Intersection properties of finite disk collections

In this work we study the intersection properties of a finite disk system in the euclidean space. We accomplish this by utilizing subsets of spheres with varying dimensions and analyze specific points within them, referred to as poles. Additionally, we introduce two applications: estimating the common scale factor for the radii that makes the re-scaled disks intersects in a single point, this is the \v{C}ech scale, and constructing the minimal Axis-Aligned Bounding Box (AABB) that encloses the intersection of all disks in the system.


INTRODUCTION
One of the new techniques developed for the analysis of large clusters of information, known as Big Data, is Topological Data Analysis (TDA).In TDA, simplicial complexes associated with the data are constructed.These structures include the Vietoris-Rips complex, the Čech complex, and the piecewise linear lower star complex, among others.Of special interest to us is the generalized Čech complex structure.Although the standard Čech complex is formed by intersecting a collection of disks with a fixed radius, the generalized version allows varying radii.This flexibility enables us to highlight specific data points by assigning or weighting them with larger and/or more rapidly expanding balls to them, while de-emphasizing others by using smaller and/or slower growing balls.This approach proves valuable for handling noisy data sets, offering an alternative to discarding data that may not meet a specific significance threshold [1].
Understanding the patterns of intersections and the timing of intersections among a set of disks in R d , each with potentially different radii, is a fundamental problem.This leads to the exploration of the generalized Čech complex structure, which captures the intersection information of these disks, regardless of their radii.Rescaling the radii by the same factor, we obtain a filtered generalized Čech complex, where the associated simplicial complexes evolve as the scale parameter varies.In particular, in [4], algorithms are provided to calculate the generalized Čech complex in R 2 , and [3] presents an algorithm to determine the Čech scale for a collection of disks in the plane.
To establish the necessary foundation for our study, Section 2 introduces crucial concepts and notation that will be used throughout the article and we focus on analyzing the intersection of a disk system in R d .We start by investigating the intersection of two disks in Subsection 2.1 and then expand our analysis to a system of m disks in Subsection 2.2.By applying Helly's Theorem, we prove that it is sufficient to examine the intersection of all subsystems consisting of d + 1 disks in order to determine if the system has a nonempty intersection.
In Section 3, we define Vietoris-Rips systems and Čech systems, together with presenting results regarding the Rips scale and the Čech scale, as well as their connections.In Subsection 3.1, we present an algorithm that can determine whether the intersection of the system is empty or nonempty.This is achieved by exclusively computing the poles of subsystems of disks (or spheres).Finally, in Subsection 3.2, we introduce the algorithm that computes an approximation to the Čech scale using the numerical bisection method.
Additionally, in Section 4 we incorporate the concept of an minimal axis-aligned bounding box (AABB) into our methodology.An AABB is a rectangular parallelepiped whose faces are perpendicular to the basis vectors.These bounding boxes frequently arise in spatial subdivision problems, such as ray tracing [5] and collision detection [2].In this paper, we study AABBs to enclose the intersection of a finite collection of disks.This approach proves valuable for discerning whether the collection intersects at a singular point or not.In this section, we also provide an algorithm for constructing the AABB of a disk system.

INTERSECTION PROPERTIES OF SPHERE SYSTEMS
Throughout this work, we will refer to a d-disk system M, or simply a disk system, as a finite collection of closed disks in R d with positive and not necessarily equal radii, i.e., Moreover, in order to study the intersection properties of a disk system M with the approach addressed in Sections 4 and 3 of this work, we will conduct a study in this section of the intersection properties of the spheres corresponding to the boundaries of each disk in M, which we call a sphere system and denote by ∂M, where ∂ denotes the topological boundary operator.
Following the notation in [6], we introduce the following generalization of the sphere.Definition 1.An i-sphere in R d is the intersection of a sphere with an affine subspace of dimension i.
Of course, the notions of a sphere (as a (d − 1)-dimensional surface) and a d-sphere in R d agree.However, an i-sphere in R d can also be viewed as the intersection of d-spheres.For instance, the intersection of two spheres typically occurs in a hyperplane, forming a (d − 1)-sphere in R d .When another d-sphere intersects this configuration, the result may be a (d − 1)-sphere, a (d − 2)-sphere, a 0-sphere (a single point), or it might even be empty, all within the same hyperplane.For a disk system M = {D i (c i ; r i )} composed of m disks, where {c 1 , . . ., c m } is a set in general position in R d , the maximum dimension of the affine subspace associated with the i-sphere, obtained from the intersection of all the spheres in ∂M, is at most d − m + 1, or equivalently, i = d − m + 1.This conclusion is drawn from [6, Theorem 2.1] and the fact that the affine hull of {c 1 , . . ., c m } is of dimension m − 1.Consequently, the following result is proven.Lemma 2. Let M = {D 1 (c 1 ; r 1 ), . . ., D m (c m ; r m )} be disk system such that {c 1 , . . ., c m } is a set in general position in R d .Then, the possibilities for the set ∩ D i ∈M ∂D i are: (1) the empty set; (2) a single point; (3) a (d − m + 1)-sphere.
Remarkable points in i-spheres that will play a key role in the rest of the article are the poles.Let π i : R d −→ R be the canonical projection on the i-th factor for i = 1, ..., d, and let {e 1 , e 2 , ..., e d } be the standard basis of R d .Definition 3. Let e q be the q-th vector of the canonical base of R d .An e q -north (south) pole of an i-sphere S in R d is a point on S whose projection on the q-th coordinate is maximum (minimum).In other words, x ∈ S is the e q -north pole if π q (y) ≤ π q (x) for all y ∈ S − {x}, where π q represents the projection onto the q-th coordinate.
We denote the e q -north pole of S by s + q and the e q -south pole by s − q .
An i-sphere can have a single e q -pole (north or south) or an infinite number of them, which occurs when a normal vector to the affine space containing the i-sphere is aligned with the vector e q .We are interested in finding the e q -poles of (d − m + 1)-spheres originating from disk systems M = {D 1 (c 1 ; r 1 ), . . ., D m (c m ; r m )}, by taking the intersection ∩ m j=1 ∂D j .Such (d − m + 1)-spheres will be denoted by S M (c; r), to emphasize the disk system M, as well as its center and radius.Lemma 4. Let M = {D 1 , . . ., D m } be a d-disk system such that m j=1 D j ̸ = ∅, and let p be a point in m j=1 D j such that π q (p) ≤ π q (x) (resp.π q (p) ≥ π q (x)) for every x in m j=1 D j .Then, there exists an i-sphere S = ∂D j 1 ∩ • • • ∩ ∂D j i such that p is in S and p is the e q -south pole (resp.e q -north pole) of S.
due to the closedness of the sets D j , for j = 1, . . ., m, and p ∈ ∂(∩ m j=1 D j ).On the other hand, since ∂(∩ m j=1 D j ) ⊂ ∪ m i=1 ∂D j , there exist indices j 1 , . . ., j i such that p ∈ ∂D jr for any r = 1, . . ., i; let Λ(p) = {j 1 , . . ., j i } ⊆ {1, . . ., m} be a maximal subset of indices such that p ∈ D j if and only if j ∈ Λ(p).We claim that p ∈ ∩ i r=1 ∂D jr is the e q -south pole of S := ∩ i r=1 ∂D jr .In effect, let V p ⊂ R d be an open neighborhood of p sufficiently small such that: (1) Every x ∈ V p ∩ ∂(∩ m j=1 D j ) has as maximal set of indices a proper subset of Λ(p), (2) S ∩ V p ⊂ ∂(∩ m j=1 D j ).The first condition can be guaranteed by the finiteness of the disk system M, and the second condition is a consequence of the maximality of the set Λ(p).Therefore, π q (p) ≤ π q (x) for every x ∈ S ∩ V p , which is equivalent to the fact that π q (p) ≤ π q (x) for every x ∈ S, in the case of i-spheres.□ 2.1.Sphere systems with two spheres.In the following two lemmas we provide the computations to determine the center, radius and poles for a (d − 1)-sphere given by the intersection of two disks in R d .Lemma 5. Let M = {D 1 (c 1 ; r 1 ), D 2 (c 2 ; r 2 )} be a disk system with two d-disks such that ∂D 1 ∩ ∂D 2 is a (d − 1)-sphere S = S M (c; r) with center c and radius r.Then, Proof.Let Π be the hyperplane containing the (d − 1)-sphere S, which is defined by the equation: (1) where c 1 = (h 1 , . . ., h d ) and c 2 = (k 1 , . . ., k d ).Then the normal vector of the hyperplane Π is given by N and the center c of S is determined by the intersection point of the hyperplane Π with the perpendicular line that passes through the center c 1 of D 1 .This line can be parameterized as γ : t → c 1 + tN = (x 1 (t), . . ., x d (t)), such that γ(0) = c 1 and γ(1) = c 2 .We can compute the intersection point c = γ(t * ) of Π and γ([0, 1]), for any t * ∈ [0, 1], by substituting it in (1), And solving for t * , we obtain that Hence, the center of S is given by: Next, we will compute the radius r of S. This radius can be determined as the height r of the triangle with base ∥c 2 − c 1 ∥ formed by the points c 1 , c 2 , and a point on S. Thus, by the Heron's formula we have that We can proceed now to compute the poles of the (d − 1)-sphere ∂D 1 ∩ ∂D 2 .
Lemma 6.Let D 1 (c 1 ; r 1 ) and D 2 (c 2 ; r 2 ) be two d-disks such that ∂D 1 ∩ ∂D 2 is a (d − 1)-sphere S = S(c; r) with center c and radius r.Then, the e q -poles of S are s ± q = c ± d i x i e i , where Proof.For simplicity, we translate the hyperplane Π, which contains the (d − 1)-sphere S, as well as the sphere itself, to the origin; in such case, the corresponding equations are given by, d i=1 where h i := π i (c 1 ) and k i := π i (c 2 ) for i = 1, . . ., d.In the case that k q − h q = π q (c 2 − c 1 ) = 0, the normal vector N = c 2 − c 1 of the hyperplane Π is orthogonal to the basis vector e q .Therefore, the e q -poles of S are c ± re q , which agree with the formulae of the lemma.
On the other hand, suppose that k q − h q ̸ = 0. To find the e q -poles of S, we will use the Lagrange multiplier method.Consider the following function: (2) x q = f(x 1 , x 2 , ..., x q , ..., subject to the restriction: Let λ be the Lagrange multiplier, we define h(x 1 , ..., x q , ..., x d , λ) = f(x 1 , ..., x q , ..., x d ) + λg(x 1 , ..., x q , ..., x d ) For any i ̸ = q, consider the following system of equations: Then Solving this system of equations for λ, we obtain that, Comparing the last expression for two indices i ̸ = ĩ, we have that, Finally, for i = q, we can use the last expression to substitute it in (2) and obtain the desired result.□ 2.2.Sphere systems with more than two spheres.Now, let us proceed with the explicit calculation of the coefficients for the center c of the (d − m + 1)-sphere S = ∩ m j=1 ∂D j .We can achieve this by considering the disk system translated to c m , denoted as {D j (c j − c m ; r j )} m j=1 , and by defining the (d − m + 1)-sphere S − {c m } = ∩ m j=1 ∂D j (c j − c m ; r j ).This sphere is positioned at the intersection of hyperplanes (for more details, refer to [6]). (3) for all k = 1, ..., m − 1. Utilizing the information that the center of S − {c m } can be expressed as a combination of the centers c k − c m and substituting it into (3), we obtain a linear system of equations with dimensions (m − 1) × (m − 1): for k = 1, ..., m − 1. Solving the system of equations for λ = (λ 1 , ..., λ m−1 ), we find the center of S as follows: The radius of the sphere S can be computed using the equation: for any k ∈ {1, ..., m − 1}.
Now that we have determined the center and radius of S, as well as the affine space that contains it, we can proceed to compute its e q -poles for each q ∈ {1, 2, ..., d}.These poles reside in the affine space that contains S and within a set that we define below.
Let S be an i-sphere in R d , and let n 1 , ..., n d−i be orthogonal vectors to the affine space L that contains S. Consider the space M generated by these vectors together with the vector e q from the canonical basis of R d .Let us denote n j = n for each j = 1, ..., d − i.Then, we can define L 0 , the set L translated to the origin, as follows: The set M is defined as: Refer to Figure 1 for a visual representation of the subspaces L and M + {c}.
As mentioned above, the e q -poles of S lie at the intersection of L and M + {c}, where c is the center of S. To simplify the calculations, we will utilize L 0 and M, and then translate them into c.The intersection of M and L 0 can be expressed as follows: Let us consider a disk system in R d , denoted {D j (c j ; r j )} m j=1 , where m < d.The intersection of their boundaries forms a (d − m + 1)-sphere S. In this case, the subspace M has dimension m, or dim(M) = m − 1 if e q ∈ M. We choose the normal vectors for the affine space containing S as n j = c j − c m , where j = 1, ..., m − 1.
By rewriting, we have -sphere with center in c and radius r, then the e q -poles of S are the e q -poles of S − {c m } but translated by c.The poles of S − {c m } are located in M ∩ L 0 .If p is an e q -pole of S − {c m }, then it can be expressed as p = m−1 j=1 λ j n j + e q λ m for some λ j ∈ R, j = 1, ..., m and the following conditions holds: Thus, if p = m−1 j=1 λ j n j + e q λ m is an e q -pole of S − {c m }, the following equations are satisfied for λ 1 , λ 2 , ..., λ m ∈ R: for all k = 1, ..., m − 1, with r the radius of the (d − m + 1)-sphere S. From (4) we have the system Let us denote A as the matrix (n i • n j ) i,j and B as the vector (−n j q ) m−1 j=1 .Then, we have Aλ = λ m B, where λ = (λ 1 , ..., λ m−1 ).Solving for λ, we obtain λ j = λ m (A −1 B)[j] for each j = 1, ..., m − 1 (where (A −1 B)[j] denotes the entry j of the (m − 1) × 1 vector (A −1 B)).By substituting the value of λ j into (5), we obtain the quadratic equation: Let us define for all i = 1, ..., d.Solving this equation, we find: Therefore, the e q -poles of S, for q = 1, ..., d, are:

VIETORIS-RIPS AND ČECH SYSTEMS
Our goal in this section is to provide a comprehensive understanding of the disk system, the Vietoris-Rips system, and the Čech system.Additionally, we introduce some results that establish a certain connection between both disk systems.Investigating the features and qualities of data and spaces can provide us with useful knowledge about their geometric and topological characteristics.
Before we look into the definitions of Vietoris-Rips and Čech systems, let us give a brief overview.These systems are essential in the field of topological data analysis for recognizing and comprehending the geometric structure of point cloud data.Vietoris-Rips complex and Čech complex share the goal of capturing the topology of the underlying metric space, both provide different ways of recognizing connections and associations among data points.The Vietoris-Rips complex tends to be more efficient and scalable for large datasets, while the Čech complex can be more accurate but computationally more expensive.The choice between the two depends on the nature of the dataset and the specific goals of the topological analysis.Now, let us move on to defining these fundamental concepts.Definition 7. Let M = {D 1 , D 2 , . . ., D m } be a d-disk system.We say M is a Vietoris-Rips system if D i ∩ D j ̸ = ∅ for each pair i, j ∈ {1, 2, . . ., m}.Furthermore, if the d-disk system M has the nonempty intersection property D i ∈M D i ̸ = ∅, then M is called a Čech system.
For each λ ≥ 0, we define a collection of d-disks M λ with the same centers as those in the d-disk system M, but with radii rescaled by λ.When λ > 0, M λ is a d-disk system again.M 1 is equal to M, and M 0 is the set of the centers of the d-disks in M.
In the field of topological data analysis, the Rips scale and the Čech scale are essential parameters for determining the closeness and connectivity between data points.These two scales offer different perspectives on how we measure and comprehend geometric relationships within point-cloud data.To understand their importance in capturing the underlying topological structure, let us look at their definitions.The Vietoris-Rips scale ν M , of a d-disk system M is the smallest λ ∈ R such that M λ is a Vietoris-Rips system.Similarly, the Čech scale µ M , of M is the smallest λ ∈ R such that M λ is a Čech system.This is Next, we present some easily observable properties for both scales.It can be easily seen that M is a Vietoris-Rips system if and only if ν M ≤ 1 (in particular ν Mν M = 1); similarly, M is a Čech system if and only if µ M ≤ 1.
Note that for a given d-disk system M = {D 1 , D 2 , . . ., D m } the Vietoris-Rips scale is where c i and r i are the center and radii of D i .An additional observation is that, in cases where the disk system consists of either one or two disks, the Vietoris-Rips scale coincides with the Čech scale.It is evident that every Čech system is also a Vietoris-Rips system; however, the reverse assertion, in general, is not true.
Conversely, if the d-disk system contains at least three disks, determining the Čech scale becomes more complex.In the context of Čech scale, the following remark is important and play a key role in implementation (see [3] for details).
Remark 8.If µ M is the Čech scale for M, then the µ M -rescaled system M µ M , has only one point in the intersection D i ∈M D i (c i ; µ M r i ).
As we have mentioned, a Čech system is also a Vietoris-Rips system, but the converse is not true.What we can affirm is that if a system is a Vietoris-Rips system, then the system rescaled by the factor 2d/(d + 1) is also a Čech system.This is established by the following lemma, the proof of which can be found in [3].
One of the implications of the previous result is that, for any given disk system M, we can bound the Čech scale using the Vietoris-Rips scale ν M .This is stated by the following corollary.
3.1.Algorithm for determining Čech system.In the previous section, we have determined the e q -poles for the intersection of any number of disks in R d .If any of these poles is in all disks of the disk system, it indicates that the system conforms to the criteria of a Čech system.It is important to recognize that this result streamlines our calculation process, focusing on specific points to establish whether the system exhibits a non-empty intersection.
Given a system of m disks in R d where m > d, it is enough to verify if every subsystem of d + 1 disks qualifies as a Čech system to conclude that the entire system of disks has a non-empty intersection.This assertion is supported by the Helly's Theorem.Now, we introduce an algorithm that determines whether a disk system qualifies as a Čech system.In simpler terms, if the disk system exhibits a non-empty intersection, the algorithm outputs "TRUE"; otherwise, it outputs "FALSE".The algorithm operates by seeking poles within the intersections of the disk boundaries, which, as we have observed, correspond to i-spheres.It initiates the search for poles within individual disks and then progresses to the pairwise intersections of the disk boundaries ((d − 2) − sphere), continuing the process iteratively.If a pole is found within the remaining disks, the system is classified as a Čech system.Theorem 11.Let M = {D 1 , . . ., D m } be a d-disk system.Then, M is a Čech system if and only if Cech.system(M) = TRUE.
Proof.If Cech.system(M) = TRUE, then the Cech.systemalgorithm (Algorithm 1) found a pole contained in the intersection ∩ m j=1 D j , therefore ∩ m j=1 D j ̸ = ∅ and it follows that M is a Čech system.
On the other hand, if m j=1 D j ̸ = ∅, let p be a point in m j=1 D j satisfying π 1 (p) ≤ π 1 (x) for every x in m j=1 D j .By Lemma 4, it follows that p belongs to an i-sphere and must be an e 1south pole.Therefore, by the exhaustive search of Algorithm 1 across all poles, its output is Cech.system(M)= TRUE.□ 3.2.Algorithm to compute the Čech scale.Finding the minimum parameter for which the rescaled system of disks has a non-empty intersection is significant because it helps identify a critical threshold at which the disks come into contact.This parameter, known as the Čech scale, provides valuable information about the proximity or overlap of the disks, which can be crucial in various applications such as collision detection in computer graphics, spatial packing problems, and modeling physical phenomena.In this section, we introduce an algorithm to compute an approximation of the Čech scale for a system of m disks in R d .
The given code presents an algorithm to compute an approximation of the Čech scale of a disk system in Euclidean space using Algorithm 1 and a precision parameter η > 0. It initializes the scale factor λ to the Rips scale ν M .If this scale satisfies Cech.system(M ν M ) = TRUE, it indicates that the Čech scale has been found.Otherwise, we initiate a cycle in which we compute Cech.system of the system rescaled by a factor λ. The Čech scale is known to fall between the Rips scale and the value 2d/(d + 1)ν M (Generalized Vietoris-Rips, Corollary 10).To approximate the Čech scale, we employ the bisection method as long as the interval enclosing the Čech scale has a length greater than η.Finally, the algorithm returns an approximation of the Čech scale.
Utilizing the previously described algorithm, we can construct the filtered generalized Čech complex for a disk system M. Let C(M) denote the set of Čech subsystems, and C λ (M) the set of Čech subsystems for the rescaled disk system M λ .The Čech filtration of the M system forms a maximal chain of Čech complexes where each λ i represents the Čech scale of the system M λ i .Since the Čech scale of a disk system indicates the factor by which we must rescale the system to make it Čech, defining a level of the filtration, C λ (M), simply requires determining the Čech scale of the system M λ .4.1.Minimal axis-aligned bounding box for two disks.Let's consider the situation when the disk system M is composed of two disks D 1 and D 2 in R d .If D 1 ∩ D 2 ̸ = ∅ and D 1 ̸ = D 2 , the subset ∂D 1 ∩ ∂D 2 can take one of three forms: an empty set, a single common point (when the disks are tangent), or a (d − 2)-dimensional sphere.In the last case, we denote the (d − 2)-dimensional sphere or the (d − 1)-sphere ∂D 1 ∩ ∂D 2 by S 1,2 .To calculate inf π i (∂(D 1 ∩ D 2 )) and sup π i (∂(D 1 ∩ D 2 )) for each i ∈ {1, 2, ..., d}, we can use: Indeed, by Lemma 4, the extremes of the AABB are the projections of certain poles, either from the (d − 1)-sphere or some d-sphere.The d-spheres represent the boundaries of each disk, with poles given by c j ± r j e i , and the (d − 1)-sphere is S 1,2 = ∂D 1 ∩ ∂D 2 , whose poles are computed using Lemma 6.It is worth noting that there are no further options for (d − m + 1)-spheres in the case of a two-disk system.
To simplify the notation, we will use B i,j to denote the AABB of the intersection of disks D i and D j .Figure 2 illustrates the AABB of the intersection of two disks in the plane.According to (6) and Lemma 5, we have a method to calculate the axis-aligned boundary box (AABB) for systems of two disks.
Knowing how to compute the AABB of two disks is not sufficient to determine the AABB for a disk system with more than two disks in R d .In the following examples, we demonstrate that the AABB of a disk system is not simply the intersection of all AABBs of two disks in R where p ∈ D satisfies π 1 (p) = inf π 1 (∂D).
Let q j be the point in ∩ k̸ =j D k such that π 1 (q j ) = inf π 1 (∩ k̸ =j D k ) for each j = 1, 2, ..., d + 1.Note that q j / ∈ D j because q j is not in D. Now, let γ j be the line segment that connects q j and p.
Choose ϵ > 0 small enough such that the hyperplane P : x 1 = π 1 (p) − ϵ does not contain any q j , and π 1 (q j ) < π 1 (p) − ϵ for all j = 1, 2, ..., d + 1 (such hyperplane P exists because π 1 (p) > π 1 (q j ) for all j = 1, ..., d + 1).Since q j is in ∩ k̸ =j D k , the hyperplane P intersects every disk in M(j) for each j, and γ j ⊂ ∩ k̸ =j D k intersects P at a point that is in ∩ k̸ =j D k (see Figure 5).Furthermore, D k ∩ P is a (d − 1)-dimensional disk.Therefore, we have a collection D = {D j ∩ P|j = 1, 2, ..., d + 1} of d + 1 disks, each of dimension d − 1, such that every subset A of D consisting of d disks has the non-empty intersection property.By Helly's Theorem, the intersection of all (d − 1)-disks in D is not empty.Therefore, there exists a point q ∈ D with π 1 (q) < π 1 (p).However, this contradicts the fact that p ∈ D is such that π 1 (p) = inf π 1 (∂D).

FIGURE 1 .
FIGURE 1. Visualization of the subspaces L and M + {c}.

Algorithm 1 : 3 Let 4 for S in S do 5 for q ← 1 to d do 6
Cech.systemInput : A d-disk system M = {D j } m j=1 Output : A logical TRUE/FALSE to indicate if M is a Čech system 1 Initialize: Is Cech System ← FALSE 2 for k ← 1 to m do S be the set of (d − k + 1)-spheres of ∂MCompute the set {s ± q } of e q -poles of S