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Symmetry 2014, 6(2), 189-209; doi:10.3390/sym6020189
Published: 8 April 2014
Abstract
: Topological symmetry groups were originally introduced to study the symmetries of non-rigid molecules, but have since been used to study the symmetries of any graph embedded in ℝ^{3}. In this paper, we determine for each complete graph K_{n} with n ≤ 6, what groups can occur as topological symmetry groups or orientation preserving topological symmetry groups of some embedding of the graph in ℝ^{3}.1. Introduction
Molecular symmetries are important in many areas of chemistry. Symmetry is used in interpreting results in crystallography, spectroscopy, and quantum chemistry, as well as in analyzing the electron structure of a molecule. Symmetry is also used in designing new pharmaceutical products. But what is meant by a “symmetry” depends on the rigidity of the molecule in question.
For rigid molecules, the group of rotations, reflections, and combinations of rotations and reflections, is an effective way of representing molecular symmetries. This group is known as the point group, of the molecule because it fixes a point of ℝ^{3}. However, some molecules can rotate around particular bonds, and large molecules can even be somewhat flexible. For example, supramolecular structures constructed through self-assembly may be somewhat conformationally flexible. Even relatively small molecules may contain rigid molecular subparts that rotate on hinges around particular bonds. For example, the left and right sides of the biphenyl derivative illustrated in Figure 1 rotate simultaneously, independent of the central part of the molecule. Because of these rotating pieces, this molecule is achiral though it cannot be rigidly superimposed on its mirror form. A detailed discussion of the achirality of this molecule can be found in [1].
In general, the amount of rigidity of a given molecule depends on its chemistry not just its geometry. Thus a purely mathematical definition of molecular symmetries that accurately reflects the behavior of all molecules is impossible. However, for non-rigid molecules, a topological approach to classifying symmetries including achirality can add important information beyond what is obtained from the point group. Such an approach could be useful to the study of supramolecular chirality, since structures constructed through self-assembly may be large and somewhat flexible or contain subparts that can rotate around covalent or non-convalent bonds.
The topological symmetry group was first introduced by Jon Simon in 1987 in order to classify the symmetries of non-rigid molecules [2]. By comparing the topological symmetry group and the orientation preserving topological symmetry group of a particular structure, one can see whether the structure is achiral and if so, understand how its achirality fits together with its other topological symmetries.
In this paper, we determine both the topological symmetry groups and the orientation preserving topological symmetry groups of structures whose underlying form is that of a complete graph with no more than six vertices. A complete graph, K_{n}, is defined to be a graph with n vertices which has an edge between every pair of vertices. In Figure 2 we illustrate embeddings of the complete graphs K_{3}, K_{4}, and K_{5}. The class of complete graphs is an interesting class to consider because the automorphism group of K_{n} is the symmetric group S_{n}, which is the largest automorphism group of any graph with n vertices. For small values of n, there exist molecules whose underlying topological structure has the form of K_{n}. For example, a tetrahedral supramolecular cluster has the underlying structure of the complete graph K_{4}. If such a cluster contains a central atom which is bonded to the four corners of the tetrahedron, then the structure has the form of the complete graph K_{5} (as illustrated on the right in Figure 2).
2. Background and Terminology
Though it may seem strange from the point of view of a chemist, the study of symmetries of embedded graphs is more convenient to carry out in the 3-dimensional sphere S^{3} = ℝ^{3} ∪ {∞} rather than in Euclidean 3-space, ℝ^{3}. In particular, in ℝ^{3} every rigid motion is a rotation, reflection, translation, or a combination of these operations. Whereas, in S^{3} glide rotations provide an additional type of rigid motion. While a topological approach to the study of symmetries does not require us to focus on rigid motions, for the purpose of illustration it is preferable to display rigid motions rather than isotopies whenever possible. Thus throughout the paper we work in S^{3} rather than in ℝ^{3}.
Definition 1
The topological symmetry group of a graph Γ embedded in S3 is the subgroup of the automorphism group of the graph, Aut(Γ), induced by homeomorphisms of the pair (S^{3}, Γ). The orientation preserving topological symmetry group, TSG_{+}(Γ), is the subgroup of Aut(Γ) induced by orientation preserving homeomorphisms of (S^{3}, Γ).
It should be noted that for any homeomorphism h of (S^{3}, Γ), there is a homeomorphism g of (S^{3}, Γ) which fixes a point p not on Γ such that g and h induce the same automorphism on Γ. By choosing p to be the point at ∞, we can restrict g to a homeomorphism of (ℝ^{3}, Γ). On the other hand if we start with an embedded graph Γ in ℝ^{3} and a homeomorphism g of (ℝ^{3}, Γ), we can consider Γ to be embedded in S^{3} = ℝ^{3} ∪ {∞} and extend g to a homeomorphism of S^{3} simply by fixing the point at ∞. It follows that the topological symmetry group of Γ in S^{3} is the same as the topological symmetry group of Γ in ℝ^{3}. Thus we lose no information by working with graphs in S^{3} rather than graphs in ℝ^{3}.
It was shown in [3] that the set of orientation preserving topological symmetry groups of 3-connected graphs embedded in S^{3} is the same up to isomorphism as the set of finite subgroups of the group of orientation preserving diffeomorphisms of S^{3}, Diff_{+}(S^{3}). However, even for a 3-connected embedded graph Γ, the automorphisms in TSG(Γ) are not necessarily induced by finite order homeomorphisms of (S^{3}, Γ).
For example, consider the embedded 3-connected graph Γ illustrated in Figure 3. The automorphism (153426) is induced by a homeomorphism that slithers the graph along itself while interchanging the inner and outer knots in the graph. On the other hand, the automorphism (153426) cannot be induced by a finite order homeomorphism of S^{3} because there is no order three homeomorphism of S^{3} taking a figure eight knot to itself [4,5] and the embedded graph in Figure 3 cannot be pointwise fixed by a finite order homeomorphism of S^{3} [6].
On the other hand, Flapan proved the following theorem which we will make use of later in the paper.
Finite Order Theorem
[7] Let φ be a non-trivial automorphism of a 3-connected graph γ which is induced by a homeomorphism h of (S^{3}, Γ) for some embedding Γ ofγ in S^{3}. Then for some embedding Γ of γ in S^{3}, the automorphism φ is induced by a finite order homeomorphism, f of (S^{3}, Γ), and f is orientation reversing if and only ifh is orientation reversing.
In the definition of the topological symmetry group, we start with a particular embedding Γ of a graph γ in S^{3} and then determine the subgroup of the automorphism group of γ which is induced by homeomorphisms of (S^{3},Γ). However, sometimes it is more convenient to consider all possible subgroups of the automorphism group of an abstract graph, and ask which of these subgroups can be the topological symmetry group or orientation preserving topological symmetry group of some embedding of the graph in S^{3}. The following definition gives us the terminology to talk about topological symmetry groups from this point of view.
Definition 2
An automorphism f of an abstract graph, γ, is said to be realizable if there exists an embedding Γ of γ in S^{3} such that f is induced by a homeomorphism of (S^{3}, Γ). A group G is said to be realizable for γ if there exists an embedding Γ of γ in S^{3} such that TSG(Γ) = G. If there exists an embedding Γ such that TSG_{+}(Γ) = G, then we say G is positively realizable for γ.
It is natural to ask whether every finite group is realizable. In fact, it was shown in [3] that the alternating group A_{m} is realizable for some graph if and only if m < 5. Furthermore, in [8] it was shown that for every closed, connected, orientable, irreducible 3-manifold M, there exists an alternating group A_{m} which is not isomorphic to the topological symmetry group of any graph embedded in M.
3. Topological Symmetry Groups of Compete Graphs
For the special class of complete graphs K_{n} embedded in S^{3}, Flapan, Naimi, and Tamvakis obtained the following result.
Complete Graph Theorem
[9] A finite group H is isomorphic to TSG_{+}(Γ) for some embedding Γ of a complete graph in S^{3} if and only if H is a finite subgroup of SO(3) or a subgroup of D_{m} × D_{m}for some odd m.
This left open the question of what topological symmetry groups and orientation preserving topological symmetry groups are possible for embeddings of a particular complete graph K_{n} in S^{3}. For each n > 6, this question was answered for orientation preserving topological symmetry groups in the series of papers [10-13]. These papers make use of a result that for n > 6, only a few types of automorphisms of K_{n} are realizable [7]. There are no comparable results available for automorphisms of K_{n} when n ≤ 6.
In the current paper, we determine which groups are realizable and which groups are positively realizable for each K_{n} with n < 6. This is the first family of graphs for which both the realizable and the positively realizable groups have been determined.
For n ≤ 3, this question is easy to answer. In particular, since K_{1} is a single vertex, the only realizable or positively realizable group is the trivial group. Since K_{2} is a single edge, the only realizable or positively realizable group is ℤ_{2}.
For n = 3, we know that Aut(K_{3}) ≅ S_{3} ≅ D_{3}, and hence every realizable or positively realizable group for K_{3} must be a subgroup of D_{3}. Note that for any embedding of K_{3} in S^{3}, the graph can be “slithered” along itself to obtain an automorphism of order 3 which is induced by an orientation preserving homeomorphism. Thus the topological symmetry group and orientation preserving topological symmetry group of any embedding of K_{3} will contain an element of order 3. Thus neither the trivial group nor Z_{2} is realizable or positively realizable for K_{3}. If Γ is a planar embedding of K_{3} in S^{3}, then TSG(Γ) = TSG_{+}(Γ) ≅ D_{3}. Recall that the trefoil knot 31 is chiral while the knot 81_{7} is negative achiral and non-invertible. Thus if Γ is the knot 8_{17}, then no orientation preserving homeomorphism of (S^{3}, Γ) inverts Γ, but there is an orientation reversing homeomorphism of (S^{3}, Γ) which inverts Γ. Whereas, if Γ is the knot 8_{17}#3_{1}, then there is no homeomorphism of (S^{3}, Γ) which inverts Γ. Table 1 summarizes our results for K_{3}.
Determining which groups are realizable and positively realizable for K_{4}, K_{5}, and K_{6} is the main point of this paper. In each case, we will first determine the positively realizable groups and then use the fact that either TSG_{+}(Γ) = TSG(Γ) or TSG_{+}(Γ) is a normal subgroup of TSG(Γ) of index 2 to help us determine the realizable groups.
4. Topological Symmetry Groups of K_{4}
In addition to the Complete Graph Theorem given above, we will make use of the following results in our analysis of positively realizable groups for K_{n} with n ≥ 4.
A_{4} Theorem
[11] A complete graph K_{m} with m ≥ 4 has an embedding Γ in S^{3} such that TSG_{+}(Γ) ≅ A_{4} if and only ifm ≡ 0, 1, 4, 5, 8 (mod 12).
A_{5} Theorem
[11] A complete graph K_{m} with m ≥ 4 has an embedding Γ in S^{3} such that TSG_{+}(Γ) ≅ A_{5} if and only ifm ≡ 0, 1, 5, 20 (mod 60).
S_{4} Theorem
[11] A complete graph K_{m} with m ≥ 4 has an embedding Γ in S^{3} such that TSG_{+}(Γ) ≅ S_{4} if and only if m ≡ 0, 4, 8, 12, 20 (mod 24).
Subgroup Theorem
[12] Let Γ be an embedding of a 3-connected graph γ in S^{3} with an edge that is not poinwise fixed by any non-trivial element of TSG_{+}(Γ). Then every subgroup of TSG_{+}(Γ) is positively realizable for γ.
It was shown in [12] that adding a local knot to an edge of a 3-connected graph is well-defined and that any homeomorphism of S^{3} taking the graph to itself must take an edge with a given knot to an edge with the same knot. Furthermore, any orientation preserving homeomorphism of S^{3} taking the graph to itself must take an edge with a given non-invertible knot to an edge with the same knot oriented in the same way Thus for n > 3, adding a distinct knot to each edge of an embedding of K_{n} in S^{3} will create an embedding Δ where TSG(Δ) and TSG_{+}(Δ) are both trivial. Hence we do not include the trivial group in our list of realizable and positively realizable groups for K_{n} when n > 3.
Finally, observe that for n > 3, for a given embedding Γ of K_{n} we can add identical chiral knots (whose mirror image do not occur in Γ) to every edge of Γ to get an embedding Γ such that TSG(Γ′) = TSG_{+}(Γ). Thus every group which is positively realizable for K_{n} is also realizable for K_{n}. We will use this observation in the rest of our analysis.
The following is a complete list of all the non-trivial subgroups of Aut(K_{4}) ≅ S_{4} up to isomorphism: S_{4}, A_{4}, D_{4}, D_{3}, D_{2}, ℤ_{4}, ℤ_{3}, ℤ_{2}.
We will show that all of these groups are positively realizable, and hence all of the groups will also be realizable. First consider the embedding Γ of K_{4} illustrated in Figure 4. The square $\overline{1234}$ must go to itself under any homeomorphism of (S^{3}, Γ). Hence TSG_{+}(Γ) is a subgroup of D_{4}. In order to obtain the automorphism (1234), we rotate the square $\overline{1234}$ clockwise by 90° and pull $\overline{24}$ under $\overline{13}$. We can obtain the transposition (13) by first rotating the figure by 180° about the axis which contains vertices 2 and 4 and then pulling $\overline{13}$ under $\overline{24}$. Thus TSG_{+}(Γ) ≅ D_{4}. Furthermore, since the edge $\overline{12}$ is not pointwise fixed by any non-trivial element of TSG_{+}(Γ), by the Subgroup Theorem the groups ℤ_{4}, D_{2} and ℤ_{2} are each positively realizable for K_{4}.
Next, consider the embedding, Γ of K_{4} illustrated in Figure 5. All homeomorphisms of (S^{3}, Γ) fix vertex 4. Hence TSG_{+}(Γ) is a subgroup of D_{3}. The automorphism (123) is induced by a rotation, and the automorphism (12) is induced by turning the figure upside down and then pushing vertex 4 back up through the centre of 123. Thus TSG_{+}(Γ) = D_{3}. Since the edge 12 is not pointwise fixed by any non-trivial element of TSG_{+}(Γ), by the Subgroup Theorem, the group Z_{3} is also positively realizable for K_{4}.
Thus every subgroup of Aut(K_{4}) is positively realizable. Now by adding appropriate equivalent chiral knots to each edge, all subgroups of Aut(K_{4}) are also realizable. We summarize our results for K_{4} in Table 2.
5. Topological Symmetry Groups of K_{5}
The following is a complete list of all the non-trivial subgroups of Aut(K_{5}) ≅ S_{5}:
S_{5}, A_{5}, S_{4}, A_{4}, ℤ_{5} × ℤ_{4}, D_{6}, D_{5}, D_{4}, D_{3}, D_{2}, ℤ_{6}, ℤ_{5}, ℤ_{4}, ℤ_{3}, ℤ_{2} (see [14] and [15]).
The lemma below follows immediately from the Finite Order Theorem [7] (stated in the introduction) together with Smith Theory [6].
Lemma 1
Let n > 3 and let φ be a non-trivial automorphism of K_{n} which is induced by a homeomorphism h of (S^{3}, Γ) for some embedding Γ of K_{n} in S^{3}. If h is orientation reversing, then φ fixes at most 4 vertices. Ifh is orientation preserving, then φ fixes at most 3 vertices, and if φ has even order, then φ fixes at most 2 vertices.
We now prove the following lemma.
Lemma 2
Let n > 3 and let Γ be an embedding of K_{n} in S^{3} such that TSG_{+}(Γ) contains an element φ of even order m > 2. Then φ does not fix any vertex or interchange any pair of vertices.
Proof
By the Finite Order Theorem, K_{n} can be re-embedded as Γ so that φ is induced on Γ by a finite order orientation preserving homeomorphism h of (S^{3}, Γ′). Suppose that φ fixes a vertex or interchanges a pair of vertices of Γ. Then fix(h) is non-empty, and hence by Smith Theory, fix(h) ≅ S^{1}. Let r = m/2. Then h^{r} induces an involution on the vertices of Γ, and this involution can be written as a product (a_{1}b_{1}) … (a_{q}b_{q}) of disjoint transpositions of vertices. Now for each i, h^{r} fixes a point on the edge a_{i}b. But fix(h^{r}) contains fix(h) and thus by Smith Theory fix(h^{r}) = fix(h). Hence h fixes a point on each edge a_{i}b_{i}. Thus h induces also (a_{1} b_{1}) … (a_{q}b_{q}) on the vertices of Γ′. But this contradicts the hypothesis that the order of φ is m > 2.
By Lemma 2, there is no embedding of K_{5} in S^{3} such that TSG_{+}(Γ) contains an element of order 4 or of order 6. It follows that TSG_{+}(Γ) cannot be D_{6}, ℤ_{6}, D_{4} or ℤ_{4}.
Consider the embedding Γ of K_{5} illustrated in Figure 6. The knotted cycle 13524 must be setwise invariant under every homeomorphism of Γ. Thus TSG_{+}(Γ) ≤ D_{5}. The automorphism (12345) is induced by rotating Γ, and (25)(34) is induced by turning the graph over. Hence TSG_{+}(Γ) = 〈(12345;, (25)(34)〉 ≅ D_{5}. Since the edge $\overline{12}$ is not pointwise fixed by any non-trivial element of TSG_{+}(Γ), by the Subgroup Theorem the groups ℤ_{5} and ℤ_{2} are also positively realizable for K_{5}.
Next consider the embedding Γ of K_{5} illustrated in Figure 7. The triangle $\overline{123}$ must go to itself under any homeomorphism. Also by Lemma 1, any orientation preserving homeomorphism which fixes vertices 1, 2, and 3 induces a trivial automorphism on K_{5}. Thus TSG_{+}(Γ) ≤ D_{3}. The automorphism (123) is induced by a rotation. Also the automorphism (45)(12) is induced by pulling vertex 4 down through the centre of triangle $\overline{123}$while pulling vertex 5 into the centre of the figure then rotating by 180° about the line through vertex 3 and the midpoint of the edge $\overline{12}$. Thus TSG_{+}(Γ) = 〈(123;, (45)(12)〉 ≅ D_{3}. Since the edge $\overline{12}$is not pointwise fixed by any non-trivial element of TSG_{+}(Γ), by the Subgroup Theorem, the group ℤ_{3} is positively realizable for K_{5}.
Lastly, consider the embedding Γ of K_{5} illustrated in Figure 8 with vertex 5 at infinity. The square $\overline{1234}$must go to itself under any homeomorphism. Hence TSG_{+}(Γ) ≤ D_{4}. The automorphism (13)(24) is induced by rotating the square by 180°. By turning over the figure we obtain (12)(34). By Lemma 2, TSG_{+}(Γ) cannot contain an element of order 4. Thus TSG_{+}(Γ) = 〈(13)(24;, (12)(34)〉 ≅ D_{2}.
We summarize our results on positive realizability for K_{5} in Table 3.
Again by adding appropriate equivalent chiral knots to each edge, all of the positively realizable groups for K_{5} are also realizable. Thus we only need to determine realizability for the groups S_{5}, S_{4}, ℤ_{5} ⋊ ℤ_{4}, D_{6}, D_{4}, ℤ_{6},andℤ_{4}.
Let Γ be the embedding of K_{5} illustrated in Figure 9. Any transposition which fixes vertex 5 is induced by a reflection through the plane containing the three vertices fixed by the transposition. To see that any transposition involving vertex 5 can be achieved, consider the automorphism (15). Pull $\overline{15}$through the triangle $\overline{234}$and then turn over the embedding so that vertex 5 is at the top, vertex 1 is in the centre and vertices 3 and 4 are switched. Now reflect in the plane containing vertices 1, 5, and 2 in order to switch vertices 3 and 4 back. All other transpositions involving vertex 5 can be induced by a similar sequence of moves. Hence TSG(Γ) ≅ S_{5}.
We create a new embedding Γ from Figure 9 by adding the achiral figure eight knot, 4_{1}, to all edges containing vertex 5. Now every homeomorphism of (S^{3}, Γ) fixes vertex 5, yet all transpositions fixing vertex 5 are still possible. Thus TSG(Γ′) ≅ S_{4}.
In order to prove D_{4} is realizable for K_{5} consider the embedding Γ illustrated in Figure 10. Every homeomorphism of (S^{3}, Γ) takes $\overline{1234}$to itself, so TSG(Γ) ≤ D_{4}. The automorphism (1234) is induced by rotating the graph by 90° about a vertical line through vertex 5, then reflecting in the plane containing the vertices 1, 2, 3, 4, and finally isotoping the knots into position. Furthermore, reflecting in the plane containing $\overline{153}$or $\overline{254}$and then isotoping the knots into position yields the transposition (24) or (13) respectively. Hence TSG(Γ) ≅ D_{4}.
We obtain a new embedding Γ by replacing the invertible 41 knots in Figure 10 with the knot 12_{427}, which is positive achiral but non-invertible [16]. Since 12_{427} is neither negative achiral nor invertible, no homeomorphism of (S^{3}, Γ) can invert $\overline{1234}$. Thus TSG(Γ′) ≅ ℤ_{4}.
Next let Γ denote the embedding of K_{5} illustrated in Figure 11. Every homeomorphism of (S^{3}, Γ) takes $\overline{123}$ to itself, so TSG(Γ) ≤ D_{6}. The 3-cycle (123) is induced by a rotation. Each transposition involving only vertices 1, 2, and 3 is induced by a reflection in the plane containing $\overline{45}$ and the remaining fixed vertex followed by an isotopy. The transposition (45) is induced by a reflection in the plane containing vertices 1, 2 and 3 followed by an isotopy. Thus TSG(Γ) ≅ D_{6}, generated by (123), (23), and (45).
We obtain a new embedding Γ′ by replacing the 4_{1} knots in Figure 11 by 12_{427} knots. Then the triangle $\overline{123}$ cannot be inverted. Thus TSG(Γ′) ≅ ℤ_{6}, generated by (123) and (45).
It is more difficult to show that ℤ_{5} × ℤ_{4} is realizable for K_{5}, so we define our embedding in two steps. First we create an embedding Γ of K_{5} on a torus T that is standardly embedded in S^{3}. In Figure 12, we illustrate Γ on a flat torus. Let f denote a glide rotation of S^{3} which rotates the torus longitudinally by 4π/5 while rotating it meridinally by 8π/5. Thus f takes Γ to itself inducing the automorphism (12345).
Let g denote the homeomorphism obtained by rotating S^{3} about a (1, 1) curve on the torus T, followed by a reflection through a sphere meeting T in two longitudes, and then a meridional rotation of T by 6π/5. In Figure 13, we illustrate the step-by-step action of g on T, showing that g takes Γ to itself inducing (2431).
The homeomorphisms f and g induce the automorphisms φ = (12345) and ψ = (2431) respectively. Observe that φ^{5} = ψ^{4} = 1 and ψφ = φψ^{2}. Thus 〈φ, ψ〉 ≅ ℤ_{5} ⋊ ℤ_{4} ≤ TSG(Γ) ≤ S_{5}. Note however that the embedding in Figure 12 is isotopic to the embedding of K_{5} in Figure 9. Thus TSG(Γ) ≅ S_{5}.
In order to obtain the group ℤ_{5} ⋊ ℤ_{4}, we now consider the embedding Γ of K_{5} whose projection on a torus is illustrated in Figure 14. Observe that the projection of Γ′ in every square of the grid given by Γ on the torus is identical. Thus the homeomorphism f which took Γ to itself inducing the automorphism φ = (12345) on Γ also takes Γ′ to itself inducing φ on Γ′.
Recall that g was the homeomorphism of (S^{3}, Γ) obtained by rotating S^{3} about a (1, 1) curve on the torus T, followed by a reflection through a sphere meeting T in two longitudes, and then a meridional rotation of T by 6π/5. In order to see what g does to Γ′, we focus on the square $\overline{1534}$ of Γ′. Figure 15 illustrates a rotation of the square $\overline{1534}$ about a diagonal, then a reflection of the square across a longitude. The result of these two actions takes the projection of the knot $\overline{1534}$ to an identical projection. Thus after rotating the torus meridionally by 6π/5, we see that g takes Γ′ to itself inducing the automorphism ψ = (2431). It now follows that ℤ_{5} ⋊ ℤ_{4} ≤ TSG(Γ′) < S_{5}.
In order to prove that TSG(Γ′) ≅ ℤ_{5} ⋊ ℤ_{4}, we need to show TSG(Γ′) ≇ S_{5}. We prove this by showing that the automorphism (15) cannot be induced by a homeomorphism of (S^{3}, Γ′).
From Figure 15 we see that the square $\overline{1534}$ is the knot 4_{1}#4_{1}#4_{1}#4_{1}. In order to see what would happen to this knot if the transposition (15) were induced by a homeomorphism of (S^{3}, Γ), we consider the square $\overline{5123}$. In Figures 16 and 17 we isotop $\overline{5134}$ to a projection with only 10 crossings. This means that $\overline{5134}$ cannot be the knot 4_{1}#4_{1}#4_{1}#4_{1}. It follows that the automorphism (15) cannot be induced by a homeomorphism of (S^{3}, Γ). Hence TSG(Γ′) ≇ S_{5}. However, the only subgroup of S_{5} that contains ℤ_{5} ⋊ ℤ_{4} and is not S_{5} is the group ℤ_{5} ⋊ ℤ_{4}. Thus in fact TSG(Γ′) ≅ ℤ_{5} ⋊ ℤ_{4}.
Thus every subgroup of Aut(K_{5}) is realizable for K_{5}. Table 4 summarizes our results for TSG(K_{5}).
6. Topological Symmetry Groups of K_{6}
The following is a complete list of all the subgroups of Aut(K_{6}) ≅ S_{6}: S_{6}, A_{6}, s_{5}, A_{5}, S_{2}[S_{3}] (B[A] represents a wreath product of A by B.), S_{4} × ℤ_{2}, A_{4} × ℤ_{2}, S_{4}, A_{4}, ℤ_{5} ⋊ ℤ_{4}, D_{3} × D_{3}, (ℤ_{3} × ℤ_{3}) ⋊ ℤ_{4}, (ℤ_{3} × ℤ_{3}) ⋊ ℤ_{2}, D_{3} × ℤ_{3}, ℤ_{3} × ℤ_{3}, D_{6}, D_{5}, D_{4}, D_{4} × ℤ_{2}, D_{3}, D_{2}, ℤ_{6}, ℤ_{5}, ℤ_{4}, ℤ_{4} × ℤ_{2}, ℤ_{3}, ℤ_{2}, ℤ_{2} × ℤ_{2} × ℤ_{2} (see [17] and independently verified using the program GAP).
Consider the embedding Γ of K_{6} illustrated in Figure 18. There are three paths in Γ on different levels that look like the letter “Z” which are highlighted in Figure 18. The top Z-path is $\overline{3146}$, the middle Z-path is $\overline{4251}$, and the bottom Z-path is $\overline{5362}$. The knotted cycle $\overline{123456}$ must be setwise invariant under every homeomorphism of Γ, and hence TSG_{+}(Γ) < D_{6}. The automorphism (123456) is induced by a glide rotation that cyclically permutes the Z-paths. Consider the homeomorphism obtained by rotating Γ by 180° about the line through vertices 2 and 5 and then pulling the edges $\overline{13}$ and $\overline{46}$ to the top level while pushing the lower edges down. The result of this homeomorphism is that the top Z-path $\overline{3146}$ goes to the top Z-path $\overline{1364}$, the middle Z-path $\overline{4251}$ goes to to middle Z-path $\overline{6253}$, and the bottom Z-path $\overline{5362}$ goes to the bottom Z-path $\overline{5142}$. Thus the homeomorphism leaves Γ setwise invariant inducing the automorphism (13)(46). It follows that TSG_{+}(Γ) = 〈(123456;, (13)(46)〉 ≅ D_{6}. Finally, since the edge $\overline{12}$ is not pointwise fixed by any non-trivial element of TSG_{+}(Γ), by the Subgroup Theorem the groups ℤ_{6}, D_{3}, ℤ_{3}, D_{2} and ℤ_{2} are positively realizable for K_{6}.
Consider the embedding, Γ of K_{6} illustrated in Figure 19 with vertex 6 at infinity. The automorphisms (13524) and (25)(34) are induced by rotations. Also since 13524 is the only 5-cycle which is knotted, $\overline{13524}$ is setwise invariant under every homeomorphism of (S^{3}, Γ). Hence TSG_{+}(Γ) ≅ D_{5}. Also since 15 is not pointwise fixed under any homeomorphism, by the Subgroup Theorem, ℤ_{5} is positively realizable for K_{6}.
Next consider the embedding, Γ of K_{6} illustrated in Figure 20. The automorphisms (123)(456) and (123)(465) are induced by glide rotations and (45)(12) is induced by turning the figure upside down. Also if we consider the circles $\overline{123}$ and $\overline{456}$ as cores of complementary solid tori, then (14)(25)(36) is induced by an orientation preserving homeomorphism that switches the two solid tori.
Observe that every homeomorphism of (S^{3}, Γ) takes the pair of triangles $\overline{123}\cup \overline{456}$ to itself, since this is the only pair of complementary triangles not containing knots. The automorphism group of the union of two triangles is S_{2}[S_{3}] [18]. Thus TSG_{+}(Γ) ≤ S_{2}[S_{3}]. Note that the transpositions (12) and (45) are each induced by a reflection followed by an isotopy. Thus TSG(Γ) ≅ S_{2}[S_{3}], since (123)(456), (123)(465), (12) and (14)(25)(36) generate S_{2}[S_{3}]. However, by the Complete Graph Theorem, TSG_{+}(Γ) ≇ S_{2}[S_{3}]. Thus TSG_{+}(Γ) must be an index 2 subgroup of S_{2}[S_{3}] containing f = (123)(456), g = (123)(465), φ = (45)(12) and ψ = (14)(25)(36). Observe that φψ is the involution (42)(51)(36), and f commutes with ψ and also fφψ = φψf^{-}^{1}, while g commutes with φψ and gψ = ψg^{-1}. Thus S_{2}[S_{3}] ≥ TSG_{+}(Γ) ≥ 〈f,φψ〉 × (g,ψ) ≅ D_{3} × D_{3}. It follows that TSG+(Γ) ≅ D_{3} × D_{3}.
The subgroup 〈f, g, ψ〉 is isomorphic to D_{3} × ℤ_{3} because ψ commutes with f and gψ = ψg^{-1}. We add the non-invertible knot 8_{17} to every edge of the triangles $\overline{123}$ and $\overline{456}$ to obtain an embedding Γ_{1}. Now the automorphism φ = (45)(12) cannot be induced by an orientation preserving homeomorphism of (S^{3}, Γ_{1}). However, f, g, and ψ are still induced by orientation preserving homeomorphisms. Thus TSG_{+}(Γ_{1}) ≅ D_{3} × ℤ_{3} since D_{3} × ℤ_{3} is a maximal subgroup of D_{3} x D_{3}.
Also 〈f, g, φ〉 is isomorphic to (ℤ_{3} × ℤ_{3}) ⋊ ℤ_{2} because fφ = φf^{-}^{1} and gφ = φg^{-1}. Again starting with Γ in Figure 20, we place 5_{2} knots on the edges of the triangle 123 so that ψ is no longer induced. Thus creating and embedding Γ_{2} with TSG_{+}(Γ_{2}) ≅ (ℤ_{3} × ℤ_{3}) ⋊ ℤ_{2} since (ℤ_{3} × ℤ_{3}) ⋊ ℤ_{2} is a maximal subgroup of D_{3} × D_{3}.
Finally 〈f, g〉 is isomorphic to ℤ_{3} × ℤ_{3}. If we place equivalent non-invertible knots on each edge of the triangle $\overline{123}$ and a another set (distinct from the first set) of equivalent non-invertible knots on each edge of $\overline{456}$ we obtain an embedding Γ_{3} with TSG_{+}(Γ_{3}) ≅ ℤ_{3} × ℤ_{3} since ℤ_{3} × ℤ_{3} is a maximal subgroup of (ℤ_{3} × ℤ_{3}) ⋊ ℤ_{2}.
We summarize our results on positively realizability for K_{6} in Table 5. Note in the last few lines of the table we list multiple groups per line, since all of these groups are not positively realizable for the same reason.
By adding appropriate equivalent chiral knots to each edge, every group which is positively realizable for K_{6} is also realizable for K_{6}. Thus we only need to determine realizability for the groups S_{6}, A_{6}, S_{5}, A_{5}, S_{4} × ℤ_{2}, A_{4} × ℤ_{2}, S_{4}, A_{4}, ℤ_{5} ⋊ ℤ_{4}, (ℤ_{3} × ℤ_{3}) ⋊ ℤ_{4}, D_{4}, D_{4} × ℤ_{2}, ℤ_{4}, ℤ_{4} × ℤ_{2}, and ℤ_{2} × ℤ_{2} × ℤ_{2}. Note that in Figure 20 we already determined that S_{2}[S_{3}] is realizable for K_{6}.
Let Γ_{4} be the embedding of K_{6} illustrated in Figure 20 with a left handed trefoil added to each edge of $\overline{123}$ and a right handed trefoil added to each edge of $\overline{456}$. The pair of triangles are setwise invariant since no other edges contain trefoils. Both (123)(456) and (123)(465) are induced by homeomorphisms of (Γ_{4},S^{3}). Also if we reflect in the plane containing vertices 4, 5, 6, and 1 then all the trefoils switch from left-handed to right-handed and vice versa. If we then interchange the complementary solid tori which have the triangles as cores followed by an isotopy, we obtain an orientation reversing homeomorphism that induces the order 4 automorphism (14)(25)(36)(23) = (14)(2536). Now 〈(14)(2536;, (123)(456), (123)(465)〉 ≅ (ℤ_{3} × ℤ_{3}) ⋊ ℤ_{4}.
We see as follows that TSG(Γ_{4}) cannot be larger than (ℤ_{3} × ℤ_{3}) ⋊ ℤ_{4}. Suppose that the automorphism (12) is induced by a homeomorphism f. By Lemma 1, f must be orientation reversing. But $f(\overline{456})=\overline{456}$, which is impossible because 456 contains only right handed trefoils. Thus TSG(Γ_{4}) ≇ S_{2}[S_{3}]. Note that the only proper subgroup of S_{2}[S_{3}] containing (ℤ_{3} × ℤ_{3}) ⋊ ℤ_{4} is (ℤ_{3} × ℤ_{3}) ⋊ ℤ_{4}. Thus TSG(Γ_{4}) ≅ (ℤ_{3} × ℤ_{3}) ⋊ ℤ_{4}.
Now let Γ be the embedding of K_{6} illustrated in Figure 21. Observe that the linking number $\text{lk}(\overline{135},\overline{246})=\pm 1$, but $\text{lk}(\overline{136},\overline{245})=0$. Thus the automorphism (56) cannot be induced by a homeomorphism of (S^{3},Γ). Since every homeomorphism of (S^{3},Γ) takes $\overline{1234}$ to itself, it follows that TSG(Γ) ≤ D_{4}. The automorphism (1234)(56) is induced by a rotation followed by a reflection and an isotopy. In addition the automorphism (14)(23)(56) is induced by turning the figure upside down. Thus TSG(Γ) ≅ D_{4} generated by the automorphisms (1234)(56) and (14)(23)(56).
Now let Γ be obtained from Figure 21 by replacing the knot 4_{1} with the non-invertible and positively achiral knot 12_{427}. Then the square $\overline{1234}$ can no longer be inverted. In this case (1234)(56) generates TSG(Γ′) and thus TSG(Γ′) ≅ ℤ _{4}.
For the next few groups we will use the following lemma.
4-Cycle Theorem
[19] For any embedding Γ of K_{6} in S^{3}, and any labelling of the vertices of K_{6} by the numbers 1 through 6, there is no homeomorphism of (S^{3},Γ) which induces the automorphism (1234).
Consider the subgroup ℤ_{5} ⋊ ℤ_{4} ≤ Aut(K_{6}). The presentation of ℤ_{5} ⋊ ℤ_{4} as a subgroup of S_{6} gives the relation x^{-1}yx = y^{2} for some elements x, y ∈ ℤ_{5} ⋊ ℤ_{4} of orders 4 and 5 respectively. Suppose that for some embedding Γ of K_{6}, we have TSG(Γ) ≅ ℤ_{5} ⋊ ℤ_{4}. Without loss of generality, we can assume that y = (12345) satisfies the relation x^{-1}yx = y^{2}. By the 4-Cycle Theorem, any order 4 element of TSG(Γ) must be of the form x = (abcd)(ef). However, there is no element in Aut(K_{6}) of the form x = (abcd)(ef) that together with y = (12345) satisfies this relation. Thus there can be no embedding Γ of K_{6} in S^{3} such that TSG(Γ) ≅ ℤ_{5} ⋊ ℤ_{4}.
Now consider the subgroup ℤ_{4} × ℤ_{2} ≤ Aut(K_{6}). By the 4-Cycle Theorem, without loss of generality we may assume that if TSG(Γ) contains an element of order 4 for some embedding Γ of K_{6}, then TSG(Γ) contains the element (1234)(56). Computation shows that the only transposition in Aut(K_{6}) that commutes with (1234)(56) is (56), which cannot be an element of TSG(Γ) since this would imply that (1234) is an element of TSG(Γ). Furthermore the only order 2 element of Aut(K_{6}) that commutes with (1234)(56) and is not a transposition is (13)(24), which is already in the group generated by (1234)(56). Thus there is no embedding Γ of K_{6} in S^{3} such that TSG(Γ) contains the group ℤ_{4} × ℤ_{2}. This rules out all of the groups S_{4} × ℤ_{2}, D_{4} × ℤ_{2} and ℤ_{4} × ℤ_{2} as possible topological symmetry groups for embeddings of K_{6} in S^{3}.
For the group ℤ_{2} × ℤ_{2} × ℤ_{2} we will use the following result.
Conway Gordon
[20] For any embedding Γ of K_{6} in S^{3}, the mod 2 sum of the linking numbers of all pairs of complementary triangles in Γ is 1.
Now suppose that for some embedding Γ of K_{6} in S^{3} we have TSG(Γ) ≅ ℤ_{2} × ℤ_{2} × ℤ_{2}. It can be shown that the subgroup ℤ_{2} × ℤ_{2} × ℤ_{2} ≤ Aut(K_{6}) contains three disjoint transpositions. Without loss of generality we can assume that TSG(Γ) contains (13), (24), and (56), which are induced by homeomorphisms h, f, and g of (S^{3}, Γ) respectively. Since any three vertices of Γ determine a pair of disjoint triangles, we can use a triple of vertices to represent a pair of disjoint triangles. For example, we use the triple 123 to denote the pair of triangles $\overline{123}$ and $\overline{456}$. With this notation, the orbits of the ten pairs of disjoint triangles in K_{6} under the group 〈(13;, (24), (56)〉 are:
Table 6 summarizes our realizability results for K_{6}. Recall that for n = 4 and n = 5 every subgroup of S_{n} is realizable for K_{n}. However, as we see from Table 6, this is not true for n = 6.
7. Conclusions
We have classified all groups which can occur as the topological symmetry group or orientation preserving topological symmetry group of an embedded complete graph with no more than six vertices. Our results show that a number of groups can occur as a topological symmetry group but not as an orientation preserving topological symmetry group for a particular K_{n}. This gives us a collection of groups which can only occur for achiral embeddings of the graph in question.
The topological symmetry group includes all of the symmetries induced by the point group together with any symmetries that occur as the result of any flexibility or rotation of subparts of a structure around specific bonds. Thus the topological symmetry group gives us more information about the symmetries and possible achirality of supramolecular structures than could be obtained from the point group. Since complete graphs with no more than six vertices may occur as supramolecular clusters, these results could be of interest in the future study of supramolecular chirality.
The first author would like to thank Claremont Graduate University for its support while he pursued the study of Topological Symmetry Groups for his Ph.D Thesis. The second author would like to thank the Institute for Mathematics and its Applications at the University of Minnesota for its hospitality while she was a long term visitor in the fall of 2013.
Conflicts of Interest
The authors declare no conflict of interest.
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Table 1. Realizable and positively realizable groups for K_{3}. |
Embedding | TSG(Γ) | TSG_{+}(Γ) |
---|---|---|
Planar | D_{3} | D_{3} |
8_{17} | D_{3} | ℤ_{3} |
8_{17} #3_{1} | ℤ_{3} | ℤ_{3} |
Table 2. Non-trivial realizable and positively realizable groups for K_{4}. |
Subgroup | Realizable/Positively Realizable | Reason |
---|---|---|
S_{4} | Yes | By S_{4} Theorem |
A_{4} | Yes | By A_{4} Theorem |
D_{4} | Yes | By Figure 4 |
D_{3} | Yes | By Figure 5 |
D_{2} | Yes | By Subgroup Theorem |
ℤ_{4} | Yes | By Subgroup Theorem |
ℤ_{3} | Yes | By Subgroup Theorem |
ℤ_{2} | Yes | By Subgroup Theorem |
Table 3. Non-trivial positively realizable groups for K_{5}. |
Subgroup | Positively Realizable | Reason |
---|---|---|
S_{5} | No | By Complete Graph Theorem |
A_{5} | Yes | By A_{5} Theorem |
ℤ_{5} ⋊ ℤ_{4} | No | By Complete Graph Theorem |
S_{4} | No | By S_{4} Theorem |
A_{4} | Yes | By A_{4} Theorem |
D_{6} | No | By Lemma 2 |
D_{5} | Yes | By Figure 6 |
D_{4} | No | By Lemma 2 |
D_{3} | Yes | By Figure 7 |
D_{2} | Yes | By Figure 8 |
ℤ_{6} | No | By Lemma 2 |
ℤ_{5} | Yes | By Subgroup Theorem |
ℤ_{4} | No | By Lemma 2 |
ℤ_{3} | Yes | By Subgroup Theorem |
ℤ_{2} | Yes | By Subgroup Theorem |
Table 4. Non-trivial realizable groups for K_{5}. |
Subgroup | Realizable | Reason |
---|---|---|
S_{5} | Yes | By Figure 9 |
A_{5} | Yes | Positively realizable |
S_{4} | Yes | By modifying Figure 9 |
A_{4} | Yes | Positively realizable |
D_{6} | Yes | By Figure 11 |
D_{5} | Yes | Positively realizable |
D_{4} | Yes | By Figure 10 |
D_{3} | Yes | Positively realizable |
D_{2} | Yes | Positively realizable |
ℤ_{6} | Yes | By modifying Figure 11 |
ℤ_{5}⋊ℤ_{4} | Yes | By Figure 14 |
ℤ_{5} | Yes | Positively realizable |
ℤ_{4} | Yes | By modifying Figure 10 |
ℤ_{3} | Yes | Positively realizable |
ℤ_{2} | Yes | Positively realizable |
Table 5. Non-trivial positively realizable groups for K_{6}. |
Subgroup | Positively Realizable | Reason |
---|---|---|
A_{5} | No | By A_{5} Theorem |
S_{4} | No | By S_{4} Theorem |
A_{4} | No | By A_{4} Theorem |
D_{6} | Yes | By Figure 18 |
D_{5} | Yes | By Figure 19 |
D_{4} | No | By Lemma 2 |
D_{3}×D_{3} | Yes | By Figure 20 |
D3×ℤ3 | Yes | By modifying Figure 20 |
D_{3} | Yes | By Subgroup Theorem |
D_{2} | Yes | By Subgroup Theorem |
ℤ_{6} | Yes | By Subgroup Theorem |
ℤ_{5} | Yes | By Subgroup Theorem |
ℤ_{4} | No | By Lemma 2 |
(ℤ_{3} × ℤ_{3}) x ℤ_{2} | Yes | By modifying Figure 20 |
ℤ_{3} ×ℤ_{3} | Yes | By modifying Figure 20 |
ℤ_{3} | Yes | By Subgroup Theorem |
ℤ_{2} | Yes | By Subgroup Theorem |
S_{6},A _{6},S_{5},S _{2}[S _{3}],S_{4}×ℤ_{2},A_{4}×ℤ_{2} | No | By Complete Graph Theorem |
ℤ_{5} ⋊ ℤ_{4}, (ℤ_{3} × ℤ_{3}) ⋊ ℤ_{4}, D_{4} × ℤ_{2} | No | By Complete Graph Theorem |
ℤ_{4} ×ℤ_{2}, ℤ_{2} ×ℤ_{2} ×ℤ_{2} | No | By Complete Graph Theorem |
Table 6. Non-trivial realizable groups for K_{6}. |
Subgroup | Realizable | Reason |
---|---|---|
S_{6} | No | TSG_{+}(K_{6}) cannot be S_{6} or A_{6} |
A_{6} | No | TSG_{+}(K_{6}) cannot be A_{6} |
S_{5} | No | TSG_{+}(K_{6}) cannot be S_{5} or A_{5} |
A_{5} | No | TSG_{+}(K_{6}) cannot be A_{5} |
S_{4} × ℤ_{2} | No | TSG+(K_{6}) cannot be S_{4} × ℤ_{2} or S_{4} |
S_{4} | No | TSG_{+}(K_{6}) cannot be S_{4} or A_{4} |
A_{4} × ℤ_{2} | No | TSG_{+}(K_{6})cannotbeA_{4} × ℤ_{2}orA_{4} |
A_{4} | No | TSG_{+}(K_{6}) cannot be A_{4} |
D_{6} | Yes | Positively realizable |
D_{5} | Yes | Positively realizable |
D_{4}×ℤ_{2} | No | TSG+(K_{6}) cannot be D_{4} × ℤ_{2}, D_{4}, ℤ_{4} × ℤ_{2}, ℤ_{2} × ℤ_{2} × ℤ_{2} |
D_{4} | Yes | By Figure 21 |
S_{2}[S_{3}] | Yes | By Figure 20 |
D_{3} × D_{3} | Yes | Positively realizable |
D_{3} × ℤ_{3} | Yes | Positively realizable |
D_{3} | Yes | Positively realizable |
D_{2} | Yes | Positively realizable |
ℤ_{6} | Yes | Positively realizable |
ℤ_{5}×ℤ_{4} | No | By 4-Cycle Theorem |
ℤ_{5} | Yes | Positively realizable |
ℤ_{4}×ℤ_{2} | No | By 4-Cycle Theorem |
ℤ_{4} | Yes | By modifying Figure 21 |
(ℤ_{3} × ℤ_{3}) ⋊ ℤ_{4} | Yes | By modifying Figure 20 |
(ℤ_{3} × ℤ_{3}) ⋊ ℤ_{2} | Yes | Positively realizable |
ℤ_{3}×ℤ_{3} | Yes | Positively realizable |
ℤ_{3} | Yes | Positively realizable |
ℤ_{2} x ℤ_{2} × ℤ_{2} | No | By Conway Gordon Theorem |
ℤ_{2} | Yes | Positively realizable |
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