 11371224).
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Ma, T. The ith p-geominimal surface area. J Inequal Appl 2014, 356 (2014). https://doi.org/10.1186/1029-242X-2014-356
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DOI: https://doi.org/10.1186/1029-242X-2014-356