Issue 
Wuhan Univ. J. Nat. Sci.
Volume 28, Number 2, April 2023



Page(s)  93  98  
DOI  https://doi.org/10.1051/wujns/2023282093  
Published online  23 May 2023 
Mathematics
CLC number: O186
A Characterization of the Polarity Mapping for Convex Bodies
School of Mathematics and Computational Science, Hunan University of Science and Technology, Xiangtan 411201, Hunan, China
^{†} To whom correspondence should be addressed. Email: wwang@hnust.edu.cn
Received:
26
June
2022
In this paper, we establish a characterization of the polarity mapping for 1dimensional convex bodies, which is a supplement to the result for such a characterization obtained by Böröczky and Schneider.
Key words: duality of convex bodies / polar convex body / involution / orderreversion
Biography: TANG Linzeng, male, Master candidate, research direction: Email: tlinzeng@163.com
Fundation item: Supported in part by the Natural Science Foundation of Hunan Province (2021JJ30235), the Scientific Research Fund of Hunan Provincial Education Department (21B0479) and Postgraduate Scientific Research Innovation Project of Hunan Province (QL20220228)
© Wuhan University 2023
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
0 Introduction
Let denote the ndimensional Euclidean vector space, equipped with its standard scalar product . We denote the set of convex bodies (compact convex subsets with nonempty interior) in which contain origin in the interior by . For , its dual or polar body is defined by (see, e.g., Ref.[1])
It is again in . Polar body is an important and fundamental notion of the dual theory of convex bodies, and this duality is one of the central concepts both in geometry and in analysis (see, e.g., Refs.[210]).
Mahler's conjecture (see, e.g., Ref.[11]), a famous open problem, is related to polar bodies. By we denote the class of all convex bodies in , and by we denote the class of all dimensional originsymmetric convex bodies in . Let , the volume product of and its polar body is defined by (see, e.g., Ref.[5]) , where denotes the dimensional volume of . Along the volume product, there is the Mahler's conjecture that: for ,
where equality holds for parallelepipeds and their polars (and other bodies). It is easily checked that for all . In 1939, Mahler^{ [12]} himself proved that for all , and in 1986, Reisner^{ [13]} characterized that equality holds only for parallelograms. Later, in 1991, Meyer^{ [14]} used some alternative methods to give a complete proof for the case , including the characterization of equality. Recently, Iriyeh and Shibata^{[15]} showed that the conjecture holds for the case n = 3 and equality holds if and only if K or K^{∗} is a parallelepiped. For the case n ≥ 4, Mahler's conjecture is still a challenging open problem.
Duality for convex functions can also be defined. For a convex function , its conjugate function is defined by (see, e.g., Ref.[16])
If is a lower semicontinuous convex function, then is also a lower semicontinuous convex function, and . This duality for lower semicontinuous convex functions can be characterized from two simple and natural properties: involution and orderreversion. ArtsteinAvidan and Milman^{ [17]} showed that any involution on the class of lower semicontinuous convex functions which is orderreversing, must be, up to linear terms, the wellknown Legendre transform. For more results on the characterizations of the duality for convex functions, sconcave functions and logconcave functions, we can refer to Refs.[1822].
Recently, Böröczky and Schneider^{ [1]} made use of an excellent tool that is lattice endomorphism from Gruber^{ [23,24]}, to characterize the duality mapping for convex bodies by interchanging the pairwise intersections and convex hulls of unions. Let denote the convex hull of unions (see Sect.1 Notations for details).
Theorem 1 Let and let be a mapping satisfying
for all . Then either is constant, or there exists a linear transformation such that for all .
It is important to point out that the property (the duality interchanges the pairwise intersections and convex hulls of unions) is sufficient for a characterization, up to a trivial exception (the constant map) and the composition with a linear transformation. A mapping is called involutive if
for all . If satisfies condition (3) and one of conditions (1) and (2), then satisfies conditions (1), (2) and (3), and is orderreversing. By replacing condition (1) with (3), Böröczky and Schneider^{ [1]} completely established a characterization of the duality mapping for convex bodies in with . Condition (3) excludes the constant map and forces the linear map appearing in the theorem to be selfadjoint.
Theorem 2 Let and let be a mapping satisfying
for all . Then there exists a selfadjoint linear transformation such that for all .
For more results on the characterization of duality and lattice endomorphism of the class of convex bodies and of convex sets, we can refer to Refs.[2529].
The main purpose of this paper is to establish a characterization of the duality mapping for convex bodies on 1dimensional Euclidean space with some additional assumptions. For simplicity, we will identify with as follows. And, obviously, if , where .
Theorem 3 Let be a mapping satisfying
for all and all real . Then, there exist constants with such that
for all ; or there exists a constant with such that
for all .
1 Notations
For reference, we collect some basic facts on convex sets and convex bodies. Excellent references are the books by Gardner^{ [30]}, Gruber^{ [31]} and Schneider^{ [16]}.
Let stand for the unit ball and the unit sphere of . A set is convex if for any two points , the line segment joining them is contained in , i.e.,
If are convex, then and are convex. A convex body is a compact convex subset of with nonempty interior.
The support function of a compact, convex is defined, for , by
It can be easily checked that the support function is sublinear, i.e., has the positive homogeneity of degree 1 and satisfies subadditive. From the definition, it follows immediately that, for , the support function of is given by .
A boundary point is said to have as an outer normal provided . The convex body is equipped with the Hausdorff metric , which is defined for convex bodies by
Let denote the convex hull of unions, i.e., for all . The set of convex bodies in containing the origin in their interior is denoted by . For , its dual or polar body is defined by
The duality mapping has a number of remarkable properties, of which we list the following; they are valid for all :
1) ;
2) implies ;
3) ;
4) ;
5) Continuity with respect to the Hausdorff metric;
6) If , then .
If is convex, then and . When , for , is convex. And for and real , is the form of the case in 6) above. For more interesting properties of the duality mapping of convex body, we can refer to Ref.[16], §1.6.
2 Main Results
Lemma 1 Let be a mapping satisfying
for all . Then, for all , the value of has the following four cases:
Case 1: ;
Case 2: ;
Case 3: ;
Case 4: ;
where are two decreasing positive functions on .
Proof Let , combining with (4), it follows that
which implies
for . Note that if , there is still . Then (6) induces
for all , where both and are decreasing positive functions with respect to and .
Suppose are arbitrary, it follows from (7) that
and
Without loss of generality, letting , . Then, we obtain from (4), (5) and (6) that
and
We now study the first result which implies the following three possibilities, and the others are similar.
(P1) If , then we have . Thus, is independent of the first variable . We obtain that can be rewritten as , which is a decreasing positive function on .
(P2) If , then we have . Thus, is independent of the second variable . We obtain that can be rewritten as , which is a decreasing positive function on .
(P3) If , then we have . Thus, is independent of the variable and . We obtain that is a constant positive function on . Since the decreasing function contains the constant function as a special case, can be rewritten as either in (P2) or in (P1).
Consequently, we may rewrite as either or , and similarly, we rewrite as either or , where are two decreasing positive functions on . Then, we conclude from (7) that has 4 different cases as desired.
A mapping is called orderreversing if
for .
Lemma 2 Let be a mapping satisfying (3) and (5) for all . Then is orderreversing and satisfies (4).
Proof Let , then, from (5), we obtain
which implies
for .
Suppose is not orderreversing and we may assume that there exist two convex bodies with such that . But, it follows from (3) that
which is a contradiction. Thus, (3) and (5) induce the property of : orderreversion.
Moreover, (3) and (5) induce
Applying (3) again, we conclude that
as desired.
Lemma 3 Let be a mapping satisfying (3) and (5) for all . Then, there exist two strictly decreasing positive functions , on with the properties that , such that
where , denote the inverse function of ,, respectively; or there exists a strictly decreasing positive function on such that
where denotes the inverse function of .
Proof is orderreversing and satisfies (3), (4) and (5) via Lemma 2. Then, the functions , must be strictly decreasing positive functions with respect to and on . Thus, together with Lemma 1, we obtain that the functions , be rewritten as , and ,, where , are two strictly decreasing positive functions on and that the forms of four cases in Lemma 1 are remained tentatively.
However, since is orderreversing, both Case 1 and Case 4 are removed when are two strictly decreasing positive functions on . We show the contradiction for Case 1 (Case 4 is similar). Suppose are arbitrary and is fixed, then it follows from (8) that
And, combining with the form of Case 1, we obtain that
and
A contradiction occurs.
Now, we further study Case 2 and Case 3 with the conditions (3) and (5).
(i) From the form of Case 2 and above, we deduce that , where , are two strictly decreasing positive functions on . Then, from (3), we have
which implies
for all . Thus,
on , where , denote the inverse function of , , respectively. Therefore, we obtain
where , are two strictly decreasing positive functions on with the properties that , .
(ii) From the form of Case 3 and above, we deduce that , where , are two strictly decreasing positive functions on . Then, from (3), we have
which implies
for all . Thus,
on , where denotes the inverse function of . Therefore, we conclude that
where is a strictly decreasing positive function on . Certainly, is also a strictly decreasing positive function on .
A mapping is called homogeneous if there exists such that
for all and all . With the additional assumption of homogeneity, we establish a characterization of polarity or duality mapping for 1dimensional convex bodies.
Proof of Theorem 3 Recall that satisfies , and for all and real . Together with Lemmas above, we finish the proof of Theorem 3 with two cases.
(i) Due to Lemma 3, if (9) holds, then
for all and real , where , are two strictly decreasing positive functions on with the properties that , . Since satisfies the homogeneity of degree 1, we obtain
for all and real . Thus,
for all and real , which implies
for all , where . Thus,
for all , where .
(ii) Due to Lemma 3, if (10) holds, then
for all and real , where is a strictly decreasing positive function on . And, from the homogeneity of , we deduce that
for all and real . Thus,
for all and real , which implies
for all , where . Thus,
for all , where .
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