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ISRN Applied Mathematics

Volume 2014 (2014), Article ID 717265, 3 pages

http://dx.doi.org/10.1155/2014/717265

## Prime Decomposition of Three-Dimensional Manifolds into Boundary Connected Sum

^{1}Department of Basis, Dalian Naval Academy, Jie Fang Road 667, Zhong Shan District, Dalian, Liaoning Province 116018, China^{2}Department of Basis, Dalian Institute of Science and Technology, Dalian, Liaoning Province, China

Received 8 November 2013; Accepted 24 December 2013; Published 19 February 2014

Academic Editors: C. Lu and F. Sartoretto

Copyright © 2014 Meili Zhang and Bo Deng. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

In 2003 Matveev suggested a new version of the Diamond Lemma suitable for topological applications. We apply this result to different situations and get a new conceptual proof of theorem on decomposition of three-dimensional manifolds into boundary connected sum of prime components.

#### 1. Introduction

Since 2003 Matveev [1–3] had suggested a new version of the Diamond Lemma [4] of great importance for various fields of mathematics, which is suitable for and efficient solving topological problems. In this paper we apply this result to get a new conceptual proof of theorem on decomposition of three-dimensional manifolds into boundary connected sum of prime components.

#### 2. Definition, Lemma, and -Irreducible Manifolds

Diamond Lemma (see [5]). *If an oriented graph ** has the properties (FP) and (MF), then each of its vertices has a unique root*.

*Definition 1. *Let be a proper disk in a compact connected 3-manifold . A disk reduction of along consists in cutting along the disk . Its result is a new manifold .

We apply nontrivial disk reductions to a given manifold as many times as possible. If this process stops, then we obtain a set of new manifolds, which is called a root of .(1)If the disk is splitting, the manifold is obtained by gluing the manifolds and together along disks on their boundaries. Then is called a boundary connected sum of the manifolds and and denoted by .(2)If the disk is nonsplitting, then is also connected.

*Definition 2. * is said to be irreducible if every properly embedded disk in is trivial.

Lemma 3 (see [6]). *Let be a -irreducible manifold. Let be the manifold obtained from by attaching a 1-handle to make the boundary connected. Then is a prime which is not irreducible.*

#### 3. Proof of Theorem 4

Theorem 4. *Any connected irreducible compact 3-manifold different from a ball and with nonempty boundary is homeomorphism to a boundary connected sum of prime manifolds. All the summands are defined uniquely up to reordering and, if is non-Orientale, replacing solid tori by solid Klein bottles .*

We apply the universal scheme [4] in two stages. First, by considering reductions along all disks we establish uniqueness of the -irreducible manifolds . Then we restrict ourselves to reductions only along splitting disks and by lemma [3] complete the proof of the theorem.

We construct the graph [5] whose vertices are compact connected irreducible manifolds, considered up to addition or deletion of three-dimensional balls. The edges of the graph correspond to reductions along both splitting and nonsplitting disks.

Lemma 5 (see [7]). *Each essential disks reduction strictly decreases , where , is the genus of a component , and the sum is taken over all components of .*

Since every set of vertexes of nonnegative integers has a minimal vertex, by Lemma 5, the process stops and we get a root. Then has the properties (FP).

Lemma 6. *The graph has a mediator function; that is, has the properties (MF).*

*Proof. *Let be a pair of edges with common beginning. Following the universal scheme, we define the value of the mediator function to be the minimal number of connected components in the intersection of disks , defining the edges and . This minimum is taken over all pairs of such disks. As usual, we assume that the disks are in general position, so that their intersection consists of a finite number of circles and arcs.

We first consider the property (MF1), that is, the case . When the reduction is carried out along disjoint disks and , then each of these disks survives under the reduction along the other disk. We can therefore assume that for and . By reducing each of the manifolds along the disk . We obtain the same vertex . This proves the property (MF1).

Next, we verify the property (MF2), that is, the case , when any two disks and defining the edges and intersect. Then these disks lie in the same connected manifold . According to the universal scheme, it is enough to show that there exists a mediator disk, that is, a nontrivial disk satisfying for .*Case 1.* Among the circles in we choose one, denoted by , which is innermost with respect to the disk . This means that the circle bounds a disk in such that . We cut along and glue up the boundaries of the cut by two parallel copies of the disk . By applying a small perturbation we obtain a new disk and a sphere whose intersection with is empty and whose intersection with consists of a smaller number of circles (since the circle has disappeared). The disks must be nontrivial in , since otherwise the disk would be trivial. Therefore, can be taken as a mediator disk.*Case 2.* Among the arcs in we choose one, denoted by , which is outermost with respect to the disk . We cut along and glue up the boundaries of the cut by two parallel copies of the disk . By applying a small perturbation we obtain a new disk and a sphere whose intersection with is empty and whose intersection with consists of a smaller number of arcs (since the circle has disappeared). At least one of these two disks (say ) must be nontrivial in , since otherwise the disk would be trivial. Therefore, can be taken as a mediator disk.

By applying the Diamond Lemma we prove that a root of any vertex of the graph exists and is unique.

We therefore obtain the following theorem, which is a particular case of Theorem 4 for manifolds without nonsplitting proper disks.

Theorem 7. *Any connected irreducible compact three-dimensional manifold different from a ball and with nonempty boundary is decomposed by disk reductions into a union of -irreducible parts. These parts are defined uniquely up to reordering.*

In order to finish the proof of Theorem 4 we change the graph constructed by keeping the vertex set intact while disallowing reductions along nonsplitting disks. The edges of the new graph therefore correspond to reductions only along splitting disks, so that the root of any vertex consists exactly of the prime summands of the manifolds corresponding to the vertex. The properties (FP) and (MF1) of are automatically inherited by the graph . The only difficulty with the proof of the property (MF2) is that after applying to the disk a surgery along the innermost circle both new spheres and and disk may be nonsplitting and therefore can not be taken as mediator disks.

*Case 1. *Assume that consists of circles. Then we connect the spheres and by a tube which does not intersect .

Since each of these spheres is obtained by connecting and by tubes contained in one of the parts into which divides , the disk is splitting and nontrivial (the latter follows from the simple observation that each simple arc connecting the sphere and disk on the boundary is trivial). It is important to note that, after a suitable small perturbation, intersects in circles and intersects in two circles. Thus, is a mediator sphere.

The cases and need to be considered separately. We show that in each of these cases there exists a mediator disk. If , then the disks and split into 4 parts , . These parts are different, since they are separated by and : any two parts lie on different sides with respect to at least one of the disks. The boundary of each part consists of a single splitting disk. At least one of these disks is nontrivial, since both and are nontrivial. Hence, it can be taken as a mediator disk.

Now let . Then the disks split either into 5 parts , (if the parts and are different), or into 4 parts (if the parts and coincide). None of the other parts can coincide, since only the parts and are not separated by . Since , then each of the parts and is bounded by a sphere is bounded by a disk, and at least one of these disks is nontrivial, since is nontrivial. This disk can be taken as a mediator. We also note that the part is bounded by a torus.

Assume that and coincide and constitute a single connected part . Then we have three candidates for a mediator sphere: the spheres and the new disk . The latter is obtained by connecting and by a tube inside . We assert that at least one of the three disks is nontrivial and therefore is a mediator.

Arguing by contradiction, assume that all three disks are trivial. This means that is ball and is homeomorphism to . Then the reduction along the disk produces two manifolds and , and the reduction along the disk also produces two manifolds and . Here the balls are viewed as handle of index 1 (if they are attached to ) or as handle of index 2 (if they are attached to ). The balls , , , , and glue up the disks on the boundaries of the corresponding manifolds. Handles of index 1 connect different disks on the boundary of the manifold , which implies that . On the other hand, the bases of handles of index 2 in the torus are isotopic; hence . Therefore, the reductions along the disks and give the same result. This contradicts the fact that in our situation when , the vertices and must be different.

*Case 2. *Assume that consists of arcs. The proof can be finished by an argument similar to that used in the proof of Case 1.

The arguments above are also applicable in the case of prime decompositions of non-Orientale manifolds, with the only difference that handles of index 1 can now be attached to the boundary of the manifold in two different ways. In one case the result is the direct product , and in the other case we get the twisted product . Nevertheless, since the manifold is non-Orientale, all summands can be replaced by .

#### Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

#### Acknowledgment

This work was supported by the Dalian Naval Academy for Basic Research.

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