7-cube
In geometry, a 7-cube is a seven-dimensional hypertope that generalizes the ordinary three-dimensional cube to higher dimensions, consisting of all points (x_1, x_2, \dots, x_7) in \mathbb{R}^7 where each |x_i| \leq 1.[1] It possesses 128 vertices located at the points (\pm 1, \pm 1, \dots, \pm 1), with edges connecting pairs of vertices that differ in exactly one coordinate.[1] The 7-cube is bounded by 14 hexeracts and encloses 1 heptacubic volume, embodying the regular {4,35} Schläfli symbol in uniform polytope notation.[1][2] The structural elements of the 7-cube follow the general formula for an n-cube, where the number of k-dimensional faces is given by \binom{n}{k} 2^{n-k}.[1] For n=7, this yields 448 edges, 672 square faces, 560 cubic cells, 280 tesseractal 4-faces, 84 penteractal 5-faces, and 14 hexeractal 6-faces.[1] Each vertex has degree 7, reflecting the seven possible directions for edges from any point, and the entire figure exhibits high symmetry under the hyperoctahedral group of order $2^7 \times 7! = 645{,}120.[1] The 7-cube can be constructed recursively as the Cartesian product of a line segment and a 6-cube, preserving its regularity and orthogonality in embedding spaces.[3] Beyond pure geometry, the 7-cube's underlying graph—known as the 7-dimensional hypercube graph Q7—serves as a model for interconnection topologies in parallel computing systems, where its logarithmic diameter and high connectivity enable efficient message routing among 128 processors.[4] This architecture has influenced designs like early hypercube-based supercomputers, highlighting the 7-cube's role in balancing scalability and communication overhead in multidimensional networks.[5]Introduction
Definition
The 7-cube is the seven-dimensional analogue of the cube, belonging to the family of hypercubes known as n-cubes, which generalize the square (2-cube) and cube (3-cube) to higher dimensions. It is a convex regular polytope embedded in seven-dimensional Euclidean space, where all edges are of equal length, all faces are congruent regular polytopes, and the figure exhibits the highest possible symmetry for its dimension. As a convex body, it contains all line segments connecting any two points within it, and its boundary consists entirely of flat (n-1)-dimensional facets.[6] This regularity ensures that the 7-cube is isometric under the Euclidean metric, meaning distances and angles are preserved by its symmetries, with mutually perpendicular sides forming an orthotope. All of its facets are identical regular 6-cubes, totaling 14 such facets that bound the polytope. The isometry group of the 7-cube acts transitively on its vertices, edges, and higher-dimensional elements, preserving the geometric structure.[6] The Schläfli symbol of the 7-cube is {4,3^5}, a notation that recursively describes its construction: it has square {4} faces with three meeting at each edge, extending this cubic pattern through five layers of three-dimensional vertex figures in higher dimensions. This symbol highlights its place among the infinite family of convex regular hypercubes, each built as the convex hull of points with coordinates \pm1 in n dimensions, though specific coordinates are derived from this abstract form.[6]Terminology and Naming
The primary name for the seven-dimensional analog of the cube is the "7-cube," a term that follows the standard convention for denoting hypercubes by their dimensionality in mathematical literature. This nomenclature extends the familiar "cube" for the three-dimensional case to higher dimensions, emphasizing its role as a regular polytope with equal edge lengths and right angles.[6] The prefix "hyper-" in "hypercube" originates from the Greek word meaning "over" or "beyond," reflecting the extension beyond three dimensions, while the specific "7-cube" adheres to the n-cube pattern established in early 20th-century geometry. An alternative designation is "hepteract," formed as a portmanteau of the Greek "hepta" (seven) and "tesseract" (the four-dimensional hypercube), introduced in contemporary polytope studies to provide mnemonic names for higher-dimensional figures.[7] The term "tetradecaexon" also appears occasionally, derived from its 14 six-dimensional facets.[2] Historically, the foundational work on n-dimensional polytopes, including what would later be called hypercubes, was laid by Ludwig Schläfli in his 1852 treatise on higher-dimensional geometry, where he classified regular polytopes and introduced symbols to describe them. This framework was expanded and systematized by H.S.M. Coxeter in the early 20th century; in his influential 1948 book Regular Polytopes, Coxeter detailed the properties of hypercubes up to high dimensions, referring to them as measure polytopes or γ_n polytopes, a naming convention originating from Elte's 1912 work on semiregular polytopes.[6][8] The 7-cube is distinguished from the more general 7-orthotope, which denotes any rectangular box in seven dimensions formed by the Cartesian product of intervals, potentially with unequal side lengths; the 7-cube specifically requires equal edges for regularity.[9] In modern geometry texts as of 2025, the "7-cube" remains the predominant term, with "hepteract" appearing in specialized discussions of uniform polytopes. Recent applications in computational geometry, such as modeling network topologies in genetic algorithms, have adopted hypercube structures like the 7-dimensional variant (hepteract) for representing chromosomal crossovers in high-dimensional search spaces.[10]Combinatorial Properties
Element Counts
The combinatorial structure of the 7-cube is quantified by the number of its k-dimensional faces for k = 0 to $7, where a k-face is a sub-polytope of dimension k embedded in the 7-dimensional space. The general formula for the number of k-faces in an n-cube, denoted f_k(n), is f_k(n) = \binom{n}{k} 2^{n-k}.[11] This arises from the coordinate representation of the n-cube as the set of points where each coordinate ranges from 0 to 1: to form a k-face, select k dimensions in which the coordinates vary freely (contributing the binomial coefficient \binom{n}{k}), and fix the remaining n-k coordinates to either 0 or 1 (contributing $2^{n-k} choices).[11] For the 7-cube (n=7), this yields the following specific counts: 128 vertices (k=0), 448 edges (k=1), 672 squares (k=2), 560 cubes (k=3), 280 tesseracts (k=4), 84 penteracts (k=5), 14 hexeracts (k=6), and 1 7-cube itself (k=7).[11] These elements highlight the exponential growth in lower-dimensional features compared to higher ones, reflecting the hypercube's expansive boundary structure. The dual polytope of the 7-cube is the 7-orthoplex, whose f-vector (the sequence of face counts) is the reverse of the 7-cube's f-vector.[12] Thus, the 7-orthoplex has 14 vertices, 84 edges, 280 squares, 560 cubes, 672 tesseracts, 448 penteracts, 128 hexeracts, and 1 cell. The table below summarizes these counts for comparison:| k (dimension) | 7-cube faces | 7-orthoplex faces |
|---|---|---|
| 0 (vertices) | 128 | 14 |
| 1 (edges) | 448 | 84 |
| 2 (squares) | 672 | 280 |
| 3 (cubes) | 560 | 560 |
| 4 (tesseracts) | 280 | 672 |
| 5 (penteracts) | 84 | 448 |
| 6 (hexeracts) | 14 | 128 |
Schläfli Symbol and Vertex Figure
The Schläfli symbol for the 7-cube is {4,3,3,3,3,3}, compactly written as {4,35}.[6] This notation recursively encodes the 7-cube's regular structure: the 2-faces are squares {4}, the 3-faces are cubes {4,3}, the 4-faces are tesseracts {4,3,3}, the 5-faces are penteracts {4,33}, the 6-faces (facets) are hexeracts {4,34}. The symbol's form, with an initial 4 followed by five 3's, distinguishes the 7-cube among regular 7-polytopes and implies its uniformity, as all elements are congruent and the arrangement is isotropic across dimensions.[13] The vertex figure of the 7-cube is a regular 6-simplex with Schläfli symbol {3,3,3,3,3} or {35}, obtained from the trailing entries of the 7-cube's symbol.[13] This 6-dimensional figure describes the local configuration around any vertex, where the seven bounding 6-cubes meet in a simplicial arrangement, with edges of length \sqrt{2} when the 7-cube's edge length is 1.[14] The 7-cube arises from the Wythoff construction applied to the Coxeter-Dynkin diagram of type B7, a linear chain of seven nodes connected by bonds labeled 4 between the first two nodes and 3 between each subsequent pair, with the initial node marked as active to generate the full symmetry orbit of vertices, edges, and facets.[15] This construction underscores the 7-cube's regularity by producing a uniform polytope through reflections in the diagram's hyperplanes, ensuring equivalent local geometries at every vertex.[16]Geometric Construction
Cartesian Coordinates
The 7-cube is realized in 7-dimensional Euclidean space \mathbb{R}^7 through its vertices, which consist of all points where each coordinate is either +1 or -1. This yields $2^7 = 128 vertices, corresponding to the $2^n combinations for an n-cube with n=7.[6][1] In this coordinate system, adjacent vertices differ in exactly one coordinate (from +1 to -1 or vice versa), resulting in a vector difference of $2 in that dimension and $0 elsewhere. The Euclidean edge length is thus \sqrt{2^2} = 2.[6][1] For applications requiring unit edge length, the coordinates can be scaled by $1/2, yielding vertices at (\pm 1/2, \pm 1/2, \dots, \pm 1/2).[6] The 7-cube itself occupies the bounded region defined by the convex hull of these vertices, specifically the set \{ \mathbf{x} \in \mathbb{R}^7 \mid -1 \leq x_i \leq 1 \ \forall i = 1, \dots, 7 \}, with interior points satisfying the strict inequality -1 < x_i < 1 for all i.[1] All lower-dimensional elements of the 7-cube are generated from these coordinates by restricting subsets of dimensions: a k-dimensional face (or k-cube) arises by fixing $7-k coordinates to either +1 or -1 and allowing the remaining k coordinates to vary freely over [-1, 1]. This construction produces \binom{7}{k} 2^{7-k} such k-faces, linking the coordinate representation to the polytope's combinatorial structure.[1]Symmetry and Coxeter Group
The full symmetry group of the 7-cube is the hyperoctahedral group B_7, a finite Coxeter group of rank 7 that acts as the group of isometries preserving the 7-cube.[17] This group has order $2^7 \times 7! = 645120.[17] The rotation subgroup, consisting of the orientation-preserving isometries, has index 2 in B_7 and thus order $2^6 \times 7! = 322560.[18] As a Coxeter group, B_7 is generated by 7 fundamental reflections corresponding to a simple system of roots in \mathbb{R}^7.[17] In the standard realization for the 7-cube (with vertices at all points having coordinates \pm 1 in each dimension), these reflections include sign flips across the coordinate hyperplanes x_i = 0 for i = 1, \dots, 7, and transpositions across the hyperplanes x_i = x_j (for $1 \leq i < j \leq 7), with the full group generated by compositions thereof.[19] The Coxeter diagram of B_7 is a linear chain of 7 nodes connected by 6 edges: the first 5 edges are single bonds (dihedral angle \pi/3, or unlabeled), and the final edge is a double bond (dihedral angle \pi/4).[17] The structure of B_7 plays a key role in classifying the facets of the 7-cube (which are 14 hexeracts) and the orbits of its lower-dimensional elements under the group action, as the transitive action on vertices, edges, and higher faces follows from the Weyl group properties of type B.[17] This classification arises from the orbits of the fundamental chamber under the reflections, determining the combinatorial equivalence classes of subpolytopes.[17]Visual Representations
Orthogonal Projections
Orthogonal projections of the 7-cube onto lower-dimensional subspaces enable visualization while preserving parallelism of edges and certain symmetry properties of the original polytope. These projections map the 128 vertices and their connections linearly, typically using a projection matrix consisting of orthonormal basis vectors for the target subspace. For instance, a basic orthogonal projection onto the first three coordinates uses the 3×7 matrix with rows as the standard basis vectors e_1, e_2, e_3 in \mathbb{R}^7, applied to the centered vertex coordinates (all combinations of \pm 1 in seven dimensions) to yield points in \mathbb{R}^3. Normalization can be achieved by scaling the matrix columns to unit length or ensuring the rows form an orthonormal set, which maintains distances up to the projection's intrinsic distortion.[20] A more revealing 3D orthogonal projection employs direction vectors chosen to highlight the internal structure, such as those derived from principal component analysis or spherical coordinates on the embedding hypersphere. In such projections, the 128 vertices appear layered concentrically according to their dot product with the viewing direction vector, with layer populations following the binomial coefficients \binom{7}{k} for k = 0 to $7, yielding the distribution 1:7:21:35:35:21:7:1 from the outermost to the innermost shells. This layering reflects the Hamming weights of the binary representations underlying the hypercube vertices, forming a nested arrangement of polytopal facets that approximates a Gaussian distribution in high dimensions. The resulting graph preserves projections of the hyperoctahedral symmetry group, manifesting as rotational and reflectional invariances in the 3D embedding.[1] For 2D shadow projections, orthogonal projection onto a plane (e.g., using a 2×7 matrix with orthonormal rows) produces a central-symmetric polygon outline, often a 14-gon or higher for generic directions, enclosing interior edges that form a complex network of parallelograms and hexagons. These shadows maintain the even-sided polygonal symmetry characteristic of hypercubic projections, with vertex densities peaking near the center due to overlapping contributions from multiple dimensions. In central configurations, where the projection direction aligns with a principal axis through opposite vertices, the two antipodal vertices may coincide at the projection center, effectively doubling the representational weight there without altering the overall topology.[20]Petrie Polygons and Alternations
In geometry, the Petrie polygon of the 7-cube, also known as the hepteract, is a skew tetradecagon—a 14-sided polygon—that forms a closed cycle visiting 14 distinct vertices without repeating any edges, such that every pair of consecutive edges lies within one of the 7-cube's square faces, while no three consecutive edges share a common face. This structure embodies a maximal-length skew cycle in the 7-cube's 1-skeleton, providing a compact representation of its edge connectivity in higher dimensions. As detailed by H.S.M. Coxeter, such polygons generalize from lower-dimensional analogues, like the hexagonal Petrie polygon of the 3-cube, to capture helical traversals that avoid planarity. The Petrie polygon can be constructed as a cycle of 14 edges where every 6 consecutive edges lie in one 6-cube facet, but no 7 do, forming a skew path through the hypercube. Alternating the 7-cube along its Petrie polygons—removing every other vertex from each such cycle—yields the demihepteract, or 7-demicube, a uniform 7-polytope with 64 vertices (half of the original 128) and 14 cells consisting of demihexeracts (alternations of the 6-cubes). This operation preserves uniformity under the full hyperoctahedral symmetry group but halves the vertex set, resulting in a star-free convex polytope whose facets interlock in a more compact arrangement than the original. Coxeter notes that such alternations generalize the tetrahedral cells of the 4-demicube (16-cell), offering a rectified-like variant that truncates the original structure without introducing new vertices. These Petrie polygons and their alternations facilitate alternative visualizations of the 7-cube by projecting the skew tetradecagon onto a plane as a regular 14-gon, revealing the polytope's rotational symmetries in a flattened form that highlights edge densities and helical windings otherwise obscured in orthogonal projections. For instance, isometric projections center the Petrie polygon as an equatorial boundary, aiding comprehension of the 7-cube's non-convex uniform derivatives and skew facet arrangements through simplified 2D traces. This approach underscores conceptual links to broader families of regular polytopes, emphasizing path-based unfoldings over volumetric renders.Related Polytopes
Dual and Lower-Dimensional Analogues
The dual of the 7-cube is the 7-orthoplex, or 7-dimensional cross-polytope, a regular convex 7-polytope with 14 vertices corresponding to the coordinate axes directions (\pm e_i for i=1 to 7) and 128 facets, each a regular 6-simplex.\] This duality arises because the faces of the 7-cube correspond to the vertices of the 7-orthoplex and vice versa, preserving the combinatorial structure through polar reciprocity in Euclidean space.\[ Lower-dimensional analogues of the 7-cube form the sequence of hypercubes, starting from the 1-cube (a line segment with two vertices and no proper facets) and progressing through the 2-cube (square, with line segment facets), 3-cube (cube, with square facets), 4-cube (tesseract, with cubic facets), 5-cube (penteract, with tesseractic facets), and 6-cube (hexeract, with penteractic facets).\] This progression illustrates increasing structural complexity, as the facets evolve from 0-dimensional points to 5-dimensional polytopes, with the dimension of facets always one less than the ambient space.\[ All hypercubes, including the 7-cube, share key properties: they are regular and convex polytopes and exhibit element counts governed by binomial coefficients, where the number of k-dimensional faces is \binom{7}{k} 2^{7-k}, reflecting exponential growth in connectivity.\] This binomial structure underscores their role as models for binary state spaces and product topologies.\[ The 7-cube belongs to the class of Hanner polytopes, which are centrally symmetric convex bodies constructed recursively via Cartesian products and polar duals of lower-dimensional instances, starting from intervals; hypercubes arise as iterated products of 1-dimensional segments, inheriting uniform convexity and extremal volume product properties within this family.$$]Higher-Dimensional Extensions
The 8-dimensional hypercube, known as the octeract, extends the 7-cube through recursive construction by duplicating a 7-cube to form two parallel copies and connecting each pair of corresponding vertices with edges perpendicular to the original 7-dimensional hyperplanes, thereby adding 128 new edges and resulting in 16 facets, each a 7-cube. This method generalizes to arbitrary dimensions greater than 7, where each n-cube (n > 7) is built from two (n-1)-cubes in the same fashion, preserving the orthogonal structure and enabling the definition of hypercubes in infinitely high dimensions.[21] The asymptotic behavior of element counts in n-cubes follows from the binomial theorem, with the number of k-faces given by \dbinom{n}{k} 2^{n-k}, representing the ways to choose k directions for the face's extent while fixing the positions in the remaining n-k directions across the two possible states per dimension. As n grows large, this yields exponential growth dominated by $2^n, tempered by polynomial factors \frac{n^k}{k!} for fixed k, illustrating how the combinatorial complexity escalates rapidly beyond the 7-cube. The Schläfli symbol generalizes to \{4, 3^{n-2}\} for the n-cube. Hypercubes play a central role in n-dimensional geometry as measure polytopes, providing a standard framework for orthogonal bases and tessellations of Euclidean space \mathbb{R}^n. In measure theory, the unit hypercube [0,1]^n serves as the prototype for the Lebesgue measure on product spaces, facilitating theorems like Fubini-Tonelli for integrating over high-dimensional domains and underpinning applications in probability, such as uniform distributions and concentration phenomena where measure concentrates near boundaries as n increases.[6][22] Higher-dimensional hypercubes differ from the 7-cube primarily in the escalated complexity of their facets, which are themselves (n-1)-cubes comprising exponentially more subelements, resulting in a vastly richer incidence structure. Despite this, regularity is maintained universally, with uniform edge lengths, congruent cells, and right dihedral angles, ensuring all n-cubes share the same abstract combinatorial type scaled to higher dimensions.[6]Abstract Structure
As a Configuration
The 7-cube can be abstracted as a regular polytope of rank 8, where the elements are partially ordered by incidence, forming a lattice of faces from vertices (rank 0) to the entire 7-cube (rank 7). This incidence structure emphasizes combinatorial relations among the elements, independent of metric properties. Since the unique rank-7 element contains all others, the configuration is conventionally described via the proper faces of ranks 0 through 6, captured in a 7×7 matrix whose entry M_{i,j} gives the number of rank-j faces incident to a fixed rank-i face.[$$ The rows and columns of the matrix are indexed by ranks 0 (vertices) to 6 (6-faces). The diagonal entries are the total numbers of elements at each rank: 128 for vertices, 448 for edges, 672 for 2-faces, 560 for 3-faces, 280 for 4-faces, 84 for 5-faces, and 14 for 6-faces; these combinatorial counts are given by the formula f_k = \binom{7}{k} 2^{7-k} for k=0 to 6 and are detailed further in the Element Counts section.\] [](https://mathworld.wolfram.com/Hypercube.html) For $i < j$, the entry $M_{i,j} = \binom{7-i}{j-i}$ counts the $j$-faces containing the $i$-face, reflecting choices of additional dimensions to vary from the fixed ones. For $i > j$, $M_{i,j} = \binom{i}{j} 2^{i-j}$ counts the $j$-faces within the $i$-face, as each $i$-face is combinatorially equivalent to an $i$-[cube](/page/Cube).\[| Rank \ Rank | 0 (Vertices) | 1 (Edges) | 2 (Squares) | 3 (Cubes) | 4 (Tesseracts) | 5 (Penteracts) | 6 (Hexaracts) |
|---|---|---|---|---|---|---|---|
| 0 | 128 | 7 | 21 | 35 | 35 | 21 | 7 |
| 1 | 2 | 448 | 6 | 15 | 20 | 15 | 6 |
| 2 | 4 | 4 | 672 | 5 | 10 | 10 | 5 |
| 3 | 8 | 12 | 6 | 560 | 4 | 6 | 4 |
| 4 | 16 | 32 | 24 | 8 | 280 | 3 | 3 |
| 5 | 32 | 80 | 80 | 40 | 10 | 84 | 2 |
| 6 | 64 | 192 | 240 | 160 | 60 | 12 | 14 |