# Note that a unit is unique: 1 = 11 = 1. Examples: Nonnegative integers under addition; all integers under multiplication.

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1 Algebra fact sheet An algebraic structure (such as group, ring, field, etc.) is a set with some operations and distinguished elements (such as 0, 1) satisfying some axioms. This is a fact sheet with definitions and properties of some of the most important algebraic structures. A substructure of a structure A (i.e., a subgroup, subring, subfield etc.) is a subset of A that is closed under all operations and contains all distinguished elements. Algebraic structures of the same type (e.g., groups) can be related to each other by homomorphisms. A homomorphism f : A B is a map that preserves all operations and distinguished elements (e.g. f(ab) = f(a)f(b)). An isomorphism is a homomorphism which is a one-to-one correspondence (bijection); then the inverse f 1 is also an isomorphism. Isomorphic algebraic structures are regarded as the same, and algebraic structures of each type are classified up to an isomorphism. Semigroup: A set G with an operation G G G, (a, b) ab, called multiplication, which is associative: (ab)c = a(bc). Examples: Positive integers with operation of addition. Monoid: A semigroup G with unit 1 G, such that 1g = g1 = g for all g G. Note that a unit is unique: 1 = 11 = 1. Examples: Nonnegative integers under addition; all integers under multiplication. Group: A monoid G with an inversion operation G G, g g 1, such that gg 1 = g 1 g = 1. Note that inverse is unique: g1 1 = g1 1 gg2 1 = g2 1. So for a semigroup, being a monoid or a group is a property, not an additional structure. Examples: (1) All integers under addition, Z. Integers modulo n under addition, Z n. (These two are called cyclic groups). The group Z N (Ndimensional vectors of integers). Rational numbers Q, real numbers R, or complex numbers C under addition. Nonzero rational, real, or complex numbers under multiplication. (2) Permutation (or symmetric) group S n on n items. The group GL n of invertible matrices with integer, rational, real, or complex entries, or with integer entries modulo n (e.g. GL n (Q)). The group of symmetries of a polytope (e.g., regular icosahedron). Abelian (commutative) group: A group G where ab = ba (commuta- 1

2 tivity). Examples: The examples from list (1) above. If A is an abelian group, one often denotes the operation by + and 1 by 0. Action of a monoid or a group on a set: A left action of a monoid (in particular, a group) G on a set X is a multiplication map G X X, (g, x) gx such that (gh)x = g(hx) and 1x = x. Similarly one defines a right action, (x, g) xg. Examples. Any monoid (in particular, group) acts on itself by left and right multiplication. The symmetric group S n acts on {1,..., n}. Matrices act on vectors. The group of symmetries of a regular icosahedron acts on the sets of its points, vertices, edges, faces and on the ambient space. Normal subgroup: A subgroup H G such that gh = Hg for all g G. Quotient group: If A is an abelian group and B a subgroup in A, then A/B is the set of subsets ab in A (where a A) with operation a 1 Ba 2 B = a 1 a 2 B; this defines a group structure on A/B. If A is not abelian, then in general A/B is just a set with a left action of A. For it to be a group (i.e., for the formula a 1 Ba 2 B = a 1 a 2 B to make sense), B needs to be a normal subgroup. This is automatic for abelian groups A. Examples. Z/nZ = Z n. S 3 /Z 3 = Z 2. Lagrange s theorem: The order (i.e., number of elements) of a subgroup H of a finite group G divides the order of G (the quotient G / H is G/H ). The order of g G is the smallest positive integer n such that g n = 1 ( if there is none). Equivalently, the order of g is the order of the subgroup generated by g. Thus by Lagrange s theorem, the order of g divides the order of G. This implies that any group of order p (a prime) is Z p. Direct (or Cartesian) product (of semigroups, monoids, groups): G H is the set of pairs (g, h), g G, h H, with componentwise operation. One can also define a direct product of more than two factors. For abelian groups, the direct product is also called the direct sum and denoted by. Generators: A group G is generated by a subset S G if any element of G is a product of elements of S and their inverses. A group is finitely generated if it is generated by a finite subset. Classification theorem of finitely generated abelian groups. Any finitely generated abelian group is a direct sum of infinite cyclic groups (Z) 2

3 and cyclic groups of prime power order. Moreover, this decomposition is unique up to order of factors (and up to isomorphism). (Unital) ring: An abelian group A with operation + which also has another operation of multiplication, (a, b) ab, under which A is a monoid, and which is distributive: a(b + c) = ab + ac, (b + c)a = ba + ca. Examples: (1) The integers Z. Rational, real, or complex numbers. Integers modulo n (Z n ). Polynomials Q[x], Q[x, y]. (2) Matrices n by n with rational, real, or complex entries, e.g. Mat n (Q). Commutative ring: A ring in which ab = ba. Examples: List (1) of examples of rings. Division ring: A ring in which all nonzero elements are invertible (i.e., form a group). Examples: Rational, real, complex numbers. Integers modulo a prime (Z p ). Quaternions. Field: A commutative division ring. Examples: Rational, real, complex numbers. Integers modulo a prime (Z p ). Characteristic of a field F : The smallest positive integer p such that (p times) is zero in F. If there is no such p, the characteristic is said to be zero. If the characteristic is not zero then it is a prime. Examples: The characteristic of Z p is p. The characteristic of Q is zero. Algebra over a field F : A ring A containing F such that elements of F commute with all elements of A. Examples: Q[x], Q[x, y], Mat 2 (Q) (2 by 2 matrices with rational enties) are algebras over Q. (Left) module over a ring A: An abelian group A with a multiplication A M M, (a, m) am which is associative ((ab)m = a(bm)) and distributive (a(m 1 + n 2 ) = am 1 + am 2, (a 1 + a 2 )m = a 1 m + a 2 m), and such that 1m = m (i.e., the monoid A acts on M, and the action is distributive in both arguments). Similarly one defines right modules (with multiplication (m, a) ma). Note that for a commutative ring, a left module is the same thing as a right module. Examples: A module over Z is the same thing as an abelian group. Also, for any ring A, A n = A... A (n times) is a module over A, left and right (called free module of rank n). More generally, if S is a set, then the free A-module A[S] with basis S is the set of formal finite sums s S a ss, a s A, where all a s but finitely many are zero. 3

4 Quotient module: If N M are A-modules, then so is the quotient M/N. Vector space: A module over a field. Examples: F n, where F is a field. The space of complex-valued functions on any set X. Basis of a vector space V : A collection of elements {v i } such that any element (vector) v V can be uniquely written as v = a i v i, a i F. Basis theorem: A basis always exists and all bases have the same number of elements (which could be infinite). This number is called the dimension of V. Theorem: Any finite field has order p n, where p is its characteristic (which is a prime). Indeed, such a field is a vector space over Z p of some finite dimension n, so its order is p n. In fact, for any prime power q there is a unique finite field of order q, denoted F q. Linear map: A homomorphism of vector spaces, i.e. a map f : V W of vector spaces over a field F such that f(a + b) = f(a) + f(b), and f(λa) = λf(a) for λ F. Examples: A matrix n by m over F defines a linear map F m F n. The derivative d/dx is a linear map from C[x] to itself. Ideal: A left ideal in a ring A is a left submodule of A, i.e., a subgroup I A such that AI = I. Similarly, a right ideal is a right submodule of A (IA = A). A two-sided ideal is a left ideal which is also a right ideal. Examples: f A, I = Af is the ideal of all multiples of f. For example, nz inside Z. If I A is a left ideal, then the quotient group A/I is a left A-module. If I is a two-sided ideal, then A/I is a ring. Examples: Z/nZ = Z n. R[x]/(x 2 + 1) = C. Lie algebra: A vector space L over a field F with a bracket operation [, ] : L L L, which is bilinear (i.e., [a, b] is linear with respect to a for fixed b and with respect to b for fixed a), skew-symmetric ([a, a] = 0, so [a, b] = [b, a]), and satisfies the Jacobi identity [a, [b, c]] + [b, [c, a]] + [c, [a, b]] = 0. Examples: Any algebra A with [a, b] = ab ba is a Lie algebra. So square matrices over a field form a Lie algebra. Other examples are the Lie algebra of matrices with trace zero and the Lie algebra of skew-symmetric matrices (X T = X). 4

5 Tensor product of modules: If A is a ring, M is a right A-module, and N a left A-module, then M A N is the quotient of the free abelian group with basis S = {m n, m M, n N} (where m n are formal symbols) by the subgroup spanned by (m 1 +m 2 ) n m 1 n m 2 n, m (n 1 +n 2 ) m n 1 m n 2, ma n m an, where a A. By doing this we force the relations saying that the expressions above are zero. Note that if A is commutative then left and right module is the same thing, and so M, N are just A-modules. Moreover, in this case the abelian group M A N is also an A-module: a (m n) = ma n = m an. Example: Z r Z Z s = Z gcd(r,s). For example, Z 2 Z Z 3 = 0. Theorem: If V, W are vector spaces over a field F with bases v i and w j then the set of elements v i w j is a basis of V F W. In particular, the dimension of V F W is the product of dimensions of V and W. So, unlike abelian groups, the tensor product of nonzero vector spaces is nonzero. 5

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