Free group
In mathematics, the free group FS over a given set S consists of all expressions that can be built from members of S, considering two expressions different unless their equality follows from the group axioms. The members of S are called generators of FS. An arbitrary group G is called free if it is isomorphic to FS for some subset S of G, that is, if there is a subset S of G such that every element of G can be written in one and only one way as a product of finitely many elements of S and their inverses. A related but different notion is a free abelian group, both notions are particular instances of a free object from universal algebra. Free groups first arose as examples of Fuchsian groups. In an 1882 paper, Walther von Dyck pointed out that these groups have the simplest possible presentations; the algebraic study of free groups was initiated by Jakob Nielsen in 1924, who gave them their name and established many of their basic properties. Max Dehn realized the connection with topology, obtained the first proof of the full Nielsen–Schreier theorem.
Otto Schreier published an algebraic proof of this result in 1927, Kurt Reidemeister included a comprehensive treatment of free groups in his 1932 book on combinatorial topology. On in the 1930s, Wilhelm Magnus discovered the connection between the lower central series of free groups and free Lie algebras; the group of integers is free. A free group on a two-element set S occurs in the proof of the Banach–Tarski paradox and is described there. On the other hand, any nontrivial finite group cannot be free, since the elements of a free generating set of a free group have infinite order. In algebraic topology, the fundamental group of a bouquet of k circles is the free group on a set of k elements; the free group FS with free generating set. S is a set of symbols, we suppose for every s in S there is a corresponding "inverse" symbol, s−1, in a set S−1. Let T = S ∪ S−1, define a word in S to be any written product of elements of T; that is, a word in S is an element of the monoid generated by T. The empty word is the word with no symbols at all.
For example, if S = T =, a b 3 c − 1 c a − 1 c is a word in S. If an element of S lies next to its inverse, the word may be simplified by omitting the c, c−1 pair: a b 3 c − 1 c a − 1 c ⟶ a b 3 a − 1 c. A word that cannot be simplified further is called reduced; the free group FS is defined to be the group of all reduced words in S, with concatenation of words as group operation. The identity is the empty word. A word is called cyclically reduced, if last letter are not inverse to each other; every word is conjugate to a cyclically reduced word, a cyclically reduced conjugate of a cyclically reduced word is a cyclic permutation of the letters in the word. For instance b−1abcb is not cyclically reduced, but is conjugate to abc, cyclically reduced; the only cyclically reduced conjugates of abc are abc and cab. The free group FS is the universal group generated by the set S; this can be formalized by the following universal property: given any function ƒ from S to a group G, there exists a unique homomorphism φ: FS → G making the following diagram commute: That is, homomorphisms FS → G are in one-to-one correspondence with functions S → G.
For a non-free group, the presence of relations would restrict the possible images of the generators under a homomorphism. To see how this relates to the constructive definition, think of the mapping from S to FS as sending each symbol to a word consisting of that symbol. To construct φ for given ƒ, first note that φ sends the empty word to the identity of G and it has to agree with ƒ on the elements of S. For the remaining words φ can be uniquely extended since it is a homomorphism, i.e. φ = φ φ. The above property characterizes free groups up to isomorphism, is sometimes used as an alternative definition, it is known as the universal property of free groups, the generating set S is called a basis for FS. The basis for a free group is not uniquely determined. Being characterized by a universal property is the standard feature of free objects in universal algebra. In the language of category theory, the construction of the free group is a functor from the category of sets to the category of groups.
This functor is left adjoint to the forgetful functor from groups to sets. Some properties of free groups follow from the definition: Any group G is the homomorphic image of some free group F. Let S be a set of generators of G; the natural map f: F → G is an epimorphism, which proves the claim. Equivalently, G is isomorphic to a quotient group of some free group F; the kernel of φ is a set of relations in the presentation of G. If S can be chosen to be finite here G is called finitely generated. If S has more than one element F is not abelian, in fact the
Lorentz group
In physics and mathematics, the Lorentz group is the group of all Lorentz transformations of Minkowski spacetime, the classical and quantum setting for all physical phenomena. The Lorentz group is named for the Dutch physicist Hendrik Lorentz. For example, the following laws and theories respect Lorentz symmetry: The kinematical laws of special relativity Maxwell's field equations in the theory of electromagnetism The Dirac equation in the theory of the electron The Standard model of particle physicsThe Lorentz group expresses the fundamental symmetry of space and time of all known fundamental laws of nature. In general relativity physics, in cases involving small enough regions of spacetime where gravitational variances are negligible, physical laws are Lorentz invariant in the same manner as that of special relativity physics; the Lorentz group is a subgroup of the Poincaré group—the group of all isometries of Minkowski spacetime. Lorentz transformations are isometries that leave the origin fixed.
Thus, the Lorentz group is an isotropy subgroup of the isometry group of Minkowski spacetime. For this reason, the Lorentz group is sometimes called the homogeneous Lorentz group while the Poincaré group is sometimes called the inhomogeneous Lorentz group. Lorentz transformations are examples of linear transformations. Mathematically, the Lorentz group may be described as the generalized orthogonal group O, the matrix Lie group that preserves the quadratic form ↦ t 2 − x 2 − y 2 − z 2 on R4; this quadratic form is, when put on matrix form, interpreted in physics as the metric tensor of Minkowski spacetime. The Lorentz group is a six-dimensional noncompact non-abelian real Lie group, not connected; the four connected components are not connected, but rather doubly connected. The identity component of the Lorentz group is itself a group, is called the restricted Lorentz group, is denoted SO+; the restricted Lorentz group consists of those Lorentz transformations that preserve the orientation of space and direction of time.
The restricted Lorentz group has been presented through a facility of biquaternion algebra. The restricted Lorentz group arises in other ways in pure mathematics. For example, it arises as the point symmetry group of a certain ordinary differential equation; this fact has physical significance. Because it is a Lie group, the Lorentz group O is both a group and admits a topological description as a smooth manifold; as a manifold, it has four connected components. Intuitively, this means; the four connected components can be categorized by two transformation properties its elements have: some elements are reversed under time-inverting Lorentz transformations, for example, a future-pointing timelike vector would be inverted to a past-pointing vector some elements have orientation reversed by improper Lorentz transformations, for example, certain vierbein Lorentz transformations that preserve the direction of time are called orthochronous. The subgroup of orthochronous transformations is denoted O+.
Those that preserve orientation are called proper, as linear transformations they have determinant +1. The subgroup of proper Lorentz transformations is denoted SO; the subgroup of all Lorentz transformations preserving both orientation and direction of time is called the proper, orthochronous Lorentz group or restricted Lorentz group, is denoted by SO+. The set of the four connected components can be given a group structure as the quotient group O/SO+, isomorphic to the Klein four-group; every element in O can be written as the semidirect product of a proper, orthochronous transformation and an element of the discrete group where P and T are the space inversion and time reversal operators: P = diag T = diag. Thus an arbitrary Lorentz transformation can be specified as a proper, orthochronous Lorentz transformation along with a further two bits of information, which pick out one of the four connected components; this pattern is typical of finite-dimensional Lie groups. The restricted Lorentz group is the identity component of the Lorentz group, which means that it consists of all Lorentz transformations that can be connected to the identity by a continuous curve lying in the group.
The restricted Lorentz group is a connected normal subgroup of the full Lorentz group with the same dimension, in this case with dimension six. The restricted Lorentz group is generated by ordinary spatial rotations and Lorentz boosts. Since every proper, orthochronous Lorentz transformation can be written as a product of a rotation and a boost, it takes 6 real parameters to specify an arbitrary proper orthochronous Lorentz transformation; this is one way. The set of all rotations forms a Lie subgroup isomorphic to the ordinary rotation group SO; the set of all boosts, does not form a sub
Cyclic group
In group theory, a branch of abstract algebra, a cyclic group or monogenous group is a group, generated by a single element. That is, it is a set of invertible elements with a single associative binary operation, it contains an element g such that every other element of the group may be obtained by applying the group operation to g or its inverse; each element can be written as a power of g in multiplicative notation, or as a multiple of g in additive notation. This element g is called a generator of the group; every infinite cyclic group is isomorphic to the additive group of the integers. Every finite cyclic group of order n is isomorphic to the additive group of Z/nZ, the integers modulo n; every cyclic group is an abelian group, every finitely generated abelian group is a direct product of cyclic groups. Every cyclic group of prime order is a simple group. In the classification of finite simple groups, one of the three infinite classes consists of the cyclic groups of prime order; the cyclic groups of prime order are thus among the building blocks from which all groups can be built.
For any element g in any group G, one can form the subgroup of all integer powers ⟨g⟩ =, called the cyclic subgroup of g. The order of g is the number of elements in ⟨g⟩. A cyclic group is a group, equal to one of its cyclic subgroups: G = ⟨g⟩ for some element g, called a generator. For a finite cyclic group with order |G| = n, this means G =, where e is the identity element and gj = gk whenever j ≡ k modulo n. An abstract group defined by this multiplication is denoted Cn, we say that G is isomorphic to the standard cyclic group Cn; such a group is isomorphic to Z/nZ, the group of integers modulo n with the addition operation, the standard cyclic group in additive notation. Under the isomorphism χ defined by χ = i the identity element e corresponds to 0, products correspond to sums, powers correspond to multiples. For example, the set of complex 6th roots of unity G = forms a group under multiplication, it is cyclic, since it is generated by the primitive root z = 1 2 + 3 2 i = e 2 π i / 6: that is, G = ⟨z⟩ = with z6 = 1.
Under a change of letters, this is isomorphic to the standard cyclic group of order 6, defined as C6 = ⟨g⟩ = with multiplication gj · gk = gj+k, so that g6 = g0 = e. These groups are isomorphic to Z/6Z = with the operation of addition modulo 6, with zk and gk corresponding to k. For example, 1 + 2 ≡ 3 corresponds to z1 · z2 = z3, 2 + 5 ≡ 1 corresponds to z2 · z5 = z7 = z1, so on. Any element generates its own cyclic subgroup, such as ⟨z2⟩ = of order 3, isomorphic to C3 and Z/3Z. Instead of the quotient notations Z/nZ, Z/, or Z/n, some authors denote a finite cyclic group as Zn, but this conflicts with the notation of number theory, where Zp denotes a p-adic number ring, or localization at a prime ideal. On the other hand, in an infinite cyclic group G = ⟨g⟩, the powers gk give distinct elements for all integers k, so that G =, G is isomorphic to the standard group C = C∞ and to Z, the additive group of the integers. An example is the first frieze group. Here there are no finite cycles, the name "cyclic" may be misleading.
To avoid this confusion, Bourbaki introduced the term monogenous group for a group with a single generator and restricted "cyclic group" to mean a finite monogenous group, avoiding the term "infinite cyclic group". The set of integers Z,with the operation of addition, forms a group, it is an infinite cyclic group, because all integers can be written by adding or subtracting the single number 1. In this group, 1 and −1 are the only generators; every infinite cyclic group is isomorphic to Z. For every positive integer n, the set of integers modulo n, again with the operation of addition, forms a finite cyclic group, denoted Z/nZ. A modular integer i is a generator of this group if i is prime to n, because these elements can generate all other elements of the group through integer addition; every finite cyclic group G is isomorphic to Z/nZ. The addition operations on integers and modular integers, used to define the cyclic groups, are the addition operations of commutative rings denoted Z and Z/nZ or Z/.
If p is a prime Z/pZ is a finite field, is denoted Fp or GF. For every positive integer n, the set of the integers modulo n that are prime to n is written as ×; this group is not always cyclic, bu
Modular group
In mathematics, the modular group is the projective special linear group PSL of 2 x 2 matrices with integer coefficients and unit determinant. The matrices A and -A are identified; the modular group acts on the upper-half of the complex plane by fractional linear transformations, the name "modular group" comes from the relation to moduli spaces and not from modular arithmetic. The modular group Γ is the group of linear fractional transformations of the upper half of the complex plane which have the form z ↦ a z + b c z + d where a, b, c, d are integers, ad − bc = 1; the group operation is function composition. This group of transformations is isomorphic to the projective special linear group PSL, the quotient of the 2-dimensional special linear group SL over the integers by its center. In other words, PSL consists of all matrices where a, b, c, d are integers, ad − bc = 1, pairs of matrices A and −A are considered to be identical; the group operation is the usual multiplication of matrices. Some authors define the modular group to be PSL, still others define the modular group to be the larger group SL.
Some mathematical relations require the consideration of the group GL of matrices with determinant plus or minus one. PGL is the quotient group GL/. A 2 × 2 matrix with unit determinant is a symplectic matrix, thus SL = Sp, the symplectic group of 2x2 matrices; the unit determinant of implies that the fractions a/b, a/c, c/d and b/d are all irreducible, having no common factors. More if p/q is an irreducible fraction a p + b q c p + d q is irreducible. Any pair of irreducible fractions can be connected in this way, i.e.: for any pair p/q and r/s of irreducible fractions, there exist elements ∈ SL such that r = a p + b q and s = c p + d q. Elements of the modular group provide a symmetry on the two-dimensional lattice. Let ω 1 and ω 2 be two complex numbers whose ratio is not real; the set of points Λ = is a lattice of parallelograms on the plane. A different pair of vectors α 1 and α 2 will generate the same lattice if and only if = for some matrix in GL, it is for this reason that doubly periodic functions, such as elliptic functions, possess a modular group symmetry.
The action of the modular group on the rational numbers can most be understood by envisioning a square grid, with grid point corresponding to the fraction p/q. An irreducible fraction is one, visible from the origin. Note that any member of the modular group maps the projectively extended real line one-to-one to itself, furthermore bijectively maps the projectively extended rational line to itself, the irrationals to the irrationals, the transcendental numbers to the transcendental numbers, the non-real numbers to the non-real numbers, the upper half-plane to the upper half-plane, et cetera. If p n − 1 / q n − 1 and p n / q n are two successive convergents of a continued fraction the matrix (
Solenoid (mathematics)
This page discusses a class of topological groups. For the wrapped loop of wire, see Solenoid. In mathematics, a solenoid is a compact connected topological space that may be obtained as the inverse limit of an inverse system of topological groups and continuous homomorphisms, fi: Si+1 → Si, i ≥ 0,where each Si is a circle and fi is the map that uniformly wraps the circle Si+1 ni times around the circle Si; this construction can be carried out geometrically in the three-dimensional Euclidean space R3. A solenoid is a one-dimensional homogeneous indecomposable continuum that has the structure of a compact topological group. In the special case where all ni have the same value n, so that the inverse system is determined by the multiplication by n self map of the circle, solenoids were first introduced by Vietoris for n = 2 and by van Dantzig for an arbitrary n; such a solenoid arises as a one-dimensional expanding attractor, or Smale–Williams attractor, forms an important example in the theory of hyperbolic dynamical systems.
Each solenoid may be constructed as the intersection of a nested system of embedded solid tori in R3. Fix a sequence of natural numbers, ni ≥ 2. Let T0 = S1 × D be a solid torus. For each i ≥ 0, choose a solid torus Ti+1, wrapped longitudinally ni times inside the solid torus Ti, their intersection Λ = ⋂ i ≥ 0 T i is homeomorphic to the solenoid constructed as the inverse limit of the system of circles with the maps determined by the sequence. Here is a variant of this construction isolated by Stephen Smale as an example of an expanding attractor in the theory of smooth dynamical systems. Denote the angular coordinate on the circle S1 by t and consider the complex coordinate z on the two-dimensional unit disk D. Let f be the map of the solid torus T = S1 × D into itself given by the explicit formula f =; this map is a smooth embedding of T into itself. If T is imagined as a rubber tube, the map f stretches it in the longitudinal direction, contracts each meridional disk, wraps the deformed tube twice inside T with twisting, but without self-intersections.
The hyperbolic set Λ of the discrete dynamical system is the intersection of the sequence of nested solid tori described above, where Ti is the image of T under the ith iteration of the map f. This set is a one-dimensional attractor, the dynamics of f on Λ has the following interesting properties: meridional disks are the stable manifolds, each of which intersects Λ over a Cantor set periodic points of f are dense in Λ the map f is topologically transitive on ΛGeneral theory of solenoids and expanding attractors, not one-dimensional, was developed by R. F. Williams and involves a projective system of infinitely many copies of a compact branched manifold in place of the circle, together with an expanding self-immersion. Solenoids are compact metrizable spaces that are connected, but not locally connected or path connected; this is reflected in their pathological behavior with respect to various homology theories, in contrast with the standard properties of homology for simplicial complexes.
In Čech homology, one can construct a non-exact long homology sequence using a solenoid. In Steenrod-style homology theories, the 0th homology group of a solenoid may have a complicated structure though a solenoid is a connected space. Protorus, a class of topological groups that includes the solenoids Pontryagin duality D. van Dantzig, Ueber topologisch homogene Kontinua, Fund. Math. 15, pp. 102–125 Hazewinkel, Michiel, ed. "Solenoid", Encyclopedia of Mathematics, Springer Science+Business Media B. V. / Kluwer Academic Publishers, ISBN 978-1-55608-010-4 Clark Robinson, Dynamical systems: Stability, Symbolic Dynamics and Chaos, 2nd edition, CRC Press, 1998 ISBN 978-0-8493-8495-0 S. Smale, Differentiable dynamical systems, Bull. of the AMS, 73, 747 – 817. L. Vietoris, Über den höheren Zusammenhang kompakter Räume und eine Klasse von zusammenhangstreuen Abbildungen, Math. Ann. 97, pp. 454–472 Robert F. Williams, Expanding attractors, Publ. Math. IHES, t. 43, p. 169–203 Semmes, Some remarks about solenoids, arXiv:1201.2647, Bibcode:2012arXiv1201.2647S
Poincaré group
The Poincaré group, named after Henri Poincaré, was first defined by Minkowski as the group of Minkowski spacetime isometries. It is a ten-dimensional non-abelian Lie group of fundamental importance in physics. A Minkowski spacetime isometry has the property. For example, if everything were postponed by two hours, including the two events and the path you took to go from one to the other the time interval between the events recorded by a stop-watch you carried with you would be the same. Or if everything were shifted five kilometres to the west, or turned 60 degrees to the right, you would see no change in the interval, it turns out that the proper length of an object is unaffected by such a shift. A time or space reversal is an isometry of this group. In Minkowski space, there are ten degrees of freedom of the isometries, which may be thought of as translation through time or space. Composition of transformations is the operator of the Poincaré group, with proper rotations being produced as the composition of an number of reflections.
In classical physics, the Galilean group is a comparable ten-parameter group that acts on absolute time and space. Instead of boosts, it features shear mappings to relate co-moving frames of reference. Poincaré symmetry is the full symmetry of special relativity, it includes: translations in time and space, forming the abelian Lie group of translations on space-time. The last two symmetries, J and K, together make the Lorentz group. Objects which are invariant under this group are said to possess Poincaré invariance or relativistic invariance; the Poincaré group is the group of Minkowski spacetime isometries. It is a ten-dimensional noncompact Lie group; the abelian group of translations is a normal subgroup, while the Lorentz group is a subgroup, the stabilizer of the origin. The Poincaré group itself is the minimal subgroup of the affine group which includes all translations and Lorentz transformations. More it is a semidirect product of the translations and the Lorentz group, R 1, 3 ⋊ O, with group multiplication ⋅ =.
Another way of putting this is that the Poincaré group is a group extension of the Lorentz group by a vector representation of it. In turn, it can be obtained as a group contraction of the de Sitter group SO ~ Sp, as the de Sitter radius goes to infinity, its positive energy unitary irreducible representations are indexed by mass and spin and are associated with particles in quantum mechanics. In accordance with the Erlangen program, the geometry of Minkowski space is defined by the Poincaré group: Minkowski space is considered as a homogeneous space for the group. In quantum field theory, the universal cover of the Poincaré group R 1, 3 ⋊ S L and the double cover R 1, 3 ⋊ S p i n are more important, because representations of S O are not able to describe fields with spin 1/2, i.e. fermions. Here S L is the group of complex matrices with unit determinant; the Poincaré algebra is the Lie algebra of the Poincaré group. It is a Lie algebra extension of the Lie algebra of the Lorentz group. More the proper, orthochronous part of the Lorentz subgroup, SO+, is connected to the identity and is thus provided by the exponentiation exp exp of this Lie algebra.
In component form, the Poincaré algebra is given by the commutation relations: where P is the generator of translations, M is the generator of Lorentz transformations, η is the Minkowski metric. The bottom commutation relation is the Lorentz group, consisting of rotations, Ji = ϵimnMmn/2, boosts, Ki = Mi0. In this notation, the entire Poincaré algebra is expressible in noncovariant language as = i ϵ
E8 (mathematics)
In mathematics, E8 is any of several related exceptional simple Lie groups, linear algebraic groups or Lie algebras of dimension 248. The designation E8 comes from the Cartan–Killing classification of the complex simple Lie algebras, which fall into four infinite series labeled An, Bn, Cn, Dn, five exceptional cases labeled E6, E7, E8, F4, G2; the E8 algebra is the most complicated of these exceptional cases. The Lie group E8 has dimension 248, its rank, the dimension of its maximal torus, is eight. Therefore, the vectors of the root system are in eight-dimensional Euclidean space: they are described explicitly in this article; the Weyl group of E8, the group of symmetries of the maximal torus which are induced by conjugations in the whole group, has order 214 35 52 7 = 696729600. The compact group E8 is unique among simple compact Lie groups in that its non-trivial representation of smallest dimension is the adjoint representation acting on the Lie algebra E8 itself. There is a Lie algebra Ek for every integer k ≥ 3, infinite dimensional if k is greater than 8.
There is a unique complex Lie algebra of type E8, corresponding to a complex group of complex dimension 248. The complex Lie group E8 of complex dimension 248 can be considered as a simple real Lie group of real dimension 496; this is connected, has maximal compact subgroup the compact form of E8, has an outer automorphism group of order 2 generated by complex conjugation. As well as the complex Lie group of type E8, there are three real forms of the Lie algebra, three real forms of the group with trivial center, all of real dimension 248, as follows: The compact form, connected and has trivial outer automorphism group; the split form, EVIII, which has maximal compact subgroup Spin/, fundamental group of order 2 and has trivial outer automorphism group. EIX, which has maximal compact subgroup E7×SU/, fundamental group of order 2 and has trivial outer automorphism group. For a complete list of real forms of simple Lie algebras, see the list of simple Lie groups. By means of a Chevalley basis for the Lie algebra, one can define E8 as a linear algebraic group over the integers and over any commutative ring and in particular over any field: this defines the so-called split form of E8.
Over an algebraically closed field, this is the only form. Over R, the real connected component of the identity of these algebraically twisted forms of E8 coincide with the three real Lie groups mentioned above, but with a subtlety concerning the fundamental group: all forms of E8 are connected in the sense of algebraic geometry, meaning that they admit no non-trivial algebraic coverings. Over finite fields, the Lang–Steinberg theorem implies that H1=0, meaning that E8 has no twisted forms: see below; the characters of finite dimensional representations of the real and complex Lie algebras and Lie groups are all given by the Weyl character formula. The dimensions of the smallest irreducible representations are: 1, 248, 3875, 27000, 30380, 147250, 779247, 1763125, 2450240, 4096000, 4881384, 6696000, 26411008, 70680000, 76271625, 79143000, 146325270, 203205000, 281545875, 301694976, 344452500, 820260000, 1094951000, 2172667860, 2275896000, 2642777280, 2903770000, 3929713760, 4076399250, 4825673125, 6899079264, 8634368000, 12692520960…The 248-dimensional representation is the adjoint representation.
There are two non-isomorphic irreducible representations of dimension 8634368000. The fundamental representations are those with dimensions 3875, 6696000, 6899079264, 146325270, 2450240, 30380, 248 and 147250; the coefficients of the character formulas for infinite dimensional irreducible representations of E8 depend on some large square matrices consisting of polynomials, the Lusztig–Vogan polynomials, an analogue of Kazhdan–Lusztig polynomials introduced for reductive groups in general by George Lusztig and David Kazhdan. The values at 1 of the Lusztig–Vogan polynomials give the coefficients of the matrices relating the standard representations with the irreducible representations; these matrices were computed after four years of collaboration by a group of 18 mathematicians and computer scientists, led by Jeffrey Adams, with much of the programming done by Fokko du Cloux. The most difficult case is