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Article

A Characterization of Three-Dimensional Convex Polytopes with Five Pairwise Antipodal Vertices

School of Mathematics, North University of China, Taiyuan 030051, China
*
Author to whom correspondence should be addressed.
These authors contributed equally to this work.
Mathematics 2025, 13(9), 1412; https://doi.org/10.3390/math13091412
Submission received: 12 March 2025 / Revised: 18 April 2025 / Accepted: 23 April 2025 / Published: 25 April 2025

Abstract

:
Concerning the antipodality properties of finite sets, we focus on convex polytopes in  R 3  with less than  2 3  vertices and characterize convex polytopes with 5 vertices that are pairwise antipodal.

1. Introduction

Let A be a subset of  R n  and  a , b A . If there are distinct parallel hyperplanes  H a  and  H b  passing through a and b, respectively, such that A lies on the slab between  H a  and  H b , then a and b are said to be (affinely) antipodal. A classical result in Convex and Discrete Geometry states that the maximum cardinality of a subset A of  R n , whose points are pairwise antipodal, is  2 n , and that a subset A of  R n  consisting of  2 n  points that are pairwise antipodal is the set of vertices of an n-dimensional parallelotope; see [1,2]. It is clear that the vertices of an n-dimensional parallelotope in  R n  are pairwise antipodal. Thus, the result mentioned above provides a characterization of n-dimensional convex polytopes with  2 n  vertices that are pairwise antipodal. Since an n-dimensional convex polytope has at least  n + 1  vertices, it is natural to ask for characterizations of n-dimensional convex polytopes with m ( n + 1 m < 2 n ) vertices that are pairwise antipodal. We refer to [3,4,5] for estimations of the cardinality of antipodal pairs of points in a finite subset in  R n .
Note that antipodality has many applications in Convex and Discrete Geometry. For example, this notion is closely related to the study of Hadwiger’s covering conjecture, a long-standing open problem in Convex and Discrete Geometry.
Let K be a convex compact set whose affine dimension is at least 1. The closure, interior, relative interior, boundary, and relative boundary of K are denoted by  cl K int K relint K bd K , and  relbd K , respectively. Let  x relbd K  and  u R n  be a direction (i.e., a nonzero vector). The point x is said to be illuminated by u if there exists a positive number  λ  such that
x + λ u relint K .
If each point of  relbd K  can be illuminated by at least one of the directions  u 1 , u 2 , , u m , then we say that  relbd K  is illuminated by  u 1 , u 2 , , u m .
Hadwiger’s covering conjecture asserts that each n-dimensional convex body (compact convex set whose affine dimension is n) that is not affinely equivalent to  [ 0 , 1 ] n  can be illuminated by at most  2 n 1  directions. While the case for  n = 2  has been fully resolved (cf. [6]), this conjecture remains open for  n 3 . For a deeper dive into this fascinating conjecture, we recommend the monographs cited in [7,8] and the survey articles in [9,10,11,12].
It is clear that two points in the boundary of a convex body K are antipodal if and only if they cannot be illuminated by the same direction (cf. Lemma 1 below). Therefore, the least number of directions needed to illuminate  bd K  is not less than the maximum number of points in  bd K  that are pairwise antipodal. Moreover, it is not difficult to verify that each n-dimensional convex polytope having at most  2 n  vertices can be illuminated by at most  2 n  directions, and  2 n  directions are necessary only if such a convex polytope is an n-dimensional parallelotope (cf. [13]). To study the illumination of an n-dimensional convex polytope K with  2 n + m  vertices when m is not too large, one can distinguish several cases based on the maximum number of vertices of such a polytope that are pairwise antipodal. Let  V 1  be the set of vertices of K that is maximally pairwise antipodal and let  V 2  be the set of the rest vertices. Then  K = conv ( conv V 1 conv V 2 ) , where  conv V 1  is an n-dimensional convex polytope whose vertices are pairwise antipodal. For this purpose, it is also important to develop characterizations of n-dimensional convex polytopes with m vertices that are pairwise antipodal.
In this paper, we present a characterization of three-dimensional convex polytopes with five pairwise antipodal vertices. Such polytopes are special cases of n-dimensional polytopes with  n + 2  vertices. Combinatorial properties of this type of polytopes can be found in Section 6.1 of [14].
For each convex body K and each  ( n 1 ) -dimensional subspace H of  R n , there exists a pair of distinct parallel supporting hyperplanes  H 1  and  H 2  of K that are parallel to H. The closed subset of  R n  bounded by  H 1  and  H 2  is called a supporting slab of K.
For each positive integer m, set
[ m ] = i Z + | 1 i m .
Let  x , y R n . The inner product of x and y is denoted as  x | y . The line segment connecting x and y is denoted by  [ x , y ] . I.e.,
[ x , y ] = { λ x + ( 1 λ ) y | λ [ 0 , 1 ] } .
When  x y , set
[ x , y ) = λ x + ( 1 λ ) y | 0 < λ 1 , ( x , y ] = λ x + ( 1 λ ) y | 0 λ < 1 , ( x , y ) = λ x + ( 1 λ ) y | 0 < λ < 1 .
The following two results follow directly from the classical result in [1].
Corollary 1.
Let  Y R n  be an m-dimensional affine subset and  X Y . Then there are at most  2 m  points in X that are pairwise antipodal.
Corollary 2.
If X is an m-dimensional convex polytope with  2 m  vertices, then the  2 m  vertices are pairwise antipodal in X if and only if X is an m-dimensional parallelotope.
Lemma 1.
Let  K R n  be a convex body. Two points  x , y bd K  are not antipodal if and only if there is a direction that illuminates both x and y.
Proof. 
Lemma 5 in [15] shows that if x and y are not antipodal, then they can be illuminated by a direction.
Conversely, suppose that x and y are illuminated by a direction u. Then there exist  λ 1 , λ 2 > 0  such that  x + λ 1 u , y + λ 2 u int K . Let  λ = min { λ 1 , λ 2 } . Then  x + λ u , y + λ u int K . Hence,  [ x , y ]  is not a longest chord of K that is parallel to the line passing through x and y. By 3.1 in [16], x and y are not antipodal. □
The proof of the following result demonstrates the usage of antipodality in the study of Hadwiger’s covering conjecture.
Lemma 2.
Let K be an n-dimensional convex polytope in  R n  having  2 n + 1  vertices. Then K can be illuminated by at most  2 n  directions.
Proof. 
Since there are at most  2 n  points in  R n  that are pairwise antipodal (see [1] again), there exists a pair x and y of vertices of K that are not antipodal. By Lemma 1, x and y can be illuminated by a single direction, and the rest of the vertices can be illuminated by at most  2 n 1  directions. □
The upper bound in Lemma 2 is not optimal if Hadwiger’s covering conjecture has an affirmative answer in  R n : the boundary of such a polytope can be illuminated by at most  2 n 1  directions. To improve the estimation in Lemma 2, one needs characterizations of convex polytopes in  R n  having  2 n 1  pairwise antipodal vertices.
Let K be a compact convex set whose affine dimension is at least 1 and  x relbd K . Let
I ( K , x ) = u R n { o } | λ > 0 s . t . x + λ u relint K .
I.e.,  I ( K , x )  is the set of directions that can illuminate x as a relative boundary point of K. If K is a convex body in  R n , then  I ( K , x )  is an open subset of  R n ; see [17].
Let  A R n  be a nonempty closed convex set and  x A . The tangent cone of A at x is the closure of the cone generated by  A { x } —i.e.,
T A ( x ) = cl d R n | u A , α 0 s . t . d = α ( u x ) .
As a closed convex set,  T A ( x )  can also be described as an intersection of closed halfspaces (cf. page 65 in [18]).
Lemma 3.
Let K be a convex body in  R n  and  x bd K . Then
I ( K , x ) = int T K ( x ) a n d cl I ( K , x ) = T K ( x ) .
Proof. 
For any  d I ( K , x ) , there exists  λ > 0  such that  x + λ d int K . Let  u = x + λ d . Then  u K  and
d = 1 λ ( u x ) T K ( x ) .
Thus,  I ( K , x ) T K ( x )  and therefore  cl I ( K , x ) T K ( x ) .
Suppose that  d = α ( u x )  where  u K  and  α 0 . We claim that  d cl I ( K , x ) . If d is the origin o, then, for any  v I ( K , x )  and any positive integer k ( 1 / k ) v  is also a direction in  I ( K , x )  and  ( 1 / k ) v o  as  k . Thus,  d cl I ( K , x ) . Now, assume that  d o . If  u int K , then obviously  d I ( K , x ) . Otherwise,  u bd K . For any positive integer k, there exists  u k B ( u , 1 / k ) int K , where  B ( u , 1 / k )  is the closed Euclidean ball centered at u having radius  1 / k . Thus,  d k = α ( u k x ) I ( K , x )  and  d k d  as  k . Hence,  d cl I ( K , x ) . It follows that
d R n | u K , α 0 s . t . d = α ( u x ) cl I ( K , x ) .
Therefore,  T K ( x ) cl I ( K , x ) . Thus,  cl I ( K , x ) = T K ( x ) . Since  I ( K , x )  is open, we have (cf. Theorem 2.38 in [19])
int T K ( x ) = int ( cl I ( K , x ) ) = int I ( K , x ) = I ( K , x ) .
The proof is complete. □
We end this section with the following lemma.
Lemma 4.
Let  K R n  be a convex body and  x 1 , x 2 bd K . Then  x 2  can be illuminated by a direction in  I ( K , x 1 )  if and only if  ( I ( K , x 1 ) + x 2 ) K .
Proof. 
If there exists a direction  u I ( K , x 1 )  such that u illuminates  x 2 , then there exists  λ > 0  such that  x 2 + λ u int K . Clearly,  λ u I ( K , x 1 ) . Thus,  ( I ( K , x 1 ) + x 2 ) K .
Conversely, assume that  ( I ( K , x 1 ) + x 2 ) K . Let c be a fixed point in  int K . There exists  u I ( K , x 1 )  such that  u + x 2 K . Since  I ( K , x 1 )  is open, there exists  γ > 0  such that the closed Euclidean ball  B ( u , γ )  centered at u having radius  γ  is contained in  I ( K , x 1 ) . Let v be a point in  [ u + x 2 , c ] int K  such that  | | v ( u + x 2 ) | | < γ . Then,
v x 2 = u + ( v ( u + x 2 ) ) I ( K , x 1 ) and v = x 2 + ( v x 2 ) int K .
I.e.,  x 2  is illuminated by a direction in  I ( K , x 1 ) . □

2. The Convex Hull of the Union of a Simplex and a Singleton

Throughout this section,  S R n  is an n-simplex with  x 0 , x 1 , , x n  as vertices and
C = conv x i | i [ n ] .
For each  j [ n ] { 0 } , let
H j = aff ( x i | i [ n ] { 0 } { x j } )
and  e j  be the unit vector in  ( H j H j )  such that  x j | e j > x | e j , x H j . Clearly,  · | e j  is constant on  H j . Let  H j +  be the open halfspace bounded by  H j  that contains  x j  and let  H j  be the other open halfspace bounded by  H j . Obviously,
H i + = x R n | x | e i > x 0 | e i , i [ n ] .
Let y be a point in  H 0  and  K y = conv ( { y } S ) . One can easily verify that both  x 0  and y are vertices of  K y .
We have the following characterizations of  I ( S , x 0 )  and its closure.
Lemma 5.
We have
I ( S , x 0 ) = x R n | x | e i > 0 , i [ n ]
and
cl I ( S , x 0 ) = x R n | x | e i 0 , i [ n ] .
Proof. 
Note that
S = x R n | x | e 0 x 1 | e 0 , x | e i x 0 | e i , i [ n ] .
By Example 5.2.6(b) in [18],
T S ( x 0 ) = x R n | x | e i 0 , i [ n ] .
From Lemma 3, the desired equalities follow. □
We shall also use the following representation of  I ( S , x ) .
Lemma 6.
We have
u I ( S , x 0 ) u + x 0 i [ n ] H i + .
Proof. 
By Lemma 5, we have
u I ( S , x 0 ) u | e i > 0 , i [ n ] u + x 0 | e i > x 0 | e i , i [ n ] u + x 0 i [ n ] H i + .
The proof is complete. □
Lemma 7.
The following statements are equivalent:
1. 
[ x 0 , y ] C ,
2. 
I ( S , x 0 ) = I ( K y , x 0 ) .
Proof. 
1⇒2. Since  S K y , we have  I ( S , x 0 ) I ( K y , x 0 ) . If  I ( K y , x 0 ) I ( S , x 0 ) , then there exists  u I ( K y , x 0 ) I ( S , x 0 ) . Since  I ( K y , x 0 )  is open, we may require further that  u I ( K y , x 0 ) cl I ( S , x 0 ) . There exists  λ > 0  such that
x : = x 0 + λ u int K y .
Then
x 0 + γ u int K y , γ ( 0 , λ ) .
Therefore we may assume, without loss of generality, that  x H 0 + .
Since  u cl I ( S , x 0 ) , by Lemma 5, there exists  i [ n ]  such that  x 0 + λ u | e i < x 0 | e i . I.e.,  x H i + . Without loss of generality, we may assume that  x H n + . By Theorem 3.13 in [19], there exist  x S  and  γ [ 0 , 1 ]  such that  x = γ y + ( 1 γ ) x . Since  x S  and  y int K y γ ( 0 , 1 ) . Then
x | e n = λ y | e n + ( 1 λ ) x | e n λ y | e n + ( 1 λ ) x 1 | e n > λ y | e n + ( 1 λ ) x | e n .
Thus,  x 1 | e n > x | e n > y | e n . Since  y H 0  and  [ x 0 , y ] C , there exist  z C  and  η > 0  such that  y = z + η ( z x 0 ) . Since z is a convex combination of  x 1 , , x n , we have
z | e n x 0 | e n = x 1 | e n .
It follows that  y | e n x 1 | e n , a contradiction.
2⇒1. Clearly,  [ x 0 , y ] H 0  is a singleton, namely,  { h } . If  h C , then there exists  i [ n ]  such that h is not in  H i + . Assume, without loss of generality, that  i = 1 . Since  h ( x 0 , y ) , there exists  γ ( 0 , 1 )  such that  h = γ y + ( 1 γ ) x 0 . We have
h | e 1 = γ y | e 1 + ( 1 γ ) x 0 | e 1 < x 0 | e 1 .
Thus,  y | e 1 < x 0 | e 1 . Let  x  be the point of intersection of  H 1  and  [ y , ( i = 0 n x i ) / ( n + 1 ) ] . Then
x = x 0 + x x 0 ( int K y ) int S ,
a contradiction to the assumption that  I ( S , x 0 ) = I ( K y , x 0 ) . □
Lemma 8.
Suppose that  relint C [ x 0 , y ] = { c 0 }  and  i [ n ] . The following statements are equivalent:
1. 
The points  x 0  and  x i  are antipodal in  K y .
2. 
y I ( S , x 0 ) + x i .
Proof. 
It is clear that  c 0 int K y . By Lemma 7,  I ( S , x 0 ) = I ( K y , x 0 ) . Take the case when  i = 1 , for example.
1⇒2. Suppose the contrary that  y I ( S , x 0 ) + x 1 . Since  I ( S , x 0 ) + x 1  is open (cf. [17]), there exists  α > 0  such that
B ( y , α ) I ( S , x 0 ) + x 1 ,
where  B ( y , α )  is the closed ball centered at y having radius  α . Let  y  be a point in  [ c 0 , y ) B ( y , α ) . Then  y int K y . Since
y I ( S , x 0 ) + x 1 = I ( K y , x 0 ) + x 1 ,
there exists a direction that illuminates  x 0  and  x 1  as boundary points of  K y  simultaneously, a contradiction to the fact that  x 0  and  x 1  are antipodal.
2⇒1. Since  c 0 int K y , we have
y I ( K y , x 0 ) + x 0 = I ( S , x 0 ) + x 0 .
Thus,  y | e i > x 0 | e i i [ n ] . By Lemma 4, it suffices to show that
( I ( K y , x 0 ) + x 1 ) K y = ( I ( S , x 0 ) + x 1 ) K y = .
For any  z K y , there exist  s S  and  λ [ 0 , 1 ]  such that  z = λ s + ( 1 λ ) y . From  y I ( S , x 0 ) + x 1 , it follows that  y | e 1 x 1 | e 1 . Actually, if  y | e 1 > x 1 | e 1 , then  y | e i > x 1 | e i  holds for all  i [ n ]  because  y | e j > x 0 | e j = x 1 | e j j = 2 , , n . This implies that  y x 1 int T S ( x 0 ) = I ( S , x 0 ) , a contradiction. Hence,
z | e 1 = λ s | e 1 + ( 1 λ ) y | e 1 λ x 1 | e 1 + ( 1 λ ) x 1 | e 1 = x 1 | e 1 .
By Lemmas 5 and 7,
I ( K y , x 0 ) + x 1 = I ( S , x 0 ) + x 1 = x R n | x | e i > x 1 | e i , i [ n ] .
Thus,  ( I ( K y , x 0 ) + x 1 ) K y = . □
Corollary 3.
If  relint C [ x 0 , y ] , then, for each  i [ n ] x 0  and  x i  are antipodal in  K y  if and only if
y i [ n ] I ( S , x 0 ) + x i .
Results in this section have potential applications for characterizing antipodality of vertices of convex polytopes in high-dimensional spaces and can be directly applied to the three-dimensional case in the next section.

3. Main Results

Set
P = { ( β 1 , β 2 , β 3 ) 0 < β 1 1 , 0 < β 2 1 , β 1 + β 2 1 , β 1 + β 3 0 , β 2 + β 3 0 , 0 β 1 + β 2 + β 3 < 1 } ,
y 0 = ( 0 , 1 , 0 ) , y 1 = ( 0 , 0 , 1 ) , y 3 = ( 0 , 0 , 0 ) , and y 4 = ( 1 , 0 , 0 ) .
Put
P = conv { y 0 , y 1 , y 2 , y 3 , y 4 } | y 2 P .
For each  K P , let
I = { 1 , 2 } , J = { 0 , 3 , 4 } , C 1 = conv y i | i I , and C 2 = conv y j | j J .
Proposition 1.
Each member of  P  is a convex polytope with five pairwise antipodal vertices.
Proof. 
Suppose that  K = conv y i | i [ 4 ] { 0 } P . Note that
H 0 : = aff { y 1 , y 3 , y 4 } = ( β 1 , β 2 , β 3 ) | β 2 = 0 , H 1 : = aff { y 0 , y 3 , y 4 } = ( β 1 , β 2 , β 3 ) | β 3 = 0 , H 3 : = aff { y 0 , y 1 , y 4 } = ( β 1 , β 2 , β 3 ) | β 1 + β 2 + β 3 = 0 ,
and
H 4 : = aff { y 0 , y 1 , y 3 } = ( β 1 , β 2 , β 3 ) | β 1 = 0 .
Since  β 3 β 1 < 0 H 1 + y 1  and  H 1 + y 2  are parallel supporting hyperplanes of K passing through  y 1  and  y 2 , respectively. Thus  y 1  and  y 2  are antipodal. From  0 < β 1 1  and  0 < β 2 1 , it follows that  H 0  and  H 0 + y 0 , and  H 4  and  H 4 + y 4  are two pairs of parallel supporting hyperplanes of K. Thus  y 0 y 3 , and  y 4  are pairwise antipodal.
Let  y 2 = ( α 1 , α 2 , α 3 )  and  { c } = C 1 aff C 2 . There exists  λ ( 0 , 1 )  such that
c = ( 1 λ ) y 1 + λ y 2 = ( λ α 1 , λ α 2 , ( 1 λ ) + λ α 3 ) .
Note that
relint C 2 = ( β 1 , β 2 , 0 ) | 0 < β 1 < 1 , 0 < β 2 < 1 , 0 < β 1 + β 2 < 1 .
Since  y 2 P , we have  0 < λ α 1 < 1 0 < λ α 2 < 1 , and
λ α 1 + λ α 2 + ( 1 λ ) + λ α 3 = ( 1 λ ) + λ ( α 1 + α 2 + α 3 ) < ( 1 λ ) + λ = 1 .
From  c aff C 2 = H 1 , it follows that  ( 1 λ ) + λ α 3 = 0 . Therefore,  λ α 1 + λ α 2 < 1 , which shows that  c relint C 2 . Hence,  C 1 relint C 2 , which implies that
y 2 P 1 : = ( β 1 , β 2 , β 3 ) | β 1 > 0 , β 2 > 0 , β 3 < 0 , β 1 + β 2 + β 3 < 1 .
Consider the three-simplex  conv { y 0 , y 1 , y 3 , y 4 }  in  R 3 . Since
aff { y 1 , y 0 , y 3 } = ( β 1 , β 2 , β 3 ) | β 1 = 0 , aff { y 1 , y 0 , y 4 } = ( β 1 , β 2 , β 3 ) | β 1 + β 2 + β 3 = 1 , aff { y 1 , y 3 , y 4 } = ( β 1 , β 2 , β 3 ) | β 2 = 0 ,
by viewing  y 1  and  y 2  as  x 0  and y in Section 2, respectively, from Lemmas 6 and 7, it follows that
I ( K , y 1 ) + y 1 = ( β 1 , β 2 , β 3 ) | β 1 > 0 , β 2 > 0 , β 1 + β 2 + β 3 < 1 .
Thus,
I ( K , y 1 ) + y 3 = ( β 1 , β 2 , β 3 ) | β 1 > 0 , β 2 > 0 , β 1 + β 2 + β 3 < 0 , I ( K , y 1 ) + y 4 = ( β 1 , β 2 , β 3 ) | β 1 > 1 , β 2 > 0 , β 1 + β 2 + β 3 < 1 , I ( K , y 1 ) + y 0 = ( β 1 , β 2 , β 3 ) | β 1 > 0 , β 2 > 1 , β 1 + β 2 + β 3 < 1 .
Let
D 1 = ( β 1 , β 2 , β 3 ) | β 2 0 , D 2 = ( β 1 , β 2 , β 3 ) | β 1 + β 2 + β 3 0 , D 3 = ( β 1 , β 2 , β 3 ) | β 1 0 , D 4 = ( β 1 , β 2 , β 3 ) | β 2 1 , D 5 = ( β 1 , β 2 , β 3 ) | β 1 + β 2 + β 3 1 , and D 6 = ( β 1 , β 2 , β 3 ) | β 1 1 .
Then
( I ( K , y 1 ) + y 3 ) c = D 1 D 2 D 3 , ( I ( K , y 1 ) + y 4 ) c = D 1 D 5 D 6 , ( I ( K , y 1 ) + y 0 ) c = D 3 D 4 D 5 .
It follows that
P 2 : = P 1 j J ( I ( K , y 1 ) + y j ) c = P 1 ( D 1 D 2 D 3 ) ( D 1 D 5 D 6 ) ( D 3 D 4 D 5 ) = ( P 1 D 2 ) ( P 1 D 6 ) ( P 1 D 4 ) = ( β 1 , β 2 , β 3 ) | 0 < β 1 1 , 0 < β 2 1 , β 3 < 0 , 0 β 1 + β 2 + β 3 < 1 .
Similarly,  conv { y 0 , y 2 , y 3 , y 4 }  is a three-simplex in  R 3 . View  y 2  and  y 1  as  x 0  and y in Section 2, respectively. Since
aff { y 2 , y 3 , y 4 } = ( β 1 , β 2 , β 3 ) | α 3 β 1 α 1 β 3 = 0 , aff { y 2 , y 0 , y 3 } = ( β 1 , β 2 , β 3 ) | α 3 β 2 α 2 β 3 = 0 , aff { y 2 , y 0 , y 4 } = ( β 1 , β 2 , β 3 ) | α 3 β 1 + α 3 β 2 + ( 1 α 1 α 2 ) β 3 = α 3 ,
by Lemmas 6 and 7, we have
I ( K , y 2 ) + y 2 = { ( β 1 , β 2 , β 3 ) | α 3 β 1 α 1 β 3 < 0 , α 3 β 2 α 2 β 3 < 0 , α 3 β 1 + α 3 β 2 + ( 1 α 1 α 2 ) β 3 > α 3 } .
Thus,
I ( K , y 2 ) + y 3 = { ( β 1 , β 2 , β 3 ) | α 3 β 1 α 1 β 3 < 0 , α 3 β 2 α 2 β 3 < 0 , α 3 β 1 + α 3 β 2 + ( 1 α 1 α 2 ) β 3 > 0 } , I ( K , y 2 ) + y 4 = { ( β 1 , β 2 , β 3 ) | α 3 β 1 α 1 β 3 < α 3 , α 3 β 2 α 2 β 3 < 0 , α 3 β 1 + α 3 β 2 + ( 1 α 1 α 2 ) β 3 > α 3 } , I ( K , y 2 ) + y 0 = { ( β 1 , β 2 , β 3 ) | α 3 β 1 α 1 β 3 < 0 , α 3 β 2 α 2 β 3 < α 3 , α 3 β 1 + α 3 β 2 + ( 1 α 1 α 2 ) β 3 > α 3 } .
Let
D 1 = ( β 1 , β 2 , β 3 ) | α 3 β 2 α 2 β 3 0 , D 2 = ( β 1 , β 2 , β 3 ) | α 3 β 1 + α 3 β 2 + ( 1 α 1 α 2 ) β 3 0 , D 3 = ( β 1 , β 2 , β 3 ) | α 3 β 1 α 1 β 3 0 , D 4 = ( β 1 , β 2 , β 3 ) | α 3 β 2 α 2 β 3 α 3 , D 5 = ( β 1 , β 2 , β 3 ) | α 3 β 1 + α 3 β 2 + ( 1 α 1 α 2 ) β 3 α 3 , D 6 = ( β 1 , β 2 , β 3 ) | α 3 β 1 α 1 β 3 α 3 .
Then
( I ( K , y 2 ) + y 3 ) c = D 1 D 2 D 3 , ( I ( K , y 2 ) + y 4 ) c = D 1 D 5 D 6 , ( I ( K , y 2 ) + y 0 ) c = D 3 D 4 D 5 .
Thus,  y 1 ( I ( K , y 2 ) + y 3 ) c  if and only if
y 2 P 3 : = ( β 1 , β 2 , β 3 ) | β 1 0 or β 2 0 or β 1 + β 2 1 .
Similarly,  y 1 ( I ( K , y 2 ) + y 4 ) c  and  y 1 ( I ( K , y 2 ) + y 0 ) c  are equivalent to
y 2 P 4 : = ( β 1 , β 2 , β 3 ) | β 2 0 or β 1 + β 2 + β 3 1 or β 1 + β 3 0
and
y 2 P 5 : = ( β 1 , β 2 , β 3 ) | β 1 0 or β 2 + β 3 0 or β 1 + β 2 + β 3 1 ,
respectively.
Since
i [ 5 ] P i = { ( β 1 , β 2 , β 3 ) | 0 < β 1 1 , 0 < β 2 1 , β 3 < 0 , β 1 + β 2 1 , 0 β 1 + β 2 + β 3 < 1 } P 4 P 5 = { ( β 1 , β 2 , β 3 ) | 0 < β 1 1 , 0 < β 2 1 , β 3 < 0 , β 1 + β 2 1 , β 1 + β 3 0 , 0 β 1 + β 2 + β 3 < 1 } P 5 = { ( β 1 , β 2 , β 3 ) | 0 < β 1 1 , 0 < β 2 1 , β 3 < 0 , β 1 + β 2 1 , β 1 + β 3 0 , β 2 + β 3 0 , 0 β 1 + β 2 + β 3 < 1 } = P ,
from  y 2 P  it follows that
y 2 j J ( I ( K , y 1 ) + y j ) c and y 1 j J ( I ( K , y 2 ) + y j ) c .
By Lemma 7 and Corollary 3, for each  i I y i  and each point in  y j | j J  are antipodal. Hence, the vertices of K are pairwise antipodal. □
Remark 1.
In fact,  P = conv { a i | i [ 5 ] } { a 2 , a 3 , a 4 } , where
a 1 = ( 1 2 , 1 2 , 1 2 ) , a 2 = ( 1 , 0 , 1 ) , a 3 = ( 1 , 1 , 1 ) , a 4 = ( 0 , 1 , 1 ) , a 5 = ( 1 , 1 , 2 ) ,
see Figure 1.
Theorem 1.
Let  K R 3  be a three-dimensional convex polytope with five vertices. Then the vertices of K are pairwise antipodal if and only if one of the following conditions holds:
1. 
K is the convex hull of the union of a singleton and a parallelogram.
2. 
K is affinely equivalent to a member of  P .
Proof. 
Suppose that  x i | i [ 4 ] { 0 }  is the set of vertices of K. By Radon’s Lemma (cf. [20]), there are two possible cases.
Case 1. There exist two disjoint two-subsets I and J of  [ 4 ] { 0 }  such that
conv x i | i I conv x j | j J .
Without loss of generality, we may assume that
x 1 , x 2 x 3 , x 4 .
It follows that
relint x 1 , x 2 relint x 3 , x 4 .
Let  C 0 = conv { x i | i [ 4 ] } .
First, suppose that the vertices of K are pairwise antipodal. From Corollaries 1 and 2, it follows that  x 0 aff C 0  and that  C 0  is a parallelogram. Therefore, K is a convex hull of the union of a singleton and a parallelogram.
Conversely, suppose that K is the convex hull of the union of a singleton and a parallelogram. If  { x 0 }  is not the singleton, then  x 0  is coplanar with three points from  x i | i [ 4 ] , which would show that  x i | i [ 4 ] { 0 }  is contained in a plane, a contradiction. Thus,  K = conv { x 0 , C 0 } , where  C 0  is a parallelogram (see Figure 2). Note that
conv x i | i [ 3 ] { 0 }
is a three-simplex with vertices  x 0 , x 1 , x 2 , x 3 . Let C H j , and  e j  be defined as in Section 2 for each  j [ 3 ] { 0 } . Since  C 0  is a parallelogram and  x 0 aff C 0 , we have
C C 0 and aff C 0 = H 0 .
Thus,  x 0 | e 0 > u | e 0 u H 0 . For any  x K , there exist  λ 0 , λ 1 , , λ 4 0  such that
i [ 4 ] { 0 } λ i = 1 and x = i [ 4 ] { 0 } λ i x i .
Therefore,
u | e 0 x | e 0 = λ 0 x 0 | e 0 + i [ 4 ] λ i x i | e 0 x 0 | e 0 , u H 0 .
This means that K lies in the supporting slab between parallel hyperplanes  H 0  and
x R 3 | x | e 0 = x 0 | e 0 .
Thus,  x 0  and each point in  C 0  are antipodal in K.
Since  C 0  is parallelogram and  x 1 , x 2 x 3 , x 4 , we have
x 4 | e 2 = x 1 | e 2 + x 4 x 1 | e 2 = x 3 | e 2 + x 2 x 3 | e 2 = x 2 | e 2 .
For any  x K , we have
u | e 2 x | e 2 = i { 2 , 4 } λ i x i | e 2 + j { 0 , 1 , 3 } λ j x j | e 2 x 2 | e 2 , u H 2 .
Hence, K lies in the supporting slab between parallel hyperplanes  H 2  and
x R 3 | x | e 2 = x 2 | e 2 .
Similarly,
x 4 | e 1 = x 2 | e 1 + x 4 x 2 | e 1 = x 3 | e 1 + x 1 x 3 | e 1 = x 1 | e 1 .
For any  x K , we have
u | e 1 x | e 1 = i { 1 , 4 } λ i x i | e 1 + j { 0 , 2 , 3 } λ j x j | e 1 x 1 | e 1 , u H 1 .
Hence, K lies in the supporting slab between parallel hyperplanes  H 1  and
x R 3 | x | e 1 = x 1 | e 1 .
It follows that  x 1 , x 2 , x 3 , and  x 4  are pairwise antipodal in K.
Case 2. There exists a two-subset I of  [ 4 ] { 0 }  such that  conv x i | i I  intersects
conv x j | j ( [ 4 ] { 0 } ) I
in its relative interior. Without loss of generality, we may assume that
I = { 1 , 2 } and J = { 0 , 3 , 4 } .
Set  C 1 = conv x i | i I  and  C 2 = conv x j | j J . Then  C 1 relint C 2 .
If K is affinely equivalent to a member of  P , then, by Proposition 1, K has pairwise antipodal vertices.
Suppose now that the vertices of K are pairwise antipodal. Let  { c } = C 1 relint C 2 . It is clear that  c int K . Since  x 1 , x 2 aff C 2 x 1  and  x 2  are strictly separated by the hyperplane determined by  aff C 2 . Then  x 1 x 3 , x 4 x 3 , x 0 x 3  form a basis of  R 3 . There exists a linear mapping  L : R 3 R 3  such that
L ( x 1 x 3 ) = ( 0 , 0 , 1 ) , L ( x 4 x 3 ) = ( 1 , 0 , 0 ) , L ( x 0 x 3 ) = ( 0 , 1 , 0 ) .
Clearly,
T : R 3 R 3 x L ( x x 3 ) = L ( x ) L ( x 3 )
is a non-singular affine transformation. Let  K  be the convex polytope in  R 3  with the set of vertices
y i R 3 | y i = T ( x i ) , i [ 4 ] { 0 } ,
where
y 0 = ( 0 , 1 , 0 ) , y 1 = ( 0 , 0 , 1 ) , y 3 = ( 0 , 0 , 0 ) , and y 4 = ( 1 , 0 , 0 ) .
Let  C 1 = conv { y 1 , y 2 }  and  C 2 = conv { y 0 , y 3 , y 4 } .
First, we claim that  K = T ( K ) . For each  x K , there exist  λ 0 , λ 1 , , λ 4 0  such that
i [ 4 ] { 0 } λ i = 1 and x = i [ 4 ] { 0 } λ i x i .
Then,
T ( x ) = T i [ 4 ] { 0 } λ i x i = i [ 4 ] { 0 } λ i T ( x i ) = i [ 4 ] { 0 } λ i y i K .
Thus,  T ( K ) K . Conversely, for any  y K , there exist  μ 0 , μ 1 , , μ 4 0  such that
j [ 4 ] { 0 } μ j = 1 and y = j [ 4 ] { 0 } μ j y j .
Then
y = j [ 4 ] { 0 } μ j y j = j [ 4 ] { 0 } μ j T ( x j ) = T j [ 4 ] { 0 } μ j x j T ( K ) ,
which shows that  K T ( K ) . Hence,  K = T ( K ) —i.e., K and  K  are affinely equivalent.
In a similar way, we have
C 1 = T ( C 1 ) and relint C 2 = T ( relint C 2 ) .
Since  c C 1 relint C 2 , we have
T ( c ) T ( C 1 ) T ( relint C 2 ) = C 1 relint C 2 ,
which implies that  C 1 relint C 2 .
Let  P 1 , P 2 , , P 5  be defined as in the proof of Proposition 1. From  C 1 relint C 2 , it follows that  y 2 P 1 . Since K is affinely equivalent to  K  and the vertices of K are pairwise antipodal, the vertices  y 0 , y 1 , y 2 , y 3 , y 4  of  K  are also pairwise antipodal. By Corollary 3, we have
y 1 j J ( I ( K , y 2 ) + y j ) c and y 2 j J ( I ( K , y 1 ) + y j ) c .
Hence
y 2 i [ 5 ] P i = P .
Therefore,  K  is a member of  P . □

Author Contributions

Conceptualization, S.W.; methodology, R.G., S.W., and C.H.; validation, S.W., L.Z., and C.H.; writing—original draft preparation, R.G.; writing—review and editing, S.W., C.H., and L.Z.; funding acquisition, S.W. All authors have read and agreed to the published version of the manuscript.

Funding

The authors were supported by the National Natural Science Foundation of China (grant numbers 12071444 and 12201581), and the Fundamental Research Program of Shanxi Province (grant numbers 202403021221109, 20210302124657, 202103021224291, and 202303021221116).

Data Availability Statement

Data sharing is not applicable to this article as no datasets were generated or analyzed during the current study.

Conflicts of Interest

The authors declare no conflicts of interest.

References

  1. Danzer, L.W.; Grünbaum, B. Über zwei Probleme bezüglich konvexer Körper von P. Erdos und von V. L. Klee. Math. Z. 1962, 79, 95–99. [Google Scholar] [CrossRef]
  2. Martini, H.; Soltan, V. Antipodality properties of finite sets in Euclidean space. Discret. Math. 2005, 290, 221–228. [Google Scholar] [CrossRef]
  3. Makai, E., Jr.; Martini, H. On the number of antipodal or strictly antipodal pairs of points in finite subsets of  R d . In Applied Geometry and Discrete Mathematics, The “Victor Klee Festschrift”. DIMACS Ser. Discr. Math. Theor. Comp. Sci. 1991, 4, 457–470. [Google Scholar]
  4. Makai, E., Jr.; Martini, H. On the number of antipodal or strictly antipodal pairs of points in finite subsets of  R d , II. Period. Math. Hung. 1993, 27, 185–198. [Google Scholar] [CrossRef]
  5. Makai, E., Jr.; Martini, H.; Nguyên, M.H.; Soltan, V.; Talata, I. On the number of antipodal or strictly antipodal pairs of points in finite subsets of  R d , III. arXiv 2021, arXiv:2103.13182. [Google Scholar]
  6. Levi, F.W. Überdeckung eines Eibereiches durch Parallelverschiebung seines offenen Kerns. Arch. Math. 1955, 6, 369–370. [Google Scholar] [CrossRef]
  7. Bezdek, K. Classical Topics in Discrete Geometry, CMS Books in Mathematics/Ouvrages de Mathématiques de la SMC; Springer: New York, NY, USA, 2010. [Google Scholar]
  8. Boltyanski, V.; Martini, H.; Soltan, P.S. Excursions into Combinatorial Geometry; Springer: Berlin/Heidelberg, Germany, 1997. [Google Scholar]
  9. Bezdek, K. Hadwiger’s covering conjecture and its relatives. Am. Math. Mon. 1992, 99, 954–956. [Google Scholar] [CrossRef]
  10. Bezdek, K.; Khan, M.A. The geometry of homothetic covering and illumination. In Discrete Geometry and Symmetry; Springer Proceedings in Mathematics & Statistics Series; Springer: Cham, Switzerland, 2018; Volume 234, pp. 1–30. [Google Scholar]
  11. Boltyanski, V.; Gohberg, I.Z. Stories about covering and illuminating of convex bodies. Nieuw Arch. Wisk. 1995, 13, 1–26. [Google Scholar]
  12. Martini, H.; Soltan, V. Combinatorial problems on the illumination of convex bodies. Aequationes Math. 1999, 57, 121–152. [Google Scholar] [CrossRef]
  13. Wu, S.; He, C. Covering functionals of convex polytopes. Linear Algebra Appl. 2019, 577, 53–68. [Google Scholar] [CrossRef]
  14. Grünbaum, B. Convex Polytopes; Springer: New York, NY, USA, 2003. [Google Scholar]
  15. Wu, S.; Zhang, K.; He, C. Homothetic covering of convex hulls of compact convex sets. Contrib. Discrete Math. 2022, 17, 31–37. [Google Scholar] [CrossRef]
  16. Soltan, V. Affine diameters of convex-bodies—A survey. Expo. Math. 2005, 23, 47–63. [Google Scholar] [CrossRef]
  17. Gao, S.; Martini, H.; Wu, S.; Zhang, L. New covering and illumination results for a class of polytopes. Arch. Math. 2024, 122, 599–607. [Google Scholar] [CrossRef]
  18. Hiriart-Urruty, J.B.; Lemaréchal, C. Fundamentals of Convex Analysis; Springer: Berlin/Heidelberg, Germany, 2001. [Google Scholar]
  19. Soltan, V. Lectures on Convex Sets; World Scientific Publishing Co. Pte. Ltd.: Hackensack, NJ, USA, 2015. [Google Scholar]
  20. Webster, R. Convexity; Oxford Univ. Press: New York, NY, USA, 1994. [Google Scholar]
Figure 1. The range of  y 2 .
Figure 1. The range of  y 2 .
Mathematics 13 01412 g001
Figure 2. K is the convex hull of the union of a singleton and a parallelogram.
Figure 2. K is the convex hull of the union of a singleton and a parallelogram.
Mathematics 13 01412 g002
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Guo, R.; He, C.; Zhang, L.; Wu, S. A Characterization of Three-Dimensional Convex Polytopes with Five Pairwise Antipodal Vertices. Mathematics 2025, 13, 1412. https://doi.org/10.3390/math13091412

AMA Style

Guo R, He C, Zhang L, Wu S. A Characterization of Three-Dimensional Convex Polytopes with Five Pairwise Antipodal Vertices. Mathematics. 2025; 13(9):1412. https://doi.org/10.3390/math13091412

Chicago/Turabian Style

Guo, Rong, Chan He, Longzhen Zhang, and Senlin Wu. 2025. "A Characterization of Three-Dimensional Convex Polytopes with Five Pairwise Antipodal Vertices" Mathematics 13, no. 9: 1412. https://doi.org/10.3390/math13091412

APA Style

Guo, R., He, C., Zhang, L., & Wu, S. (2025). A Characterization of Three-Dimensional Convex Polytopes with Five Pairwise Antipodal Vertices. Mathematics, 13(9), 1412. https://doi.org/10.3390/math13091412

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