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On containment measure and the mixed isoperimetric inequality
Journal of Inequalities and Applicationsvolume 2013, Article number: 540 (2013)
We first investigate whether for given convex domains , in the Euclidean plane, for any rotation α, there is a translation x so that or . Then, we estimate the mixed isoperimetric deficit of domains and via the known kinematic formulas of Poincaré and Blaschke in integral geometry. We obtain the sufficient condition for domain to contain, or to be contained in, convex domain . Finally, we obtain the mixed isoperimetric inequality and some Bonnesen-style mixed inequalities. Those Bonnesen-style mixed inequalities obtained are the known Bonnesen-style inequalities if one of the domains is a disc. As a direct consequence, we obtain the strengthened Bonnesen isoperimetric inequality.
1 Introductions and preliminaries
A set of points K in the Euclidean space is convex if for all and , . The convex hull of K is the intersection of all convex sets that contain K. The Minkowski sum of convex sets K and L is defined by
and the scalar product of convex set K for is defined by
A homothety of a convex set K is of the form for , . A convex body is a compact convex set with nonempty interior. A domain is a set with nonempty interior.
One may be interested in the following strong containment problem: Whether for given convex domains and , there exists a translation x so that or for any rotation α. It should be noted that this containment problem is much stronger than Hadwiger’s one. Therefore, the strong containment problem could lead to general and fundamental geometric inequalities (cf. [1–9]).
The well-known classical isoperimetric problem says that the disc encloses the maximum area among all domains of fixed perimeters in the Euclidean plane .
Proposition 1 Let Γ be a simple closed curve of length P in the Euclidean plane , then the area A of the domain K enclosed by Γ satisfies
The equality sign holds if and only if Γ is a circle.
Its analytic proofs root back to centuries ago. One can find some simplified and beautiful proofs that lead to generalizations of the discrete case, higher dimensions, the surface of constant curvature and applications to other branches of mathematics (cf. [1, 3–5, 10–53]).
The isoperimetric deficit
measures the difference between domain K of area A and perimeter P, and a disc of radius .
During the 1920s, Bonnesen proved a series of inequalities of the form
where the quantity is an invariant of geometric significance having the following basic properties:
is vanish only when K is a disc.
Many s are found during the past. The main interest is still focusing on those unknown invariants of geometric significance. See references [3–5, 12, 17, 23, 31, 32, 36] for more details. The following Bonnesen’s isoperimetric inequality is well known.
Proposition 2 Let K be a domain of area A, bounded by a simple closed curve of perimeter P in the Euclidean plane . Let r and R be, respectively, the maximum inscribed radius and minimum circumscribed radius of K. Then we have the following Bonnesen’s isoperimetric inequality:
where the equality holds if and only if K is a disc.
Since for any domain K in , its convex hull increases the area and decreases the perimeter , that is, and , then we have , that is, . Therefore, the isoperimetric inequality and the Bonnesen-style inequality are valid for all domains in if these inequalities are valid for convex domains.
In this paper, we first investigate the stronger containment problem: Whether for given convex bodies , in the Euclidean plane , there is a translation x so that or for any rotation α. Then we investigate the mixed isoperimetric deficit of domains and .
Definition 1 Let and be two domains of areas and , and of perimeters and , respectively. Then the mixed isoperimetric deficit of and is defined as
Since the convex hull of a set K in the Euclidean plane decreases the circum perimeter and increases the area, we have
Therefore, we can only consider the convex domains when we estimate the mixed isoperimetric deficit low bound.
Via the kinematic formulas of Poincaré and Blaschke in integral geometry, we obtain sufficient conditions for convex domain to contain, or to be contained in, another convex domain for a translation x and any rotation α (Theorem 1 and Theorem 2). We obtain the mixed isoperimetric inequality and some Bonnesen-style mixed inequalities (Theorem 3, Theorem 4, Corollary 2, Corollary 3, Corollary 4, Theorem 5 and Theorem 6). One immediate consequence of our results is the strengthening Bonnesen isoperimetric inequality (Corollary 3). These new Bonnesen-style mixed inequalities obtained are fundamental and generalize some known Bonnesen-style inequalities (Corollary 5).
2 The containment measure
Let () be two domains of areas with simple boundaries of perimeters in the Euclidean plane . Let dg denote the kinematic density of the group of rigid motions, that is, translations and rotations, in . Let be convex, and let () be a homothetic copy of , then we have the known kinematic formula of Poincaré (cf. [3, 36])
where denotes the number of points of intersection .
Let be the kinematic measure of the set of positions g, for which has exactly n intersection points with , i.e., . Notice that the measure for the odd n, then the formula of Poincaré can be rewritten as
We consider the homothetic copy () of .
The formula of Poincaré (7) and the formula of Blaschke (9) give
Since all are non-negative, we have
On the other hand, since domains () are assumed to be simply connected and bounded by simple curves, we have the number of connected components of the intersection . The fundamental kinematic formula of Blaschke (8) can be rewritten as
If μ denotes set of all positions of , in which either or , then the above formula of Blaschke can be rewritten as
When , each component of is bounded by at least an arc of and an arc of . Therefore, . Then by formulas of Poincaré and Blaschke, we obtain
Theorem 1 Let () be two domains of areas with simple boundaries of perimeters in . Let be convex. A sufficient condition for to contain, or to be contained in, another domain for a translation and any rotation, is
Moreover, if , then contains .
Theorem 2 Let () be two convex domains with areas and perimeters . Denote by the isoperimetric deficit of . Then a sufficient condition for , a homothetic copy of the convex domain , to contain domain for a translation and any rotation, is
Proof Condition (15) means that and
By Theorem 1, we conclude that either contains or is contained in . This inequality also leads to
The isoperimetric inequality guarantees that . We complete the proof of the theorem. □
3 Bonnesen-style mixed inequalities
Let , the maximum inscribed radius of with respect to , and , the minimum circum scribed radius of with respect to . Note that , are, respectively, the maximum inscribed radius, the minimum circum radius of when is the unit disc. It is obvious that . Therefore, for neither contains nor it is contained in . Then by Theorem 1, we have the following.
Theorem 3 Let () be two convex domains with areas and perimeters . Then
Corollary 1 Let K be a convex domain with a simple boundary ∂K of length P and area A. Denote by R and r, respectively, the radius of the minimum circumscribed disc and radius of the maximum inscribed disc of K. Then
By the two special cases of inequality (17):
we obtain the following.
Theorem 4 Let () be two convex domains in the Euclidean plane with areas and perimeters . If is convex, then
where the equality holds if and only if , that is, and are discs.
Proof By inequalities (19), we have
We complete the proof of Theorem 4. □
Corollary 2 (Kotlyar)
Let () be two domains in with areas and perimeters . If is convex, then
where the equality holds if and only if both and are discs.
Let be the unit disc, then Theorem 4 immediately leads to the following inequality that strengthens the Bonnesen isoperimetric inequality (4).
Corollary 3 Let K be a domain of area A, bounded by a simple closed curve of length P in the Euclidean plane . Let r and R be, respectively, the inscribed radius and circumscribed radius of K, then
where the equality holds if and only if K is a disc.
One immediate consequence of Theorem 4 is the following mixed isoperimetric inequality:
where the equality holds if and only if and are discs.
One may wish to consider the following Bonnesen-style mixed inequality:
where is an invariant of and . is, of course, assumed to be nonnegative and vanishes only when both and are discs.
The inequality (17) can be rewritten as the following several inequalities:
Therefore, we obtain the following Bonnesen-style mixed inequalities.
Corollary 4 Let () be two convex domains in the Euclidean plane with areas and perimeters . Then for , we have
Each inequality holds as an equality if and only if both and are discs.
On the other hand, let us consider the following Bonnesen quadratic polynomial
It is clear that and . If is convex, then the mixed isoperimetric inequality guarantees that two roots of exist and satisfy
The condition for existence of root(s) of the Bonnesen quadratic equation is the following symmetric mixed isoperimetric inequality:
The Bonnesen function attains minimum value at . The Bonnesen quadratic trinomial has only one root when . This means that both and are discs. This immediately leads to the following results.
Theorem 5 Let () be two convex domains of areas and perimeters in . Then
Theorem 6 Let () be two convex domains of areas and perimeters in the Euclidean plane . Then we have
Each equality holds if and only if and are discs.
Corollary 5 Let K be a plane domain of area A, bounded by a simple closed curve of length P. Let r and R be, respectively, the in-radius and out-radius of K. Then for any disc of radius t, , we have the following Bonnesen-style inequalities:
Each equality holds if and only if K is a disc.
It should be noted that the first inequality in (27) is due to Bonnesen, and he only derived some inequalities for 2-dimensional case and never had any progress for higher dimensions or 2-dimensional surface of constant curvature. One would be interested in the situations in higher dimensional space and in the surface of constant curvature. Related development in those areas can be found in [26, 35, 37, 63–66] and . More details for the isoperimetric inequality and Bonnesen style inequalities can be found in [67–80].
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Authors would like to thank two anonymous referees for many helpful comments and suggestions that directly lead to the improvement of the original manuscript. The corresponding author is supported in part by the NSFC (No. 11271302) and the Ph.D. Program of Higher Education Research Fund (No. 2012182110020).
The authors declare that they have no competing interests.
All authors read and approved the final manuscript.