simplex

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In
geometry Geometry (from the grc, γεωμετρία; ' "earth", ' "measurement") is, with , one of the oldest branches of . It is concerned with properties of space that are related with distance, shape, size, and relative position of figures. A mat ...

, a simplex (plural: simplexes or simplices) is a generalization of the notion of a
triangle A triangle is a polygon In geometry, a polygon () is a plane (mathematics), plane Shape, figure that is described by a finite number of straight line segments connected to form a closed ''polygonal chain'' (or ''polygonal circuit''). The b ...

or
tetrahedron In geometry, a tetrahedron (plural: tetrahedra or tetrahedrons), also known as a triangular Pyramid (geometry), pyramid, is a polyhedron composed of four triangular Face (geometry), faces, six straight Edge (geometry), edges, and four vertex (g ...

to arbitrary
dimensions thumb , 236px , The first four spatial dimensions, represented in a two-dimensional picture. In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature ...

. The simplex is so-named because it represents the simplest possible
polytope In elementary geometry Geometry (from the grc, γεωμετρία; ''wikt:γῆ, geo-'' "earth", ''wikt:μέτρον, -metron'' "measurement") is, with arithmetic, one of the oldest branches of mathematics. It is concerned with propertie ...
in any given space. For example, * a 0-simplex is a
point Point or points may refer to: Places * Point, LewisImage:Point Western Isles NASA World Wind.png, Satellite image of Point Point ( gd, An Rubha), also known as the Eye Peninsula, is a peninsula some 11 km long in the Outer Hebrides (or Western I ...
, * a 1-simplex is a
line segment 250px, The geometric definition of a closed line segment: the intersection of all points at or to the right of ''A'' with all points at or to the left of ''B'' In geometry Geometry (from the grc, γεωμετρία; ''wikt:γῆ, geo-'' ...

, * a 2-simplex is a
triangle A triangle is a polygon In geometry, a polygon () is a plane (mathematics), plane Shape, figure that is described by a finite number of straight line segments connected to form a closed ''polygonal chain'' (or ''polygonal circuit''). The b ...

, * a 3-simplex is a
tetrahedron In geometry, a tetrahedron (plural: tetrahedra or tetrahedrons), also known as a triangular Pyramid (geometry), pyramid, is a polyhedron composed of four triangular Face (geometry), faces, six straight Edge (geometry), edges, and four vertex (g ...

, * a 4-simplex is a . Specifically, a ''k''-simplex is a ''k''-dimensional
polytope In elementary geometry Geometry (from the grc, γεωμετρία; ''wikt:γῆ, geo-'' "earth", ''wikt:μέτρον, -metron'' "measurement") is, with arithmetic, one of the oldest branches of mathematics. It is concerned with propertie ...
which is the
convex hull In geometry Geometry (from the grc, γεωμετρία; ''wikt:γῆ, geo-'' "earth", ''wikt:μέτρον, -metron'' "measurement") is, with arithmetic, one of the oldest branches of mathematics. It is concerned with properties of space t ...

of its ''k'' + 1 vertices. More formally, suppose the ''k'' + 1 points $u_0, \dots, u_k \in \mathbb^$ are
affinely independent In mathematics, an affine space is a geometric Structure (mathematics), structure that generalizes some of the properties of Euclidean spaces in such a way that these are independent of the concepts of distance and measure of angles, keeping onl ...
, which means $u_1 - u_0,\dots, u_k-u_0$ are
linearly independent In the theory of vector spaces, a set of vectors is said to be if at least one of the vectors in the set can be defined as a linear combinationIn mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics ...
. Then, the simplex determined by them is the set of points :$C = \left\$ This representation in terms of weighted vertices is known as the
barycentric coordinate system In geometry Geometry (from the grc, γεωμετρία; ''wikt:γῆ, geo-'' "earth", ''wikt:μέτρον, -metron'' "measurement") is, with arithmetic, one of the oldest branches of mathematics. It is concerned with properties of space ...
. A regular simplex is a simplex that is also a
regular polytope In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). It ...
. A regular ''k''-simplex may be constructed from a regular (''k'' − 1)-simplex by connecting a new vertex to all original vertices by the common edge length. The standard simplex or probability simplex is the ''k - 1'' dimensional simplex whose vertices are the ''k'' standard unit vectors, or :$\left\.$ In
topology s, which have only one surface and one edge, are a kind of object studied in topology. In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structu ...

and
combinatorics Combinatorics is an area of mathematics primarily concerned with counting, both as a means and an end in obtaining results, and certain properties of finite set, finite Mathematical structure, structures. It is closely related to many other area ...
, it is common to "glue together" simplices to form a
simplicial complex In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). It ...
. The associated combinatorial structure is called an
abstract simplicial complex In combinatorics Combinatorics is an area of mathematics primarily concerned with counting, both as a means and an end in obtaining results, and certain properties of finite set, finite Mathematical structure, structures. It is closely relate ...
, in which context the word "simplex" simply means any
finite set In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). It ...
of vertices.

# History

The concept of a simplex was known to
William Kingdon Clifford William Kingdon Clifford (4 May 18453 March 1879) was an English mathematician and philosopher. Building on the work of Hermann Grassmann, he introduced what is now termed geometric algebra, a special case of the Clifford algebra named in his ...

, who wrote about these shapes in 1886 but called them "prime confines".
Henri Poincaré Jules Henri Poincaré ( S: stress final syllable ; 29 April 1854 – 17 July 1912) was a French French (french: français(e), link=no) may refer to: * Something of, from, or related to France France (), officially the French Repu ...
algebraic topology 250px, A torus, one of the most frequently studied objects in algebraic topology Algebraic topology is a branch of mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathemat ...
in 1900, called them "generalized tetrahedra". In 1902
Pieter Hendrik Schoute Pieter Hendrik Schoute (21 January 1846, Wormerveer – 18 April 1923, Groningen (city), Groningen) was a Netherlands, Dutch mathematician known for his work on regular polytopes and Euclidean geometry. He started his career as a civil engine ...
described the concept first with the
Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the Roman Republic, it became the dominant la ...

superlative ''simplicissimum'' ("simplest") and then with the same Latin adjective in the normal form ''simplex'' ("simple"). The regular simplex family is the first of three
regular polytope In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). It ...
families, labeled by
Donald Coxeter Harold Scott MacDonald "Donald" Coxeter, (February 9, 1907 – March 31, 2003) was a British and later also Canadian geometer. He is regarded as one of the greatest geometers of the 20th century. Biography Coxeter was born in Kensington to ...
as ''αn'', the other two being the
cross-polytope In geometry Geometry (from the grc, γεωμετρία; ''wikt:γῆ, geo-'' "earth", ''wikt:μέτρον, -metron'' "measurement") is, with arithmetic, one of the oldest branches of mathematics. It is concerned with properties of space th ...
family, labeled as ''βn'', and the
hypercube In geometry Geometry (from the grc, γεωμετρία; ''wikt:γῆ, geo-'' "earth", ''wikt:μέτρον, -metron'' "measurement") is, with arithmetic, one of the oldest branches of mathematics. It is concerned with properties of space th ...

s, labeled as ''γn''. A fourth family, the tessellation of ''n''-dimensional space by infinitely many hypercubes, he labeled as ''δn''.

# Elements

The convex hull of any
nonempty In mathematics, the empty set is the unique Set (mathematics), set having no Element (mathematics), elements; its size or cardinality (count of elements in a set) is 0, zero. Some axiomatic set theories ensure that the empty set exists by includ ...

subset In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). ...

of the ''n'' + 1 points that define an ''n''-simplex is called a face of the simplex. Faces are simplices themselves. In particular, the convex hull of a subset of size ''m'' + 1 (of the ''n'' + 1 defining points) is an ''m''-simplex, called an ''m''-face of the ''n''-simplex. The 0-faces (i.e., the defining points themselves as sets of size 1) are called the vertices (singular: vertex), the 1-faces are called the edges, the (''n'' − 1)-faces are called the facets, and the sole ''n''-face is the whole ''n''-simplex itself. In general, the number of ''m''-faces is equal to the
binomial coefficient In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers ( and ), formulas and related structures (), shapes and spaces in which they are contained (), and quantities and their changes ( and ). There is no g ...
$\tbinom$. Consequently, the number of ''m''-faces of an ''n''-simplex may be found in column (''m'' + 1) of row (''n'' + 1) of
Pascal's triangle In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). It h ...
. A simplex ''A'' is a coface of a simplex ''B'' if ''B'' is a face of ''A''. ''Face'' and ''facet'' can have different meanings when describing types of simplices in a
simplicial complex In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). It ...
; see for more detail. The number of 1-faces (edges) of the ''n''-simplex is the ''n''-th
triangle number A triangular number or triangle number counts objects arranged in an equilateral triangle. Triangular numbers are a type of figurate number, other examples being square numbers and Cube (algebra)#In integers, cube numbers. The th triangular number ...
, the number of 2-faces of the ''n''-simplex is the (''n'' − 1)th tetrahedron number, the number of 3-faces of the ''n''-simplex is the (''n'' − 2)th 5-cell number, and so on. In layman's terms, an ''n''-simplex is a simple shape (a polygon) that requires ''n'' dimensions. Consider a line segment ''AB'' as a "shape" in a 1-dimensional space (the 1-dimensional space is the line in which the segment lies). One can place a new point ''C'' somewhere off the line. The new shape, triangle ''ABC'', requires two dimensions; it cannot fit in the original 1-dimensional space. The triangle is the 2-simplex, a simple shape that requires two dimensions. Consider a triangle ''ABC'', a shape in a 2-dimensional space (the plane in which the triangle resides). One can place a new point ''D'' somewhere off the plane. The new shape, tetrahedron ''ABCD'', requires three dimensions; it cannot fit in the original 2-dimensional space. The tetrahedron is the 3-simplex, a simple shape that requires three dimensions. Consider tetrahedron ''ABCD'', a shape in a 3-dimensional space (the 3-space in which the tetrahedron lies). One can place a new point ''E'' somewhere outside the 3-space. The new shape ''ABCDE'', called a 5-cell, requires four dimensions and is called the 4-simplex; it cannot fit in the original 3-dimensional space. (It also cannot be visualized easily.) This idea can be generalized, that is, adding a single new point outside the currently occupied space, which requires going to the next higher dimension to hold the new shape. This idea can also be worked backward: the line segment we started with is a simple shape that requires a 1-dimensional space to hold it; the line segment is the 1-simplex. The line segment itself was formed by starting with a single point in 0-dimensional space (this initial point is the 0-simplex) and adding a second point, which required the increase to 1-dimensional space. More formally, an (''n'' + 1)-simplex can be constructed as a join (∨ operator) of an ''n''-simplex and a point, ( ). An (''m'' + ''n'' + 1)-simplex can be constructed as a join of an ''m''-simplex and an ''n''-simplex. The two simplices are oriented to be completely normal from each other, with translation in a direction orthogonal to both of them. A 1-simplex is the join of two points: ( ) ∨ ( ) = 2 ⋅ ( ). A general 2-simplex (scalene triangle) is the join of three points: ( ) ∨ ( ) ∨ ( ). An
isosceles triangle In geometry Geometry (from the grc, γεωμετρία; ''wikt:γῆ, geo-'' "earth", ''wikt:μέτρον, -metron'' "measurement") is, with arithmetic, one of the oldest branches of mathematics. It is concerned with properties of space ...

is the join of a 1-simplex and a point:  ∨ ( ). An
equilateral triangle In geometry, an equilateral triangle is a triangle in which all three sides have the same length. In the familiar Euclidean geometry, an equilateral triangle is also equiangular polygon, equiangular; that is, all three internal angles are also con ...

is 3 ⋅ ( ) or . A general 3-simplex is the join of 4 points: ( ) ∨ ( ) ∨ ( ) ∨ ( ). A 3-simplex with mirror symmetry can be expressed as the join of an edge and two points:  ∨ ( ) ∨ ( ). A 3-simplex with triangular symmetry can be expressed as the join of an equilateral triangle and 1 point: 3.( )∨( ) or ∨( ). A
regular tetrahedron In geometry, a tetrahedron (plural: tetrahedra or tetrahedrons), also known as a triangular Pyramid (geometry), pyramid, is a polyhedron composed of four triangular Face (geometry), faces, six straight Edge (geometry), edges, and four vertex (g ...
is 4 ⋅ ( ) or and so on. In some conventions, the empty set is defined to be a (−1)-simplex. The definition of the simplex above still makes sense if ''n'' = −1. This convention is more common in applications to algebraic topology (such as
simplicial homology In geometry, a simplex (plural: simplexes or simplices) is a generalization of the notion of a triangle or tetrahedron to arbitrary dimensions. The simplex is so-named because it represents the simplest possible polytope in any given space. For e ...
) than to the study of polytopes.

# Symmetric graphs of regular simplices

These
Petrie polygon In geometry Geometry (from the grc, γεωμετρία; ''wikt:γῆ, geo-'' "earth", ''wikt:μέτρον, -metron'' "measurement") is, with arithmetic, one of the oldest branches of mathematics. It is concerned with properties of space tha ...
s (skew orthogonal projections) show all the vertices of the regular simplex on a
circle A circle is a consisting of all in a that are at a given distance from a given point, the ; equivalently it is the curve traced out by a point that moves in a plane so that its distance from a given point is . The distance between any po ...

, and all vertex pairs connected by edges.

# The standard simplex

The standard ''n''-simplex (or unit ''n''-simplex) is the subset of R''n''+1 given by : $\Delta^n = \left\$ The simplex Δ''n'' lies in the
affine hyperplane In geometry Geometry (from the grc, γεωμετρία; ''wikt:γῆ, geo-'' "earth", ''wikt:μέτρον, -metron'' "measurement") is, with arithmetic, one of the oldest branches of mathematics. It is concerned with properties of space t ...
obtained by removing the restriction ''t''''i'' ≥ 0 in the above definition. The ''n'' + 1 vertices of the standard ''n''-simplex are the points ''e''''i'' ∈ R''n''+1, where :''e''0 = (1, 0, 0, ..., 0), :''e''1 = (0, 1, 0, ..., 0), : ⋮ :''e''''n'' = (0, 0, 0, ..., 1). There is a canonical map from the standard ''n''-simplex to an arbitrary ''n''-simplex with vertices (''v''0, ..., ''v''''n'') given by :$\left(t_0,\ldots,t_n\right) \mapsto \sum_^n t_i v_i$ The coefficients ''t''''i'' are called the barycentric coordinates of a point in the ''n''-simplex. Such a general simplex is often called an affine ''n''-simplex, to emphasize that the canonical map is an
affine transformation In Euclidean geometry Euclidean geometry is a mathematical system attributed to Alexandrian Greek mathematics , Greek mathematician Euclid, which he described in his textbook on geometry: the ''Euclid's Elements, Elements''. Euclid's method con ...
. It is also sometimes called an oriented affine ''n''-simplex to emphasize that the canonical map may be orientation preserving or reversing. More generally, there is a canonical map from the standard $\left(n-1\right)$-simplex (with ''n'' vertices) onto any
polytope In elementary geometry Geometry (from the grc, γεωμετρία; ''wikt:γῆ, geo-'' "earth", ''wikt:μέτρον, -metron'' "measurement") is, with arithmetic, one of the oldest branches of mathematics. It is concerned with propertie ...
with ''n'' vertices, given by the same equation (modifying indexing): :$\left(t_1,\ldots,t_n\right) \mapsto \sum_^n t_i v_i$ These are known as generalized barycentric coordinates, and express every polytope as the ''image'' of a simplex: $\Delta^ \twoheadrightarrow P.$ A commonly used function from R''n'' to the interior of the standard $\left(n-1\right)$-simplex is the softmax function, or normalized exponential function; this generalizes the standard logistic function.

## Examples

* Δ0 is the point . * Δ1 is the line segment joining (1, 0) and (0, 1) in R2. * Δ2 is the
equilateral triangle In geometry, an equilateral triangle is a triangle in which all three sides have the same length. In the familiar Euclidean geometry, an equilateral triangle is also equiangular polygon, equiangular; that is, all three internal angles are also con ...

with vertices (1, 0, 0), (0, 1, 0) and (0, 0, 1) in R3. * Δ3 is the
regular tetrahedron In geometry, a tetrahedron (plural: tetrahedra or tetrahedrons), also known as a triangular Pyramid (geometry), pyramid, is a polyhedron composed of four triangular Face (geometry), faces, six straight Edge (geometry), edges, and four vertex (g ...
with vertices (1, 0, 0, 0), (0, 1, 0, 0), (0, 0, 1, 0) and (0, 0, 0, 1) in R4.

## Increasing coordinates

An alternative coordinate system is given by taking the indefinite sum: :$\begin s_0 &= 0\\ s_1 &= s_0 + t_0 = t_0\\ s_2 &= s_1 + t_1 = t_0 + t_1\\ s_3 &= s_2 + t_2 = t_0 + t_1 + t_2\\ &\;\;\vdots\\ s_n &= s_ + t_ = t_0 + t_1 + \cdots + t_\\ s_ &= s_n + t_n = t_0 + t_1 + \cdots + t_n = 1 \end$ This yields the alternative presentation by ''order,'' namely as nondecreasing ''n''-tuples between 0 and 1: :$\Delta_*^n = \left\.$ Geometrically, this is an ''n''-dimensional subset of $\mathbb^n$ (maximal dimension, codimension 0) rather than of $\mathbb^$ (codimension 1). The facets, which on the standard simplex correspond to one coordinate vanishing, $t_i=0,$ here correspond to successive coordinates being equal, $s_i=s_,$ while the Interior (topology), interior corresponds to the inequalities becoming ''strict'' (increasing sequences). A key distinction between these presentations is the behavior under permuting coordinates – the standard simplex is stabilized by permuting coordinates, while permuting elements of the "ordered simplex" do not leave it invariant, as permuting an ordered sequence generally makes it unordered. Indeed, the ordered simplex is a (closed) fundamental domain for the group action, action of the symmetric group on the ''n''-cube, meaning that the orbit of the ordered simplex under the ''n''! elements of the symmetric group divides the ''n''-cube into $n!$ mostly disjoint simplices (disjoint except for boundaries), showing that this simplex has volume $1/n!$ Alternatively, the volume can be computed by an iterated integral, whose successive integrands are $1,x,x^2/2,x^3/3!,\dots,x^n/n!$ A further property of this presentation is that it uses the order but not addition, and thus can be defined in any dimension over any ordered set, and for example can be used to define an infinite-dimensional simplex without issues of convergence of sums.

## Projection onto the standard simplex

Especially in numerical applications of probability theory a Graphical projection, projection onto the standard simplex is of interest. Given $\left(p_i\right)_i$ with possibly negative entries, the closest point $\left\left(t_i\right\right)_i$ on the simplex has coordinates :$t_i= \max\,$ where $\Delta$ is chosen such that $\sum_i\max\=1.$ $\Delta$ can be easily calculated from sorting $p_i$. The sorting approach takes $O\left( n \log n\right)$ complexity, which can be improved to $O\left(n\right)$ complexity via Selection algorithm, median-finding algorithms. Projecting onto the simplex is computationally similar to projecting onto the $\ell_1$ ball.

## Corner of cube

Finally, a simple variant is to replace "summing to 1" with "summing to at most 1"; this raises the dimension by 1, so to simplify notation, the indexing changes: :$\Delta_c^n = \left\.$ This yields an ''n''-simplex as a corner of the ''n''-cube, and is a standard orthogonal simplex. This is the simplex used in the simplex method, which is based at the origin, and locally models a vertex on a polytope with ''n'' facets.

# Cartesian coordinates for a regular ''n''-dimensional simplex in R''n''

One way to write down a regular ''n''-simplex in R''n'' is to choose two points to be the first two vertices, choose a third point to make an equilateral triangle, choose a fourth point to make a regular tetrahedron, and so on. Each step requires satisfying equations that ensure that each newly chosen vertex, together with the previously chosen vertices, forms a regular simplex. There are several sets of equations that can be written down and used for this purpose. These include the equality of all the distances between vertices; the equality of all the distances from vertices to the center of the simplex; the fact that the angle subtended through the new vertex by any two previously chosen vertices is $\pi/3$; and the fact that the angle subtended through the center of the simplex by any two vertices is $\arccos\left(-1/n\right)$. It is also possible to directly write down a particular regular ''n''-simplex in R''n'' which can then be translated, rotated, and scaled as desired. One way to do this is as follows. Denote the basis (linear algebra), basis vectors of R''n'' by e1 through e''n''. Begin with the standard -simplex which is the convex hull of the basis vectors. By adding an additional vertex, these become a face of a regular -simplex. The additional vertex must lie on the line perpendicular to the barycenter of the standard simplex, so it has the form for some real number ''α''. Since the squared distance between two basis vectors is 2, in order for the additional vertex to form a regular ''n''-simplex, the squared distance between it and any of the basis vectors must also be 2. This yields a quadratic equation for ''α''. Solving this equation shows that there are two choices for the additional vertex: :$\frac \left\left(1 \pm \sqrt \right\right) \cdot \left(1, \dots, 1\right).$ Either of these, together with the standard basis vectors, yields a regular ''n''-simplex. The above regular ''n''-simplex is not centered on the origin. It can be translated to the origin by subtracting the mean of its vertices. By rescaling, it can be given unit side length. This results in the simplex whose vertices are: :$\frac\mathbf_i - \frac\bigg\left(1 \pm \frac\bigg\right) \cdot \left(1, \dots, 1\right),$ for $1 \le i \le n$, and :$\mp\frac \cdot \left(1, \dots, 1\right).$ Note that there are two sets of vertices described here. One set uses $+$ in the first $n$ coordinate calculations and $-$ in the last calculation. The other set replaces $-$ for $+$ and vice versa. This simplex is inscribed in a hypersphere of radius $\sqrt$. A different rescaling produces a simplex that is inscribed in a unit hypersphere. When this is done, its vertices are :$\sqrt\cdot\mathbf_i - n^\left(\sqrt \pm 1\right) \cdot \left(1, \dots, 1\right),$ where $1 \le i \le n$, and :$\mp n^ \cdot \left(1, \dots, 1\right).$ The side length of this simplex is $\sqrt$. A highly symmetric way to construct a regular -simplex is to use a representation of the cyclic group by orthogonal matrix, orthogonal matrices. This is an orthogonal matrix such that is the identity matrix, but no lower power of is. Applying powers of this matrix (mathematics), matrix to an appropriate vector will produce the vertices of a regular -simplex. To carry this out, first observe that for any orthogonal matrix , there is a choice of basis in which is a block diagonal matrix :$Q = \operatorname\left(Q_1, Q_2, \dots, Q_k\right),$ where each is orthogonal and either or . In order for to have order , all of these matrices must have order divisor, dividing . Therefore each is either a matrix whose only entry is or, if is parity (mathematics), odd, ; or it is a matrix of the form :$\begin \cos \frac & -\sin \frac \\ \sin \frac & \cos \frac \end,$ where each is an integer between zero and inclusive. A sufficient condition for the orbit of a point to be a regular simplex is that the matrices form a basis for the non-trivial irreducible real representations of , and the vector being rotated is not stabilized by any of them. In practical terms, for parity (mathematics), even this means that every matrix is , there is an equality of sets :$\ = \,$ and, for every , the entries of upon which acts are not both zero. For example, when , one possible matrix is :$\begin \cos\left(2\pi/5\right) & -\sin\left(2\pi/5\right) & 0 & 0 \\ \sin\left(2\pi/5\right) & \cos\left(2\pi/5\right) & 0 & 0 \\ 0 & 0 & \cos\left(4\pi/5\right) & -\sin\left(4\pi/5\right) \\ 0 & 0 & \sin\left(4\pi/5\right) & \cos\left(4\pi/5\right) \end.$ Applying this to the vector results in the simplex whose vertices are :$\begin 1 \\ 0 \\ 1 \\ 0 \end, \begin \cos\left(2\pi/5\right) \\ \sin\left(2\pi/5\right) \\ \cos\left(4\pi/5\right) \\ \sin\left(4\pi/5\right) \end, \begin \cos\left(4\pi/5\right) \\ \sin\left(4\pi/5\right) \\ \cos\left(8\pi/5\right) \\ \sin\left(8\pi/5\right) \end, \begin \cos\left(6\pi/5\right) \\ \sin\left(6\pi/5\right) \\ \cos\left(2\pi/5\right) \\ \sin\left(2\pi/5\right) \end, \begin \cos\left(8\pi/5\right) \\ \sin\left(8\pi/5\right) \\ \cos\left(6\pi/5\right) \\ \sin\left(6\pi/5\right) \end,$ each of which has distance √5 from the others. When is odd, the condition means that exactly one of the diagonal blocks is , equal to , and acts upon a non-zero entry of ; while the remaining diagonal blocks, say , are , there is an equality of sets :$\left\ = \left\,$ and each diagonal block acts upon a pair of entries of which are not both zero. So, for example, when , the matrix can be :$\begin 0 & -1 & 0 \\ 1 & 0 & 0 \\ 0 & 0 & -1 \\ \end.$ For the vector , the resulting simplex has vertices :$\begin 1 \\ 0 \\ 1/\surd2 \end, \begin 0 \\ 1 \\ -1/\surd2 \end, \begin -1 \\ 0 \\ 1/\surd2 \end, \begin 0 \\ -1 \\ -1/\surd2 \end,$ each of which has distance 2 from the others.

# Geometric properties

## Volume

The volume of an ''n''-simplex in ''n''-dimensional space with vertices (''v''0, ..., ''v''''n'') is :$\mathrm = \frac \left, \det \begin v_1-v_0 && v_2-v_0 && \cdots && v_n-v_0 \end\$ where each column of the ''n'' × ''n'' determinant is the difference between the vector (geometry), vectors representing two vertices. This formula is particularly useful when $v_0$ is the origin. A more symmetric way to write it is :$\mathrm = \left, \det \begin v_0 & v_1 & \cdots & v_n \\ 1 & 1 & \cdots & 1 \end\ = \left\left(\det \left\left[ \begin v_0^T & 1 \\ v_1^T & 1 \\ \vdots & \vdots \\ v_n^T & 1 \end \begin v_0 & v_1 & \cdots & v_n \\ 1 & 1 & \cdots & 1 \end \right\right]\right\right)^\,,$ where the last expression works even when the ''n''-simplex's vertices are in a Euclidean space with more than ''n'' dimensions. Another common way of computing the volume of the simplex is via the Distance geometry#Cayley–Menger determinants, Cayley–Menger determinant. It can also compute the volume of a simplex embedded in a higher-dimensional space, e.g., a triangle in $\mathbb^3$. Without the 1/''n''! it is the formula for the volume of an ''n''-parallelepiped#Parallelotope, parallelotope. This can be understood as follows: Assume that ''P'' is an ''n''-parallelotope constructed on a basis $\left(v_0, e_1, \ldots, e_n\right)$ of $\R^n$. Given a permutation $\sigma$ of $\$, call a list of vertices $v_0,\ v_1, \ldots, v_n$ a ''n''-path if :$v_1 = v_0 + e_,\ v_2 = v_1 + e_,\ldots, v_n = v_+e_$ (so there are ''n''! ''n''-paths and $v_n$ does not depend on the permutation). The following assertions hold: If ''P'' is the unit ''n''-hypercube, then the union of the ''n''-simplexes formed by the convex hull of each ''n''-path is ''P'', and these simplexes are congruent and pairwise non-overlapping. In particular, the volume of such a simplex is : $\frac = \frac 1 .$ If ''P'' is a general parallelotope, the same assertions hold except that it is no longer true, in dimension > 2, that the simplexes need to be pairwise congruent; yet their volumes remain equal, because the ''n''-parallelotope is the image of the unit ''n''-hypercube by the linear isomorphism that sends the canonical basis of $\R^n$ to $e_1,\ldots, e_n$. As previously, this implies that the volume of a simplex coming from a ''n''-path is: : $\frac = \frac.$ Conversely, given an ''n''-simplex $\left(v_0,\ v_1,\ v_2,\ldots v_n\right)$ of $\mathbf R^n$, it can be supposed that the vectors $e_1 = v_1-v_0,\ e_2 = v_2-v_1,\ldots e_n=v_n-v_$ form a basis of $\mathbf R^n$. Considering the parallelotope constructed from $v_0$ and $e_1,\ldots, e_n$, one sees that the previous formula is valid for every simplex. Finally, the formula at the beginning of this section is obtained by observing that :$\det\left(v_1-v_0, v_2-v_0,\ldots, v_n-v_0\right) = \det\left(v_1-v_0, v_2-v_1,\ldots, v_n-v_\right).$ From this formula, it follows immediately that the volume under a standard ''n''-simplex (i.e. between the origin and the simplex in R''n''+1) is : The volume of a regular ''n''-simplex with unit side length is :$\frac$ as can be seen by multiplying the previous formula by ''x''''n''+1, to get the volume under the ''n''-simplex as a function of its vertex distance ''x'' from the origin, differentiating with respect to ''x'', at $x=1/\sqrt$  (where the ''n''-simplex side length is 1), and normalizing by the length $dx/\sqrt$ of the increment, $\left(dx/\left(n+1\right),\ldots, dx/\left(n+1\right)\right)$, along the normal vector.

## Dihedral angles of the regular n-simplex

Any two (''n'' − 1)-dimensional faces of a regular ''n''-dimensional simplex are themselves regular (''n'' − 1)-dimensional simplices, and they have the same dihedral angle of cos−1(1/''n''). This can be seen by noting that the center of the standard simplex is $\left(\frac, \dots, \frac\right)$, and the centers of its faces are coordinate permutations of $\left(0, \frac, \dots, \frac\right)$. Then, by symmetry, the vector pointing from $\left(\frac, \dots, \frac\right)$ to $\left(0, \frac, \dots, \frac\right)$ is perpendicular to the faces. So the vectors normal to the faces are permutations of $\left(-n, 1, \dots, 1\right)$, from which the dihedral angles are calculated.

## Simplices with an "orthogonal corner"

An "orthogonal corner" means here that there is a vertex at which all adjacent edges are pairwise orthogonal. It immediately follows that all adjacent Face (geometry), faces are pairwise orthogonal. Such simplices are generalizations of right triangles and for them there exists an ''n''-dimensional version of the Pythagorean theorem: The sum of the squared (''n'' − 1)-dimensional volumes of the facets adjacent to the orthogonal corner equals the squared (''n'' − 1)-dimensional volume of the facet opposite of the orthogonal corner. :$\sum_^n , A_k, ^2 = , A_0, ^2$ where $A_1 \ldots A_n$ are facets being pairwise orthogonal to each other but not orthogonal to $A_0$, which is the facet opposite the orthogonal corner. For a 2-simplex the theorem is the Pythagorean theorem for triangles with a right angle and for a 3-simplex it is de Gua's theorem for a tetrahedron with an orthogonal corner.

## Relation to the (''n'' + 1)-hypercube

The Hasse diagram of the face lattice of an ''n''-simplex is isomorphic to the graph of the (''n'' + 1)-
hypercube In geometry Geometry (from the grc, γεωμετρία; ''wikt:γῆ, geo-'' "earth", ''wikt:μέτρον, -metron'' "measurement") is, with arithmetic, one of the oldest branches of mathematics. It is concerned with properties of space th ...

's edges, with the hypercube's vertices mapping to each of the ''n''-simplex's elements, including the entire simplex and the null polytope as the extreme points of the lattice (mapped to two opposite vertices on the hypercube). This fact may be used to efficiently enumerate the simplex's face lattice, since more general face lattice enumeration algorithms are more computationally expensive. The ''n''-simplex is also the vertex figure of the (''n'' + 1)-hypercube. It is also the Facet (geometry), facet of the (''n'' + 1)-orthoplex.

## Topology

Topology, Topologically, an ''n''-simplex is topologically equivalent, equivalent to an ball (mathematics), ''n''-ball. Every ''n''-simplex is an ''n''-dimensional manifold with corners.

## Probability

In probability theory, the points of the standard ''n''-simplex in (''n'' + 1)-space form the space of possible probability distributions on a finite set consisting of ''n'' + 1 possible outcomes. The correspondence is as follows: For each distribution described as an ordered (''n'' + 1)-tuple of probabilities whose sum is (necessarily) 1, we associate the point of the simplex whose barycentric coordinates are precisely those probabilities. That is, the ''k''th vertex of the simplex is assigned to have the ''k''th probability of the (''n'' + 1)-tuple as its barycentric coefficient. This correspondence is an affine homeomorphism.

## Compounds

Since all simplices are self-dual, they can form a series of compounds; * Two triangles form a hexagram . * Two tetrahedra form a compound of two tetrahedra or stellated octahedron, stella octangula. * Two 5-cells form a compound of two 5-cells in four dimensions.

# Algebraic topology

In
algebraic topology 250px, A torus, one of the most frequently studied objects in algebraic topology Algebraic topology is a branch of mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathemat ...
, simplices are used as building blocks to construct an interesting class of topological spaces called
simplicial complex In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). It ...
es. These spaces are built from simplices glued together in a combinatorics, combinatorial fashion. Simplicial complexes are used to define a certain kind of homology (mathematics), homology called
simplicial homology In geometry, a simplex (plural: simplexes or simplices) is a generalization of the notion of a triangle or tetrahedron to arbitrary dimensions. The simplex is so-named because it represents the simplest possible polytope in any given space. For e ...
. A finite set of ''k''-simplexes embedded in an open subset of R''n'' is called an affine ''k''-chain. The simplexes in a chain need not be unique; they may occur with Multiplicity (mathematics), multiplicity. Rather than using standard set notation to denote an affine chain, it is instead the standard practice to use plus signs to separate each member in the set. If some of the simplexes have the opposite orientability, orientation, these are prefixed by a minus sign. If some of the simplexes occur in the set more than once, these are prefixed with an integer count. Thus, an affine chain takes the symbolic form of a sum with integer coefficients. Note that each facet of an ''n''-simplex is an affine (''n'' − 1)-simplex, and thus the boundary (topology), boundary of an ''n''-simplex is an affine (''n'' − 1)-chain. Thus, if we denote one positively oriented affine simplex as :$\sigma=\left[v_0,v_1,v_2,\ldots,v_n\right]$ with the $v_j$ denoting the vertices, then the boundary $\partial\sigma$ of ''σ'' is the chain :$\partial\sigma = \sum_^n \left(-1\right)^j \left[v_0,\ldots,v_,v_,\ldots,v_n\right].$ It follows from this expression, and the linearity of the boundary operator, that the boundary of the boundary of a simplex is zero: :$\partial^2\sigma = \partial \left\left( \sum_^n \left(-1\right)^j \left[v_0,\ldots,v_,v_,\ldots,v_n\right] \right\right) = 0.$ Likewise, the boundary of the boundary of a chain is zero: $\partial ^2 \rho =0$. More generally, a simplex (and a chain) can be embedded into a manifold by means of smooth, differentiable map $f\colon\R^n \to M$. In this case, both the summation convention for denoting the set, and the boundary operation commute with the embedding. That is, :$f \left\left(\sum\nolimits_i a_i \sigma_i \right\right) = \sum\nolimits_i a_i f\left(\sigma_i\right)$ where the $a_i$ are the integers denoting orientation and multiplicity. For the boundary operator $\partial$, one has: :$\partial f\left(\rho\right) = f \left(\partial \rho\right)$ where ρ is a chain. The boundary operation commutes with the mapping because, in the end, the chain is defined as a set and little more, and the set operation always commutes with the function (mathematics), map operation (by definition of a map). A continuous function (topology), continuous map $f: \sigma \to X$ to a topological space ''X'' is frequently referred to as a singular ''n''-simplex. (A map is generally called "singular" if it fails to have some desirable property such as continuity and, in this case, the term is meant to reflect to the fact that the continuous map need not be an embedding.)

# Algebraic geometry

Since classical algebraic geometry allows to talk about polynomial equations, but not inequalities, the ''algebraic standard n-simplex'' is commonly defined as the subset of affine (''n'' + 1)-dimensional space, where all coordinates sum up to 1 (thus leaving out the inequality part). The algebraic description of this set is :$\Delta^n := \left\,$ which equals the Scheme (mathematics), scheme-theoretic description $\Delta_n\left(R\right) = \operatorname\left(R\left[\Delta^n\right]\right)$ with :$R\left[\Delta^n\right] := R\left[x_1,\ldots,x_\right]\left/\left\left(1-\sum x_i \right\right)\right.$ the ring of regular functions on the algebraic ''n''-simplex (for any ring (mathematics), ring $R$). By using the same definitions as for the classical ''n''-simplex, the ''n''-simplices for different dimensions ''n'' assemble into one simplicial object, while the rings $R\left[\Delta^n\right]$ assemble into one cosimplicial object $R\left[\Delta^\bullet\right]$ (in the category (mathematics), category of schemes resp. rings, since the face and degeneracy maps are all polynomial). The algebraic ''n''-simplices are used in higher K-theory and in the definition of higher Chow groups.

# Applications

*In statistics, simplices are sample spaces of compositional data and are also used in plotting quantities that sum to 1, such as proportions of subpopulations, as in a ternary plot. *In applied statistics#industrial, industrial statistics, simplices arise in problem formulation and in algorithmic solution. In the design of bread, the producer must combine yeast, flour, water, sugar, etc. In such mixtures, only the relative proportions of ingredients matters: For an optimal bread mixture, if the flour is doubled then the yeast should be doubled. Such mixture problem are often formulated with normalized constraints, so that the nonnegative components sum to one, in which case the feasible region forms a simplex. The quality of the bread mixtures can be estimated using response surface methodology, and then a local maximum can be computed using a nonlinear programming method, such as sequential quadratic programming. *In operations research, linear programming problems can be solved by the simplex algorithm of George Dantzig. *In geometric design and computer graphics, many methods first perform simplicial triangulation (topology), triangulations of the domain and then interpolation, fit interpolating polynomial and rational function modeling, polynomials to each simplex. *In chemistry, the hydrides of most elements in the p-block can resemble a simplex if one is to connect each atom. Neon does not react with hydrogen and as such is Monatomic gas, a point, fluorine bonds with one hydrogen atom and forms a line segment, oxygen bonds with two hydrogen atoms in a Bent molecular geometry, bent fashion resembling a triangle, nitrogen reacts to form a Trigonal pyramidal molecular geometry, tetrahedron, and carbon forms Tetrahedral molecular geometry, a structure resembling a Schlegel diagram of the 5-cell. This trend continues for the heavier analogues of each element, as well as if the hydrogen atom is replaced by a halogen atom. *In some approaches to quantum gravity, such as Regge calculus and causal dynamical triangulations, simplices are used as building blocks of discretizations of spacetime; that is, to build simplicial manifolds.

* 3-sphere * Aitchison geometry * Causal dynamical triangulation * Complete graph * Delaunay triangulation * Distance geometry * Hill tetrahedron * Hypersimplex * List of regular polytopes * Metcalfe's law * Other regular ''n''-
polytope In elementary geometry Geometry (from the grc, γεωμετρία; ''wikt:γῆ, geo-'' "earth", ''wikt:μέτρον, -metron'' "measurement") is, with arithmetic, one of the oldest branches of mathematics. It is concerned with propertie ...
s ** Cross-polytope ** Hypercube ** Tesseract * Polytope * Schläfli orthoscheme * Simplex algorithm—a method for solving optimization problems with inequalities. * Simplicial complex * Simplicial homology * Simplicial set * Spectrahedron * Ternary plot

# References

* ''(See chapter 10 for a simple review of topological properties.)'' * * * ** pp. 120–121, §7.2. see illustration 7-2A ** p. 296, Table I (iii): Regular Polytopes, three regular polytopes in ''n'' dimensions (''n'' ≥ 5) * * A
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