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Symmetry
  • Feature Paper
  • Article
  • Open Access

6 May 2023

Recursive Symmetries: Chemically Induced Combinatorics of Colorings of Hyperplanes of an 8-Cube for All Irreducible Representations

School of Molecular Sciences, Arizona State University, Tempe, AZ 85287-1604, USA
This article belongs to the Collection Feature Papers in Chemistry

Abstract

We outline symmetry-based combinatorial and computational techniques to enumerate the colorings of all the hyperplanes (q = 1–8) of the 8-dimensional hypercube (8-cube) and for all 185 irreducible representations (IRs) of the 8-dimensional hyperoctahedral group, which contains 10,321,920 symmetry operations. The combinatorial techniques invoke the Möbius inversion method in conjunction with the generalized character cycle indices for all 185 IRs to obtain the generating functions for the colorings of eight kinds of hyperplanes of the 8-cube, such as vertices, edges, faces, cells, tesseracts, and hepteracts. We provide the computed tables for the colorings of all the hyperplanes of the 8-cube. We also show that the developed techniques have a number of chemical, biological, chiral, and other applications that make use of such recursive symmetries.

1. Introduction

N-dimensional hypercubes in general and the eight-dimensional hypercube (8-cube) in particular are of interest not only because they represent the potential energy surfaces of water clusters, including the octamer (H2O)8, but because they also have several novel applications in many other fields [,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,]. Moreover, hypercubes in general provide representations of the periodic table, encompassing superheavy elements and elements that are yet to be discovered [,,,]. Hypercubes are employed in quantum similarity measures, quantum chemistry, computational chemistry, chirality, image processing, quantitative measures of shapes and stereochemistry, and so forth [,,,,,,,,,,,,,,,,,,,,,,]. There are several representations of hypercubes and the various hyperplanes of hypercubes. For example, the associated octeract, a representation of the 8-cube, is an isomorphic representation of the potential energy surfaces of a completely non-rigid water octamer, (H2O)8 [], in which 256 vertices correspond to the potential minima in the potential energy surface. Consequently, the classifications of the rovibronic levels [] of a fully nonrigid (H2O)8 require the automorphism group of the 8-cube, which is isomorphic with the eight-dimensional hyperoctahedral group or the wreath product group, S8[S2], which contains 10,321,920 permutation operations for which S8 is the permutation group of eight objects consisting of 8! operations. In a more general context, n-dimensional hypercubes, polycubes, recursive structures, and their properties have been studied over the years [,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,]. They find numerous applications in vast areas, such as the representation theory of nonrigid molecules, genetic regulatory networks, biological modeling, finite automata, isomerization reactions, computer graphics, DNA synthetic bases, chirality, protein–protein interactions, parallel computing, visualizations, big data, and so forth [,,,,,,,,,,,,,,,,,,,].
The double groups [] of the wreath products are applicable to the classification of the potential energy surfaces of relativistic polymeric clusters [,,,] that could be made in supersonic expansions, for example, (PoH2)n, (LvH2)8, (FlH2)8, Sn3, gallium arsenide clusters and their heavier analogs, and so forth. Relativistic effects [,,,] make significant contributions to such molecules that contain very heavy atoms; thus, the coupling of the spin and orbital angular momenta results in double group symmetries []. Combinatorial enumerations of the colorings of structures or enumerations under group actions, especially those that are pertinent to hypercubes, polytopes, and color symmetries, have been the subject matter of several studies [,,,,,,,,,,]. Such enumerations have several molecular, biological, and other applications to phylogentic trees, pandemic trees, intrinsically disordered proteins, genetic regulatory networks, dynamic chirality, spontaneous generation of optical activity, configuration interaction computations in quantum chemistry, and so forth [,,,,,,,,,,,,,].
Combinatorial enumerations of the colorings of the various hyperplanes of the 8-cube have not been considered until now. Such enumerations are extremely challenging as the generating functions must be obtained for 185 IRs for each of the eight hyperplanes of the 8-cube. The 8-cube exhibits eight types of (8-q) hyperplanes, with q ranging from 1 to 8. For example, q = 1 corresponds to 16 hepteracts, q = 2 yields 112 hexeracts…q = 7 provides 1024 edges, and q = 8 is simply 256 vertices of the 8-cube. In fact, Pólya’s theorem [,] of enumeration under group action becomes a special case of the combinatorial enumeration considered here when it is reduced to the totally symmetric A1 representation of the hypercube. Several other variants of Pólya’s theorem [,,,,,] have also been considered in the literature. Here, we have considered enumerations and generating functions for all 185 IRs and 185 conjugacy classes of the 8-cube. When the enumeration is generalized to all IRs, the results are extremely useful in a number of applications, for example, the enumeration of the nuclear spin statistics and nuclear spin species of the rovibronic levels of nonrigid molecules, multiple-quantum NMR spin functions and energy levels, the enumeration of isomerization reactions, dynamic chirality, mathematical methods pertinent to drug discovery, etc. [,,,]. Moreover, there exists no one-to-one correspondence between Pólya’s cycle types and the conjugacy classes for the hyperoctahedral groups. We therefore employ the matrix representations of the conjugacy classes in combination with the Möbius inversion technique [] to generate the cycle types of all eight hyperplanes of the 8-cube for all 185 IRs of the wreath product group S8[S2]. Consequently, for (8-q) hyperplanes (q = 1 to 8) and for each IR of the wreath group, we obtain a generalized character cycle index (GCCI) in order to derive the combinatorial generating functions for the colorings of (n-q) hyperplanes of the 8-cube. We construct the combinatorial enumeration tables for the 4-colorings and 2-colorings of the hyperplanes of the 8-cube for all 185 IRs.

2. Recursive Symmetries, Wreath Products, and Combinatorial and Computational Techniques for the 8-Cube

Hypercubes are recursive structures, and their symmetries are also recursively defined in terms of wreath product groups. Recursivity is a topic that has been explored at multiple levels, including its philosophical implications []. In the present context, recursivity implies that the symmetry of the larger system is constructed from the previous levels of smaller systems in a nested manner, as defined in the ensuing sentences. We define an n-dimensional hypercube whose graph is represented by Qn recursively through the use of the cartesian product shown below:
Qn = Qn−1 × K2 for n ≥ 2 with Q1 = K2,
where K2 is a graph with two vertices connected by an edge. The adjacency matrices of hypercubes are also generated recursively using the above recursive construction. The adjacency matrix of Qn+1, AQn+1, is recursively expressible in terms of the corresponding matrices of Qn, as follows:
A Q n + 1 = A Q n I I A Q n
where I is simply an identity matrix of the order 2n × 2n.
The automorphism group of a graph is defined as a set of permutations of the vertices of the graph that preserve the adjacency matrix. Subsequently, for n-cubes, the permutations of the vertices must not break or make any edges. The automorphism group of an n-cube is simply given by the wreath product group, Sn[S2].
We restrict ourselves to particular details concerning the 8-cube and the enumeration of the colorings of eight possible hyperplanes for 185 irreducible representations of the S8[S2] group. The (8-q) hyperplanes (1 ≤ q ≤ 8) of the 8-cube are characterized by an 8 × 8 Coxeter’s configuration matrix []:
: 256 8 28 56 70 56 28 8 2 1024 7 21 35 35 21 7 4 4 1792 6 15 20 15 6 8 12 6 1792 5 10 10 5 16 32 24 8 1120 4 6 4 32 80 80 40 10 448 3 3 64 192 240 160 60 12 112 2 128 448 672 560 280 84 4 16
The number of (n-q) hyperplanes for an nD-hypercube is given by
N q = n q 2 q
It can be seen that the diagonal elements of the 8 × 8 configuration matrix yield the number of hyperplanes, with the first row corresponding to the vertices and the last row providing the number of hepteracts. The off-diagonal element Cij of the 8 × 8 configuration matrix provides the number of times hyperplane j occurs in hyperplane i of the 8-cube. Thus, C51 = 16 means that each penteract of the 8-cube contains 16 vertices, and so forth. Hence, the configuration matrix is fundamental to the combinatorial enumeration of the colorings of the hyperplanes of the 8-cube.
Figure 1 demonstrates the first row of the Coxeter [] configuration matrix (q = 8) for the 8-cube, which shows 256 vertices and 1024 edges with C12 = 8, providing the degree of each vertex in Figure 1 taken from []. Transitive graphs, such as the one in Figure 1, have been considered by the author [] as well as Balaban in the context of chemical isomerization graphs, such as the well-known Balaban cages [,]. Likewise, for the case under consideration, Figure 1 represents the isomerization rearrangements of the water octamer in the fully fluxional limit, where the breaking and remaking of all hydrogen bonds between any two water molecules can take place. This happens at a higher temperature at which facile rearrangements among the H-bonded water molecules lead to a totally nonrigid limit of the water octamer. The automorphism group of the graph in Figure 1 contains all permutations of the vertices such that no edges are made or broken, and it is provided by the wreath product group, S8[S2]. The generalized character cycle indices for all 185 IRs of the S8[S2] group and the 8 hyperplanes of the 8-cube are constructed next using the generalized matrix cycle types of the permutations.
Figure 1. The octeract graph of an 8-cube, displaying the relationship among the 256 vertices of the 8-cube. The automorphism group of this graph is the 8-dimensional hyperoctahedral group or the wreath product S8[S2] comprising 10,321,920 permutations and 185 irreducible representations. (Figure reproduced from Ref. []).
The Möbius inversion technique [,] is one of the most powerful methods for obtaining a number of generating functions; in the present case, it facilitates the construction of permutational cycle types for all eight hyperplanes of the 8-cube under the action of the S8[S2] group. We accomplish this task by using the matrix generators for the conjugacy classes of the S8[S2] group in combination with the Möbius inversion method. Table 1 shows 185 different 2 × 8 matrix cycle types for the conjugacy classes of the S8[S2] group. The matrix cycle types are powerful generalizations of the Pólya cycle types used in the construction of the ordinary cycle index of a group. The matrix cycle types for the 185 conjugacy classes of the S8[S2] group are constructed using a combinatorial technique that considers the group actions of the composing groups in the wreath product S8[S2]. Let a permutation g ∈ S8 act on a set Ω of eight objects and generate a1 cycles of length 1, a2 cycles of length 2, a3 cycles of length 3, …, a8 cycles of length 8; the group action can then be represented by 1a12a23a3.…. 8a8 or a cycle type of g denoted as Tg = (a1, a2, a3, …, a8). From this structure, one obtains a 2 × 8 matrix cycle type for a conjugacy class of the wreath product in which the first row represents the action of {(g; e)} permutations for which e is the identity operation of the S2 group, with g ∈ S8, and the second row corresponds to the permutations {(g;π)} for π (12) ∈ S2 and g ∈ S8. Consequently, the matrix cycle type of any conjugacy class of the wreath product S8[S2], denoted by T(g;π), is a 2 × 8 matrix generated using the orbit structure of g ∈ Sn and the corresponding conjugacy class of S2. Thus, for (g;π), the matrix types of the conjugacy classes of S8[S2] are given by
T(g;π) = aik (1 ≤ i ≤ 2), (1 ≤ k ≤ 8),
Table 1. Conjugacy Classes of the 8-dimensional hyperoctahedral group & cycle types for each hyperplane.
To exemplify, take the conjugacy class {(1)(2)(3)(4)(5678);(12)} of S8[S2] (class number 38 in Table 1) given by (2)
T [ { ( 1 ) ( 2 ) ( 3 ) ( 4 ) ( 5678 ) ; ( 12 ) } ] = 4 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0
Likewise, the conjugacy class number 162 in Table 1, which corresponds to {(12345)(6)(7)(8);(12)}, is given by (3):
T [ { ( 12345 ) ( 6 ) ( 7 ) ( 8 ) ; ( 12 ) } ] = 3 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0
Table 1 shows the results of all the matrix cycles thus obtained for all 185 conjugacy classes of S8[S2]. Moreover, Table 1 displays the order of each conjugacy class of the S8[S2] group and the cycle types of the eight hyperplanes (1 ≤ q ≤ 8) obtained using the Möbius inversion method [,]. The label R is assigned to each conjugacy class in Table 1 if the symmetry operation is a proper rotation of the 8-cube. This facilitates the further discrimination of colorings into chiral or achiral colorings. The order of each conjugacy class is shown in the third column of Table 1, which is readily obtained from the corresponding 2 × 8 matrix cycle type shown in Table 1. Let P(n) represent the number of partitions of an integer n, given that P(0) = 1. The order any conjugacy class of S8[S2] with the matrix type T(g;π) = aik is given by (4):
| T g ; π | = 8 ! 2 8 i , k a i k ! ( 2 k ) a i k
For example, for the conjugacy class in Equation (3) (conjugacy class 162 in Table 1), the order is given by (5):
3 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 = 8 ! 2 8 3 ! ( 2.1 ) 3 1 ! 2.5 1 = 21,504
The above algorithm is iterated for all 185 conjugacy classes of the 8-cube to generate the orders of the conjugacy classes shown in the third column of Table 1. Each matrix type shown in Table 1 for the conjugacy class of the S8[S2] group generates a permutation upon its action on the set of hyperplanes for each q of the 8-cube. These actions are computed in a single generating function through the Möbius inversion technique, as illustrated by Lemmis [] for a 4D hypercube. Thus, the generating functions for the cycle types of the (8-q) hyperplanes are computed as coefficients of xq in the Qp(x) polynomial obtained using the Möbius inversion method shown in (6):
Q p x = 1 p d / p μ p / d F d x
The summation is over all divisors d of p, and μ p / d is the Möbius function defined for any integer m as:
μ(m) = 1 if one of m’s prime factors is not a perfect square and m contains an even number of prime factors;
μ(m) = −1 if m satisfies the same perfect-square condition as before but m contains odd number of prime factors;
μ(m) = 0 if m has a perfect square as one of its factors. Consequently,
μ(m) = 1, −1, −1, 0, −1, 1, −1, 0, 0, 1 … for m = 1 to 10.
In Equation (7), Fd(x) is a polynomial in x obtained from the 2 × 8 matrix cycle types shown in the second column of Table 1. We consider only the nonzero columns of the matrix cycle types of the S8[S2] group. Suppose p is the period of the matrix type shown in Table 1, and let g = gcd(k; p), p’ = k/g, h = gcd(2k; p). and p” = 2k/h; then, the function Fp(x) is given by (8):
F p ( x ) = k n c 1 + 2 x p g a 1 k 1 + 2 x p h a 2 k 2 ,   i f   h   d o e s   n o t   d i v i d e   k ; F p ( x ) = k n c 1 + 2 x p g a 1 k ,   i f   h   d i v i d e   k ,
where we take the product only over nc non-zero columns of the 2 × 8 matrix cycle type for each of the 185 classes displayed in Table 1. The coefficient of xq in Qp(x) can be obtained by substituting the various Fd polynomials in the Möbius sum (7) in which ds are divisors of p. Thus, the cycle types of all eight hyperplanes of the 8-cube are obtained in a single generating function for each conjugacy class by collecting the coefficients of xq to obtain the cycle types for (8-q) hyperplanes of the 8-cube. We now illustrate this with an example, using the matrix type in Equation (2) for conjugacy class 38 in Table 1 for the S8[S2] group.
As can be seen from the matrix shown in Equation (2), the first and fourth columns of the matrix contain non-zero values; consequently, we consider only these two columns for computing the cycle types of the hyperplanes of the 8-cube. Thus, the maximum period to consider is 8, and the possible F polynomials are therefore F8, F4, F2, and F1, as divisors of 8 are 1, 2, 4, and 8. Applying the GCD, followed by the use of Equation (10), we obtain each of these polynomials as
F1 = (1 + 2x)4
F2 = (1 + 2x)4
F4 = (1 + 2x)4
F8 = (1 + 2x)8
The Qp polynomials are obtained using the Möbius sum, (Equation (6)), as follows:
Q1 = F1 = 1 + 8x + 24x2 + 32x3 + 16x4
Q2 = 1/2 {μ(2)F1 + μ(1)F2} = 1/2 {F2 − F1} = 1/2 {(1 + 2x)4 − (1 + 2x)4} = 0
Q4 = 1/4 {μ(1)F4 + μ(2)F2 + μ(4)F1} = 1/4 {F4 − F2} = 0
Q8 = 1/8 {μ(1)F8 + μ(2)F4 + μ(4)F2 + μ(8)F1} = 1/8 {F8 − F4}
   = x + 11x2 + 52x3 + 138x4 + 224x5 + 224x6 + 128x7 + 32x8
The coefficients of the xq terms thus obtained are sorted in a tabular form shown below for all possible Qp polynomials. Once the coefficient of xq is collected for each column, we obtain the cycle type of the (8-q) hyperplanes:
Qpxx2x3x4x5x6x7x8
Q18243216
Q2
Q4
Q81115213822422412832
Cycle type1881248111328521168138822482248128832
Hyperplaneq = 1 (hepteracts)q = 2 hexeractsq = 3 penteractsq = 4 tesseractsq = 5 cubic cellsq = 6 facesq = 7 edgesq = 8 vertices
The above technique was repeated for all 185 conjugacy classes of the 8-cube, and the results are shown in columns 5–12 (Table 1) for the (8-q) hyperplanes for q = 1 through 8, respectively. The generating functions for the colorings of the the hyperplanes for each irreducible representation are obtained using the generalized character cycle index (GCCI) for the character χ of the S8 [S2] group defined by
P G χ = 1 G g ε G χ ( g ) s 1 b 1 s 2 b 2 s n b n
where the sum is taken over all permutations g ∈ G = S8 [S2] with cycle type 1b12b23b3….8b8 upon its action on the (8-q) hyperplanes of the 8-cube. The GCCIs for all 185 irreducible representations and for each of the cycle types of the (8-q) hyperplanes (Table 1) are constructed for the S8 [S2] group. Multinomial generating functions are then computed for the colorings of the (8-q) hyperplanes of the 8-cube for all irreducible representations. The generating function is defined as follows. Let [n] be an ordered partition of eight (compositions of n = 8) into p parts such that n1 ≥ 0, n2 ≥ 0, …, np ≥ 0, i = 1 p n i = n . Then, a multinomial generating function in λs, in which λs represent arbitrary weights, is computed as
λ 1 + λ 2 + + λ p n = n p n 1 n 2 n n p λ 1 n 1 λ 2 n 2 . . λ p 1 n p 1 λ p n p
where n 1 n 2 n n p are multinomials given by
n 1 n 2 n n p = n ! n 1 ! n 2 ! n p 1 ! n p !
Define D as the set of (8-q) hyperplanes which are to be colored, and let R be the set of different colors. Furthermore, let wi be a weight assigned to each color r in R. Following Pólya, the weight of a function f from D to R can then be defined as
W f = i = 1 | R | w ( f d i )
Hence, the generating functions for each of the 185 irreducible representations of the 8-cube with character χ are provided as follows:
G F χ w 1 , w 2 . . w p = P G χ { s k w 1 k + w 2 k + . + w p 1 k + w p k }
We compute the GFs for all irreducible representations of the 8-cube’s group for all 8 hyperplanes, resulting in 1480 such combinatorial GFs for the 8-cube. The coefficient of a typical term, w1n1w2n2, …, wpnp, computes the number of colorings in the set RD that transform according to the irreducible representation with character χ. Pólya’s theorem becomes a special case for one of these 185 IRs; that is, for the totally symmetric irreducible representation A1 which has the unit character values of all 185 conjugacy classes of the 8-cube. The sizes of the multinomial generators rapidly increase for such a large number of 8-cube hyperplanes, and we therefore consider only two colors in the set R for all cases except q = 1, for which 4-colorings are considered. All the computations were carried out in FORTRAN ’95, invoking the quadruple precision arithmetic, which provides over 32 digits of accuracy for the computed results.

3. Results and Discussions

Table 2, Table 3, Table 4, Table 5, Table 6, Table 7, Table 8 and Table 9 display our computed results for the colorings of eight different hyperplanes for q = 1–8, respectively. As there are 185 IRs, we do not show all the results for all IRs to prevent the Tables from becoming too large. Thus, we show the restricted results for the various hyperplanes. Table 2 shows the 4-colorings for the hepteracts (q = 1) for all one- and seven-dimensional irreducible representations. Table 3, Table 4, Table 5, Table 6, Table 7, Table 8 and Table 9 show the binomial colorings for the hyperplanes q = 8 through q = 2 in the order of vertices (Table 3) to hexeracts (Table 9). The reason for the restriction of the binomial colorings is that as q moves away from 1, too many partitions exist as the number of hyperplanes increases both binomially and exponentially.
Table 2. Generating functions for the one-dimensional and 7-dimensional IRs for coloring hepteracts (q = 1) for 4 colors.
Table 3. Generating functions for the binomial colorings of the vertices (q = 8) for one-dimensional IRs of the 8-cube.
Table 4. Generating functions for the binomial colorings of edges (q = 7) for one-dimensional IRs of the 8-cube.
Table 5. Generating functions for the binomial colorings of faces (q = 6) for one-dimensional IRs of the 8-cube.
Table 6. Generating functions for the binomial colorings of cubic cells (q = 5) for one-dimensional IRs of the 8-cube.
Table 7. Generating functions for the binomial colorings of tesseracts (q = 4) for one-dimensional IRs of the 8-cube.
Table 8. Generating functions for the binomial colorings of penteracts (q = 3) for one-dimensional IRs of the 8-cube.
Table 9. Generating functions for the binomial colorings of hexeracts (q = 2) for one-dimensional IRs of the 8-cube.
As can be seen from Table 2, among four one-dimensional IRs (A1–A4), only three appear in the table for the 4-colorings of the hepteracts of the 8-cube as N(A2) becomes zero for all 4-color partitions. Moreover, for the A4 representation, only the last few rows of the color partitions give rise to nonzero values. The N(A1) numbers simply yield Pólya’s equivalence classes for four colors. As can be seen from Table 2, there are two classes for the [2,14] and [3,13] color partitions. Among the 2-colorings, there are five equivalence classes for the [8,8] color partition, or there are five inequivalent ways to color the hepteracts of the 8–cube with eight black and eight white colors. Likewise, there are 138 inequivalent ways to color the hepteracts of the 8-cube with four blue colors, four red colors, four yellow colors, and four white colors (Table 2). Thus, the frequency for each color partition shown in Table 2 provides the numbers of IRs contained in the set of all the colorings of the hepteracts of the 8-cube that transform in accordance with that IR. The representations labeled A5 to A8 in Table 2 are seven-dimensional IRs of the wreath product group S8[S2]. The A6 and A7 seven-dimensional IRs appear less frequently in colorings compared to A5 and A8. This is analogous to the one-dimensional A2 and A4 IRs, which are less frequent, and A4 appears only for the highly distributed color partitions, while A2 does not appear at all for all four-colorings of the sixteen hepteracts.
Table 3 displays the 2-colorings of the vertices of the 8-cube (q = 8). As there are 256 vertices, the color partitions are of the form [n1, 256-n1]; thus, in Table 2, we have shown only n1 or, for example, the number of black colors remaining as 256-n1 whites. As all the vertices of the 8-cube are equivalent, N(A1) becomes 1 for n1 = 1. As can be seen from Table 2, there are 8 (N(A1)) inequivalent ways to color the vertices of the 8-cube with 2 black colors, 32 ways for 3 black colors, 373 ways for 4 black colors, and so forth. The maximum is reached at [128,128], which is too large to be shown accurately, even with quadruple precision. Among the results shown in Table 3, there are 1850439767413213381676300410605 inequivalent ways to color with 28 black colors. Although the A2 IR does not present for the hepteracts, all one-dimensional IRs appear for most of the binomial colorings of the vertices of the 8-cube. The only exceptions are 1–3 black colors for the A2 and A4 IRs. As the number of black colors increases, almost every one-dimensional IR appears with nearly the same frequency. This also implies that almost every coloring becomes chiral as the number of black colors increases, approaching the peak at the [128,128] color partition. There is a similar pairing of the A1 and A3 IRs, while A2 and A4 exhibit similar pairs (Table 3).
Table 4 shows the edge colorings of the 8-cube for a set of two colors. As there are 1024 edges, the number of colorings for different IRs increases quite rapidly. As can be seen from Table 4, there are 14 inequivalent ways to color with 2 black colors, 216 ways for 3 black colors, 13,143 ways to color with 4 black colors, and so forth for the edge colorings of the 8-cube. The first nonzero frequency for the A2 IR occurs for a minimum of four black colors for the A2 IR, while for A3 and A4, one would need two and four black colors, respectively. A common feature of the binomial distribution is exhibited by all edge colorings for all IRs peaking at the [512,512] color partition.
Table 5, Table 6, Table 7, Table 8 and Table 9 show the colorings of the faces, cells, tesseracts, penteracts, and hexeracts of the 8-cube for two colors. For example, there are 17 inequivalent ways to color the faces of the 8-cube with 2 black colors, while the corresponding numbers are 17, 14, 9, and 5 for the cubic cells, tesseracts, penteracts, and hexeracts, respectively. A similar pattern is exhibited by these hyperplanes for other numbers of black colors. Likewise, the similarity of the (A1, A3) and (A2, A4) pairs is shared by all hyperplanes (Table 5, Table 6, Table 7, Table 8 and Table 9). When comparing the frequencies of the A2 IR among hyperplanes, the hexeracts and hepteracts stand out, as A2 does not appear in any 2-colorings of the hepteracts, while at least six black colors are needed to produce a coloring that contains the A2 IR for the hexeracts. An important point for all hyperplanes is that there is only one equivalence class for the A1 representation when only one black color is used. Consequently, all hyperplanes of the 8-cube or any n-cube are equivalent. This follows from the highly symmetric, vertex-, edge-, and arc-transitive natures of the n-cube. Thus, the combinatorial numbers are consistent with these general features of the n-cube.

4. Chemical, Biological and other Applications of the Colorings of 8-Cube

The colorings of the hyperplanes of n-cubes have several applications in a variety of fields, such as chemistry, biology, image processing, latent symmetries in computational psychiatry, phylogenetic networks, pandemic networks, Bethe lattices, Cayley trees, and so forth. In this section, we focus on chemical and biological applications, with slight coverage of other types of applications. An obvious connection of the 8-cube is to Boolean strings of a length of 8 for which each such string is represented by the vertex of the 8-cube, giving rise to 256 Boolean strings of a length of 8. An important chemical and spectroscopic application of the 8-cube is to the water octamer, (H2O)8. Figure 1 displays the graph of the octeract with 256 vertices and 1024 edges. If every hydrogen bond of the water octmer is allowed to be broken and remade, which happens at higher temperatures, then the cluster becomes fully nonrigid. At that limit, the graph in Figure 1 would represent the isomerization graph of the water octamer. The dynamic stereochemistry, isomerization pathways, and their intimate relations to graph theory through finite topologies and borel fields were explored earlier in the context of rapid internal rotations around the C-C single bonds []. The isomerization graphs provide insights into reaction pathways and phenomena such as the spontaneous generation of chirality and dynamic or transient chirality. This is attributed to the existence of dl-edges or edges between chiral pairs in the dynamic isomerization graphs.
The nuclear spin statistics and combinatorics of nuclear spin functions that have a wide-range of applications, from multiple quantum NMR spectroscopy to predicting the tunneling splittings of rovibronic levels, are critical to the interpretation of the observed spectra of nonrigid molecules including water clusters and the water octamer in particular. That is, as the molecule becomes fluxional as it surmounts several potential energy barriers, its overall symmetry is described in the nonrigid molecular group, which becomes the automorphism group of the hypercube for water clusters. For fluxional complexes such as water clusters, ammonia clusters, and methane clusters, the tunneling splittings of the rovibronic levels are obtained through the use of induced representations from the irreducible representations of the rigid molecular group to the ones in the symmetry group of the nonrigid molecule. While the magnitudes of the tunneling splittings would depend on the extent of the fluxionality, the tunneling levels are predicted using the induced representations from the smaller rigid group to the larger fluxional group. Moreover, nuclear spin functions and the nuclear spin populations of the rovibronic tunneling levels are predicted from the colorings of the hyperplanes of the n-cube.
The nuclear spin functions can be envisaged as colorings from the set of the nuclei in the molecule to a set of spin colors which correspond to the various nuclear spin orientations. The possible nuclear spin orientations depend on the isotopes of the nuclei present in the molecule and the overall nuclear spin quantum number of the isotope. For example, the naturally abundant isotope of bismuth, 209Bi, exhibits a spin of 9/2 with 10 different spin orientations or 10 colors; therefore, the set of different colors would have a cardinality of 10. On the other hand, for the normal water clusters, the proton nuclei exhibit a spin of 1/2, and thus the two spin orientations of each proton of the octamer become two distinct colors; this corresponds to the 2-coloring of the hyperplanes of the 8-cube considered herein. The most stable naturally occurring isotope of oxygen, 16O, carries no nuclear spin, although another naturally occurring 17O isotope carries a nuclear spin of 5/2, giving rise to six nuclear spin orientations or colors. The 17O NMR is the technique of choice for probing the dynamics of oxygen-containing molecules. Consequently, the combinatorial numbers enumerated in Table 2 for the 4-colorings of hepteracts contain the information needed to classify the protonic nuclear spin functions of the water octamer in the nonrigid limit. That is, there are 216 proton nuclear spin functions for the water octamer, and together, they transform as reducible representations in the symmetry group of the 8-cube. The irreducible representations contained in the set of protonic nuclear spin functions are enumerated by the combinatorial numbers in Table 2 when they are extended for all 185 IRs of the 8-cube. Likewise, when the technique is extended for 6 colors and applied to all 185 IRs, we obtain the frequencies of the IRs of the nuclear spin functions for the 17O isotopes of (H217O)8. The set of such 17O nuclear functions has a cardinality of 68 for (H217O)8. Consequently, the combinatorial numbers enumerated in Tables contain rich information pertinent to a significant amount of spectroscopically important information, for example, nuclear spin species, the nuclear spin statistical weights of the rovibronic levels, and nuclear spin multiplets and thus the intensities and hyperfine structures of the rovibronic spectra of the water octamer.
In order to demonstrate the utility of the combinatorial colorings of the hyperplanes of the 8-cube, let us consider the deuterated water octamer, (D2O)8. As deuterium is a spin-1 nucleus, its bosonic spin functions can be denoted by λ, μ, and ν. Thus, there are 316 nuclear spin functions corresponding to the 3-colorings of the hepteracts of the 8-cube, and the results are contained in Table 2 for eight of the IRs. That is, all the color partitions with three or fewer parts correspond to 3-colorings, and the subsets of numbers in Table 3 therefore provide the frequencies of the corresponding IRs in the set of the 316 nuclear spin functions of (D2O)8. As yet another example, consider the 185th IR, which has a dimension of 672 in the hyperoctahedral group of the 8-cube; it shall be denoted as Γ672-2 as it is the second IR with a dimension of 672 in the character table of S8[S2]. The generating function for Γ672-2 for (D2O)8 is shown in Table 10. In Table 10, N(Γ672-2) is the frequency of Γ672-2 in the set of bosonic spin functions of the deuterium nuclei of (D2O)8 that encompass 316 nuclear spin functions. An ordered partition of [], for example, [7 6 3], provides the spin distribution λ7μ6 ν3. From Table 10, it can be readily seen that the frequency of Γ672-2 for the color partition [7 6 3] spin distribution is 62. In this manner, the generating function obtained using the GCCI for Γ672-2 then yields all frequencies of all spin distributions. One can sort these frequencies into spin multiplets; hence, we obtain the following nuclear spin multiplets with frequencies in parentheses for the Γ672-2, as shown below:
1Γ672-2(17), 3Γ672-2(46), 5Γ672-2(64), 7Γ672-2(67), 9Γ672-2(59), 11Γ672-2(44), 13Γ672-2(28),
15Γ672-2(15), 17Γ672-2(6), 19Γ672-2(2)                       
Table 10. Generating function for the second IR with dimension 672 for (D2O)8 where three bosnic colors are shown as ordered partitions of [] into 3 parts for 3 colors.
We iterate the above process to generate the nuclear spin multiplets for each of the 185 IRs of the hyperoctahedral group of the 8-cube. Finally, we use either the Fermi–Dirac or Bose–Einstein stipulations for the overall wave function’s symmetry. In this case, as all deuterium nuclei are bosons, we stipulate that the direct product of the rovibronic wave function and the nuclear spin function must be totally symmetric, which would then give the overall nuclear spin statistical weights for each tunneling level of the (D2O)8.
We expect the 8-cube to be applicable to relativistic measures of time [] as hypercubes serve as time representation holders. Relativistic effects are known to make significant contributions to the electronic states and spectroscopic properties of molecules that possess very heavy atoms [,,]. Consequently, the incorporation of spin–orbit coupling changes the nonrigid molecular symmetry into double groups of hypercube groups. The double group symmetry corresponds to the coupling of the spin angular and orbital angular momenta; hence, two states with the same symmetry in the double group mix. Such topics can be the subject matter of future studies.
Another important application of the colorings of n-cube hyperplanes is in chirality and transient chirality. One can define an object to be chiral if it does not possess an improper axis of rotation. For this reason, we assign a symbol R to those conjugacy classes in Table 1 to designate proper rotations. The absence of R would then imply that the operation is an improper rotation. This is important in determining the chirality of the enumerated coloring. The symbol R (Table 1) is assigned for each conjugacy class by stipulating that a conjugacy class with the matrix type [aik] is a proper rotation if and only if the sum shown below is even:
k e v e n a 1 k + k o d d a 2 k
where the first sum is restricted to even columns, while the second sum is restricted to odd columns. Consequently, the developed computer code carries out the above sum for each of the 185 conjugacy classes of the 8-cube to determine if the sum is even or odd, and then the code assigns the symbol R if the sum is even to designate the operation as a proper rotation. Consequently, a coloring of the (8-q) hyperplane of the 8-cube is chiral if and only if the coloring function transforms as the chiral irreducible representation for a given ordered color partition, [n1 n2]. The chiral irreducible representation of the 8-cube is as a one-dimensional IR with +1 character value for all proper rotations or the ones that carry the label R in Table 1 and −1 for all improper rotations. The A2 irreducible representation of the S8[S2] group is chiral; hence, it can be seen from Table 2, Table 3, Table 4, Table 5, Table 6, Table 7, Table 8 and Table 9 that the number of chiral colorings for the (8-q) hyperplanes corresponds to the frequencies of the A2 irreducible representation in Table 2, Table 3, Table 4, Table 5, Table 6, Table 7, Table 8 and Table 9. As can be inferred from Table 2, the A2 representation does not occur at all for the 2-colorings or for up to four colors, suggesting that there are no chiral colorings for the hepteracts of the 8-cube if one uses up to four kinds of colors (blue, yellow, red, and white). However, there are chiral colorings for some of the other hyperplane colorings, including vertex colorings and edge colorings, as can be seen from Table 3, Table 4, Table 5, Table 6, Table 7, Table 8 and Table 9.
There are several biological and biochemical applications of the colorings of n-cube hyperplanes. In particular, the 2-colorings of the vertices of the 8-cube are critical to the combinatorics of genetic networks [], as well as the combinatorics of the colorings of phylogenic trees [] and pandemic trees []. The symmetries involved in both these applications are recursive in nature, and they are expressible as wreath product groups. Each level of the phylogenetic tree involves a recursive construction relative to the previous level. On the other hand, genetic regulatory networks are important combinatorial characterizations of the evolutionary processes which are also recursive; therefore, they are represented by hypercubes []. Consequently, the colorings of the vertices (q = 8) of the hypercube provide information about the equivalence classes that result in considerable simplification in computing the properties that are pertinent to the genetic regulatory networks. The equivalence classes are enumerated by the totally symmetric A1 colorings of the vertices among the 185 IRs that were considered here for the 8-cube. Wallace [,] has illustrated several applications of hypercube symmetries for understanding the spontaneous symmetry-breaking in intrinsically disordered proteins and the dynamics of proteins in general. Furthermore, the moonlighting functions of such proteins can be better understood through the use of the recursive symmetries displayed by hypercubes. It would be interesting to explore the applications of the chiral colorings of the hyperplanes of the hypercubes, which are enumerated by the generating functions obtained herein for the chiral IR. This aspect could become the subject of future studies.
Moreover, the colorings of the 8-cube hyperplanes have applications in completely different areas, such as X-ray diffraction patterns, neutron scattering studies, quarks, magnetic symmetry, and other physics applications [,,,]. As shown by several investigators [,,,], the analysis of complex X-ray patterns, magnetic structures, their symmetries, and the neutron diffractions of materials exhibiting distortions and understanding the dynamics of different phases could be benefited by the insights derived from wreath product and other types of approaches. There are several other chemical and material applications of dynamic symmetries, for example, to O8 clusters [,] and polytwistane type nanomaterials []. Finally such combinatorial enumerations pertinent to the nuclear spin statistics of donut type polyaromatic structures [] and holey nanographenes should be of future interest.

5. Conclusions

In the present study on recursive symmetries arising from the wreath products of the 8-cube, we have employed combinatorial and computational techniques to seek generating functions for the colorings of 8 hyperplanes of the 8-cube for all 185 IRs. These techniques combined Möbius inversion with the generalized character cycle indices and computer codes in order to enumerate the colorings of (8-q) hyperplanes (for q = 1–8). Several applications were outlined for the prediction of the tunneling splittings of rovibronic levels and their nuclear spin species and nuclear spin statistics. A few biological and material science applications were also pointed out. It is hoped that the present study will generate further interest and applications, especially in dynamic chirality, transient chirality, and the observed spontaneous generation of optical activity and other dynamic-symmetry-induced phenomena. Graph theoretical and combinatorial techniques can be especially useful in enumerating the number of phases and the number of chiral phases that are generated during phase transitions. The dynamic chirality is such an interesting phenomenon because an achiral system could separate into distinct enantiomorphic phases during phase transitions. Such applications would require the juxtaposition of the colorings of the hyperplanes of hypercubes enumerated in higher symmetries to various subgroups that would correspond to the symmetries of different phases. Such exciting topics that involve the combinatorics of hypercubes to lower symmetries could be the subject matter of future studies.

Funding

This research received no external funding.

Data Availability Statement

All data used are contained in the manuscript.

Conflicts of Interest

The author declares no conflict of interest.

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