Research Article | Open Access

Volume 2008 |Article ID 254637 | https://doi.org/10.1155/2008/254637

Basudeb Datta, Nandini Nilakantan, "Three-Dimensional Pseudomanifolds on Eight Vertices", International Journal of Mathematics and Mathematical Sciences, vol. 2008, Article ID 254637, 21 pages, 2008. https://doi.org/10.1155/2008/254637

# Three-Dimensional Pseudomanifolds on Eight Vertices

Revised11 Jun 2008
Accepted25 Jun 2008
Published18 Sep 2008

#### Abstract

A normal pseudomanifold is a pseudomanifold in which the links of simplices are also pseudomanifolds. So, a normal 2-pseudomanifold triangulates a connected closed 2-manifold. But, normal -pseudomanifolds form a broader class than triangulations of connected closed -manifolds for . Here, we classify all the 8-vertex neighbourly normal 3-pseudomanifolds. This gives a classification of all the 8-vertex normal 3-pseudomanifolds. There are 74 such 3-pseudomanifolds, 39 of which triangulate the 3-sphere and other 35 are not combinatorial 3-manifolds. These 35 triangulate six distinct topological spaces. As a preliminary result, we show that any 8-vertex 3-pseudomanifold is equivalent by proper bistellar moves to an 8-vertex neighbourly 3-pseudomanifold. This result is the best possible since there exists a 9-vertex nonneighbourly 3-pseudomanifold which does not allow any proper bistellar moves.

#### 1. Introduction

Recall that a simplicial complex is a collection of nonempty finite sets (sets of vertices) such that every nonempty subset of an element is also an element. For , the elements of size are called the -simplices (or -faces) of the complex.

A simplicial complex is usually thought of as a prescription for construction of a topological space by pasting geometric simplices. The space thus obtained from a simplicial complex is called the geometric carrier of and is denoted by . We also say that triangulates. A combinatorial -manifold (resp., combinatorial-sphere) is a simplicial complex which triangulates a closed surface (resp., the 2-sphere ).

For a simplicial complex , the maximum of such that has a -simplex, is called the dimension of . A -dimensional simplicial complex is called pure if each simplex of is contained in a -simplex of . A -simplex in a pure -dimensional simplicial complex is called a facet. A -dimensional pure simplicial complex is called a weak pseudomanifold if each -simplex of is contained in exactly two facets of .

With a pure simplicial complex of dimension , we associate a graph as follows. The vertices of are the facets of and two vertices of are adjacent if the corresponding facets intersect in a -simplex of . If is connected, then is called strongly connected. A strongly connected weak pseudomanifold is called a pseudomanifold. Thus, for a -pseudomanifold is a connected -regular graph. This implies that has no proper subcomplex which is also a -pseudomanifold; (or else, the facets of such a subcomplex would provide a disconnection of ).

For any set with (), let be the simplicial complex whose simplexes are all the nonempty proper subsets of . Then is a -pseudomanifold and triangulates the -sphere . This -pseudomanifold is called the standard -sphere and is denoted by (or ). By convention, is the only -pseudomanifold.

If is a face of a simplicial complex , then the link of in , denoted by (or ), is by definition the simplicial complex whose faces are the faces of such that is disjoint from and is a face of . Clearly, the link of an -face in a weak -pseudomanifold is a weak -pseudomanifold. For , a connected weak -pseudomanifold is said to be a normal -pseudomanifold if the links of all the simplices of dimension are connected. Thus, any connected triangulated -manifold (triangulation of a closed -manifold) is a normal -pseudomanifold. Clearly, the normal 2-pseudomanifolds are just the connected combinatorial -manifolds; but, normal -pseudomanifolds form a broader class than connected triangulated -manifolds for .

Observe that if is a normal pseudomanifold, then is a pseudomanifold. (If is not connected, then, since is connected, has two components and and two intersecting facets , such that . Choose among all such pairs such that is maximum. Then and is not connected, a contradiction.) Notice that all the links of positive dimensions (i.e., the links of simplices of dimension ) in a normal -pseudomanifold are normal pseudomanifolds. Thus, if is a normal 3-pseudomanifold, then the link of a vertex in is a combinatorial 2-manifold. A vertex of a normal 3-pseudomanifold is called singular if the link of in is not a 2-sphere. The set of singular vertices is denoted by Clearly, the space is a pl 3-manifold. If (i.e., the link of each vertex is a 2-sphere), then is called a combinatorial -manifold. A combinatorial -sphere is a combinatorial 3-manifold which triangulates the topological 3-sphere .

Let be a weak -pseudomanifold. If is a -face of such that and is not a face of (such a face is said to be a removable face of ), then consider the weak -pseudomanifold (denoted by ) whose facet-set is . The operation is called a bistellar -move. For , a bistellar -move is called a proper bistellar move. If is a proper bistellar -move and , then is a removable -face of (with ) and is an bistellar -move. For a vertex , if , then the bistellar -move deletes the vertex (we also say that is obtained from by collapsing the vertex ). The operation is called a bistellar -move (we also say that is obtained from by starring the vertex in the facet of ). The 10-vertex combinatorial 3-manifold in Example 3.15 is not neighbourly and does not allow any bistellar 1-move. In , Bagchi and Datta have shown that if the number of vertices in a nonneighbourly combinatorial 3-manifold is at most 9, then the 3-manifold admits a bistellar 1-move. Existence of the 9-vertex 3-pseudomanifold in Example 3.16 shows that Bagchi and Datta's result is not true for 9-vertex 3-pseudomanifolds. Here we prove the following theorem.

Theorem 1.1. If is an -vertex -pseudomanifold, then there exists a sequence of bistellar -moves , for some , such that is a neighbourly -pseudomanifold.

In , Altshuler has shown that every combinatorial 3-manifold with at most 8 vertices is a combinatorial 3-sphere. In , Grünbaum and Sreedharan have shown that there are exactly 37 polytopal 3-spheres on 8 vertices (namely, in Examples 3.1 and 3.3). They have also constructed the nonpolytopal sphere . In , Barnette proved that there is only one more nonpolytopal 8-vertex 3-sphere (namely, ). In , Emch constructed an 8-vertex normal 3-pseudomanifold (namely, in Example 3.5) as a block design. This is not a combinatorial 3-manifold and its automorphism group is (cf. ). In , Altshuler has constructed another 8-vertex normal 3-pseudomanifold (namely, in Example 3.5). In , Lutz has shown that there exist exactly three 8-vertex normal 3-pseudomanifolds which are not combinatorial 3-manifolds (namely, and in Example 3.5) with vertex-transitive automorphism groups. Here we prove the following theorem.

Theorem 1.2. Let be as in Examples 3.1 and 3.5.
(i)Then and for and .(ii)If is an -vertex neighbourly normal -pseudomanifold, then is isomorphic to one of .

Corollary 1.3. There are exactly combinatorial -manifolds on vertices, all of which are combinatorial -spheres.

Corollary 1.4. There are exactly normal -pseudomanifolds on vertices which are not combinatorial -manifolds. These are defined in Examples 3.5 and 3.8.

The topological properties of these normal -pseudomanifolds are given in Section 3.

#### 2. Preliminaries

All the simplicial complexes considered in this paper are finite (i.e., with finite vertex-set). The vertex-set of a simplicial complex is denoted by . We identify the 0-faces of a complex with the vertices. The -faces of a complex are also called the edges of .

If are two simplicial complexes, then an isomorphism from to is a bijection such that for is a face of if and only if is a face of . Two complexes , are called isomorphic when such an isomorphism exists. We identify two complexes if they are isomorphic. An isomorphism from a complex to itself is called an automorphism of . All the automorphisms of form a group under composition, which is denoted by .

For a face in a simplicial complex , the number of vertices in is called the degree of in and is denoted by (or by ). If every pair of vertices of a simplicial complex form an edge, then is called neighbourly. For a simplicial complex , if , then denotes the induced complex of on the vertex-set .

If the number of -faces of a -dimensional simplicial complex is then the number is called the Euler characteristic of .

A graph is a simplicial complex of dimension . A finite 1-pseudomanifold is called a cycle. An -cycle is a cycle on vertices and is denoted by (or by if the edges are ).

For a simplicial complex , the graph consisting of the edges and vertices of is called the edge-graph of and is denoted by . The complement of is called the nonedge graph of and is denoted by . For a weak 3-pseudomanifold and an integer , we define the graph as follows. The vertices of are the vertices of . Two vertices and form an edge in if is an edge of degree in . Clearly, if and are isomorphic, then and are isomorphic for each .

If is a weak 3-pseudomanifold and is a bistellar 1-move, then, from the definition, and for any vertex . If is a bistellar -move, then .

Consider the binary relation “” on the set of weak 3-pseudomanifolds as if there exists a finite sequence of bistellar -moves , for some , such that . Clearly, this is a partial order relation.

Two weak -pseudomanifolds and are bistellar equivalent (denoted by ) if there exists a finite sequence of bistellar operations leading from to . If there exists a finite sequence of proper bistellar operations leading from to , then we say and are properly bistellar equivalent and we denote this by . Clearly, “" and “" are equivalence relations on the set of pseudomanifolds. It is easy to see that implies that and are pl homeomorphic.

For two simplicial complexes and with disjoint vertex sets, the simplicial complex is called the join of and .

Let be an -vertex (weak) -pseudomanifold. If is a vertex of and is not a vertex of , then consider the simplicial complex on the vertex set whose set of facets is is a facet of and is a facet of . Then is a (weak) -pseudomanifold and is the topological suspension of (cf. ). It is easy to see that the links of and in are isomorphic to . This is called the one-point suspension of .

For two -pseudomanifolds and , a simplicial map is called a -fold branched covering (with discrete branch locus) if is a -fold covering for some . (We say that is a branched cover of and is a branched quotient of .) The smallest such (so that is a covering) is called the branch locus. If is a -fold branched quotient of and is obtained from by collapsing a vertex (resp., starring a vertex in a facet), then is the branched quotient of , where can be obtained from by collapsing vertices (resp., starring vertices in facets). For proper bistellar moves we have the following lemma.

Lemma 2.1. Let and be two -pseudomanifolds and be a -fold branched covering. For , if is a removable -face, then consists of removable -faces and is a -fold branched cover of .

Proof. Let . Since the dimension of is consists of -faces, (say) of . Let and for . Since is simplicial, is not a face of and hence is removable for each . Since , it follows that is neighbourly. For , if , then is an edge in and hence the number of edges in is less than , a contradiction. So, for . This implies that is not a face in and hence is removable in for . The result now follows.

Remark 3.14 shows that Lemma 2.1 is not true for (i.e., for bistellar 1-moves) in general.

Example 2.2. In Figure 2, we present some weak -pseudomanifolds on at most seven vertices. The degree sequences are presented parenthetically below the figures. Each of triangulates the 2-sphere, each of triangulates the real projective plane and triangulates the torus. Observe that are not pseudomanifolds.
We know that if is a weak 2-pseudomanifold with at most six vertices, then is isomorphic to or (cf. ). In , we have seen the following.

Proposition 2.3. There are exactly distinct -dimensional weak pseudomanifolds on vertices, namely, , and .

#### 3. Examples

We identify a weak pseudomanifold with the set of facets in it.

Example 3.1. These four neighbourly 8-vertex combinatorial 3-manifolds were found by Grünbaum and Sreedharan (in , these are denoted by and , resp.). It follows from Lemma 3.4 that these are combinatorial 3-spheres. It was shown in  that the first three of these are polytopal 3-spheres and the last one is a nonpolytopal sphere:

Lemma 3.2. for .

Proof. Observe that , , and , where . Since implies , for .

Example 3.3. Some nonneighbourly 8-vertex combinatorial 3-manifolds. It follows from Lemma 3.4 that these are combinatorial 3-spheres. For , the sphere is isomorphic to the polytopal sphere in  and the sphere is isomorphic to the nonpolytopal sphere found by Barnette in . We consecutively define

Lemma 3.4. (a) , for , is a combinatorial -sphere for , and for .

Proof. For , let denote the set of 's with nonedges. Then and .
From the proof of Lemma 4.7, . Thus, for . Now, if , then, from the definition of . This proves part (a).
Since is a join of spheres, is a combinatorial 3-sphere. Clearly, if and is a combinatorial -sphere, then is so. Part (b) now follows from part (a).
Since the nonedge graphs of the members of (resp., ) are pairwise nonisomorphic, the members of (resp., ) are pairwise nonisomorphic.
For and imply or . Since implies and the members of are pairwise nonisomorphic.
For and imply or . Let Since the nonedge graph of a member in is nonisomorphic to the nonedge graph of a member of for , a member of is nonisomorphic to a member of . Observe that and for implies . Since , the members of are pairwise nonisomorphic.
Since for , the members of are pairwise nonisomorphic. By the same reasoning, the members of are pairwise nonisomorphic.
By Lemma 3.2, the members of are pairwise nonisomorphic. Since a member of is nonisomorphic to a member of for , the above imply part (c).

Example 3.5. Some 8-vertex neighbourly normal 3-pseudomanifolds: All the vertices of are singular and their links are isomorphic to the 7-vertex torus . There are two singular vertices in and their links are isomorphic to . The singular vertices in are 8, 3, 4, 2, 5 and their links are isomorphic to and , respectively. There is only one singular vertex in whose link is isomorphic to . All the vertices of (resp., ) are singular and their links are isomorphic to (resp., ). Each of has exactly two singular vertices and their links are 7-vertex 's. Thus, each is a normal 3-pseudomanifold.
It follows from the definition that for . Here we prove the following lemmas.

Lemma 3.6. (a) The geometric carriers of , and are distinct (non-homeomorphic), for , .

Proof. For a normal 3-pseudomanifold , let denote the number of singular vertices. Clearly, if and are two normal 3-pseudomanifolds with homeomorphic geometric carriers, then . Now, , , , . This proves part .
Part (b) follows from the fact that is neighbourly and has no removable edge and, hence, there is no proper bistellar move from for .
Let be obtained from by starring a new vertex 0 in the facet . Let , then is isomorphic to via the map . This proves part (c).

Lemma 3.7. for .

Proof. Let be as above. Clearly, if and are two isomorphic 3-pseudomanifolds, then . Now, , and for . Since the links of each vertex in is isomorphic to and the links of each vertex in is isomorphic to , it follows that . Thus, for , , .
Observe that the singular vertices in are 3 and 8 for . Moreover, (i) , (ii) and , (iii) and , (iv) and , (v) and , (vi) and , (vii) and , , (viii) and . (xi) and . These imply that there is no isomorphism between and for . This completes the proof.

Example 3.8. Some 8-vertex nonneighbourly normal 3-pseudomanifolds:

Lemma 3.9. for and for .

Proof. For , let denote the set of 3-pseudomanifolds defined in Examples 3.5 and 3.8 with nonedges. Then , and . The singular vertices in are 3 and 8 for .
By Lemma 3.7, the members of are pairwise nonisomorphic.
Observe that (i) and , (ii) , (iii) and , (iv) and , (v) and , (vi) and , (vii) and , (viii) and , (ix) . These imply that there is no isomorphism between any two members of .
Observe that (i) and , (ii) and , (iii) and , (iv) and , (v) and , (vi) and , (vii) . These imply that there is no isomorphism between any two members of .
Observe that (i) , (ii) , (iii) , (iv) and . These imply that there is no isomorphism between any two members of .
Since a member of is nonisomorphic to a member of for , the above imply part (a). Part (b) follows from the definition of for .

The 3-dimensional Kummer variety is the torus modulo the involution . It has 8 singular points corresponding to 8 elements of order 2 in the abelian group . In , Kühnel showed that triangulates . For a topological space denotes a cone with base . Let denote the solid torus. As a consequence of the above lemmas we get.

Corollary 3.10. All the -vertex normal -pseudomanifolds triangulate seven distinct topological spaces, namely, for and for .

Proof. Let be an -vertex normal -pseudomanifold. If is a combinatorial -sphere, then it triangulates the 3-sphere .
If is not a combinatorial -sphere, then, by Lemma 3.9(b), is (pl) homeomorphic to , or . Since is homeomorphic to the suspension . In , the facets not containing the vertex 8 form a solid torus whose boundary is the link of 8. This implies that . It follows from Lemma 3.6(c) that is (pl) homeomorphic to . Since is isomorphic to the suspension , . Therefore, by Lemma 3.9(b), is (pl) homeomorphic to for . The result now follows from Lemma 3.6(a).

A 3-dimensional pseudocomplex is an ordered pair , where is a finite collection of disjoint tetrahedra and is a family of affine isomorphisms between pairs of 2-faces of the tetrahedra in . Let denote the quotient space obtained from the disjoint union by setting for . The quotient of a tetrahedron in is called a -simplex in and is denoted by . Similarly, the quotient of 2-faces, edges, and vertices of tetrahedra are called -simplices, edges, and vertices in , respectively. If is homeomorphic to a topological space , then is called a pseudotriangulation of . A 3-dimensional pseudocomplex is said to be regular if the following hold: (i) each 3-simplex in has four distinct vertices, and (ii) for , no two distinct -simplices in have the same set of vertices. So, for , an -simplex in is uniquely determined by its vertices and denoted by , where are vertices of . (But, the edges in may not form a simple graph.) So, we can identify a regular pseudocomplex with . Simplices and edges in are said to be simplices and edges of . Clearly, a pure 3-dimensional simplicial complex is a regular pseudocomplex.

Let be a regular pseudotriangulation of and be two 3-simplices in . If are not 2-simplices in , then is also a regular pseudotriangulation of . We say that is obtained from by the generalized bistellar -move. If there is no edge between and in , then is called a bistellar -move. If there exist 3-simplices of the form in a regular pseudotriangulation of and is not a 2-simplex, then is also a regular pseudotriangulation of . We say that is obtained from by the generalized bistellar -move, where is the common edge in and . If is the only edge between and in , then is called a bistellar -move.

Let be a pseudotriangulation of a closed 3-manifold and a 3-pseudomanifold. A simplicial map is said to be a -fold branched covering (with discrete branch locus) if there exists such that is a -fold covering. The smallest such (so that is a covering) is called the branch locus. It is known that can be regarded as a branched quotient of a regular hyperbolic tessellation (cf. ). In , Kühnel has shown that is a 2-fold branched quotient of a pseudotriangulation of the 3-dimensional torus. Here we prove the following theorem.

Theorem 3.11. (a) is a -fold branched quotient of a -vertex combinatorial -sphere.
(b) For is a -fold branched quotient of a -vertex regular pseudotriangulation of the -sphere.

Lemma 3.12. Let be a regular pseudotriangulation of a -manifold and be a normal -pseudomanifold. Let be a -fold branched covering with at most two vertices in the branch locus. If is a bistellar -move, then there exist generalized bistellar -moves such that is a -fold branched cover of .

Proof. Let . Let consist of the edges . Let the end points of be , , the 3-simplices containing be , and for . Since is not a simplex in , it follows that is not a 2-simplex in . Let be the pseudocomplex consists of and . Since the number of vertices in the branched locus is at most 2, it follows that the number of vertices common in and is at most 2 for . In particular, . Therefore, is not a 2-simplex in . So, we can perform generalized bistellar 2-move on