Outline of algebraic structures

In mathematics, there are many types of algebraic structures which are studied. Abstract algebra is primarily the study of specific algebraic structures and their properties. Algebraic structures may be viewed in different ways, however the common starting point of algebra texts is that an algebraic object incorporates one or more sets with one or more binary operations or unary operations satisfying a collection of axioms.

Another branch of mathematics known as universal algebra studies algebraic structures in general. From the universal algebra viewpoint, most structures can be divided into varieties and quasivarieties depending on the axioms used. Some axiomatic formal systems that are neither varieties nor quasivarieties, called nonvarieties, are sometimes included among the algebraic structures by tradition.

Concrete examples of each structure will be found in the articles listed.

Algebraic structures are so numerous today that this article will inevitably be incomplete. In addition to this, there are sometimes multiple names for the same structure, and sometimes one name will be defined by disagreeing axioms by different authors. Most structures appearing on this page will be common ones which most authors agree on. Other web lists of algebraic structures, organized more or less alphabetically, include Jipsen and PlanetMath. These lists mention many structures not included below, and may present more information about some structures than is presented here.

Study of algebraic structures

Algebraic structures appear in most branches of mathematics, and one can encounter them in many different ways.

• Beginning study: In American universities, groups, vector spaces and fields are generally the first structures encountered in subjects such as linear algebra. They are usually introduced as sets with certain axioms.
• Abstract algebra studies properties of specific algebraic structures.
• Universal algebra studies algebraic structures abstractly, rather than specific types of structures.
• Category theory studies interrelationships between different structures, algebraic and non-algebraic. To study a non-algebraic object, it is often useful to use category theory to relate the object to an algebraic structure.

Types of algebraic structures

In full generality, an algebraic structure may use any number of sets and any number of axioms in its definition. The most commonly studied structures, however, usually involve only one or two sets and one or two binary operations. The structures below are organized by how many sets are involved, and how many binary operations are used. Increased indentation is meant to indicate a more exotic structure, and the least indented levels are the most basic.

One set with no binary operations

• Set: a degenerate algebraic structure S having no operations.
• Pointed set: S has one or more distinguished elements, often 0, 1, or both.
• Unary system: S and a single unary operation over S.
• Pointed unary system: a unary system with S a pointed set.

One binary operation on one set

Group-like structures
Totalityα Associativity Identity Inverse Commutativity
Semigroupoid Unneeded Required Unneeded Unneeded Unneeded
Small category Unneeded Required Required Unneeded Unneeded
Groupoid Unneeded Required Required Required Unneeded
Magma Required Unneeded Unneeded Unneeded Unneeded
Quasigroup Required Unneeded Unneeded Required Unneeded
Unital magma Required Unneeded Required Unneeded Unneeded
Semigroup Required Required Unneeded Unneeded Unneeded
Loop Required Unneeded Required Required Unneeded
Monoid Required Required Required Unneeded Unneeded
Group Required Required Required Required Unneeded
Commutative monoid Required Required Required Unneeded Required
Abelian group Required Required Required Required Required
The closure axiom, used by many sources and defined differently, is equivalent.

The following group-like structures consist of a set with a binary operation. The binary operation can be indicated by any symbol, or with no symbol (juxtaposition). The most common structure is that of a group. Other structures involve weakening or strengthening the axioms for groups, and may additionally use unary operations.

Two binary operations on one set

The main types of structures with one set having two binary operations are ring-like or ringoids and lattice-like or simply lattices. Ringoids and lattices can be clearly distinguished despite both having two defining binary operations. In the case of ringoids, the two operations are linked by the distributive law; in the case of lattices, they are linked by the absorption law. Ringoids also tend to have numerical models, while lattices tend to have set-theoretic models.

In ring-like structures or ringoids, the two binary operations are often called addition and multiplication, with multiplication linked to addition by the distributive law.

• Semiring: a ringoid such that S is a monoid under each operation. Addition is typically assumed to be commutative and associative, and the monoid product is assumed to distribute over the addition on both sides, and the additive identity 0 is an absorbing element in the sense that 0 x = 0 for all x.
• Near-ring: a semiring whose additive monoid is a (not necessarily abelian) group.
• Ring: a semiring whose additive monoid is an abelian group.
• Commutative ring: a ring in which the multiplication operation is commutative.
• Division ring: a nontrivial ring in which division by nonzero elements is defined.
• Integral domain: A nontrivial commutative ring in which the product of any two nonzero elements is nonzero.
• Field: a commutative division ring (i.e. a commutative ring which contains a multiplicative inverse for every nonzero element).
• Nonassociative rings: These are like rings, but the multiplication operation need not be associative.
• Boolean ring: a commutative ring with idempotent multiplication operation.
• Kleene algebras: a semiring with idempotent addition and a unary operation, the Kleene star, satisfying additional properties.
• *-algebra or *-ring: a ring with an additional unary operation (*) known as an involution, satisfying additional properties.
• Arithmetic: addition and multiplication on an infinite set, with an additional pointed unary structure. The unary operation is injective successor, and has distinguished element 0.
• Robinson arithmetic. Addition and multiplication are recursively defined by means of successor. 0 is the identity element for addition, and annihilates multiplication. Robinson arithmetic is listed here even though it is a variety, because of its closeness to Peano arithmetic.
• Peano arithmetic. Robinson arithmetic with an axiom schema of induction. Most ring and field axioms bearing on the properties of addition and multiplication are theorems of Peano arithmetic or of proper extensions thereof.

Lattice-like structures have two binary operations called meet and join, connected by the absorption law.

• Latticoid: meet and join commute but need not associate.
• Skew lattice: meet and join associate but need not commute.
• Lattice: meet and join associate and commute.
• Complete lattice: a lattice in which arbitrary meet and joins exist.
• Bounded lattice: a lattice with a greatest element and least element.
• Complemented lattice: a bounded lattice with a unary operation, complementation, denoted by postfix . The join of an element with its complement is the greatest element, and the meet of the two elements is the least element.
• Modular lattice: a lattice whose elements satisfy the additional modular identity.
• Distributive lattice: a lattice in which each of meet and join distributes over the other. Distributive lattices are modular, but the converse does not hold.
• Boolean algebra: a complemented distributive lattice. Either of meet or join can be defined in terms of the other and complementation. This can be shown to be equivalent with the ring-like structure of the same name above.
• Heyting algebra: a bounded distributive lattice with an added binary operation, relative pseudo-complement, denoted by infix →, and governed by the axioms:
• x → x = 1
• x (x → y) = x y
• y (x → y) = y
• x → (y z) = (x → y) (x → z)

Module-like structures on two sets

The following module-like structures have the common feature of having two sets, A and B, so that there is a binary operation from A×A into A and another operation from A×B into A. Modules, counting the ring operations, have at least three binary operations.

Algebra-like structures on two sets

These structures are defined over two sets, a ring R and an R-module M equipped with an operation called multiplication. This can be viewed as a system with five binary operations: two operations on R, two on M and one involving both R and M. Many of these structures are hybrid structures of the previously mentioned ones.

Algebraic structures with additional non-algebraic structure

There are many examples of mathematical structures where algebraic structure exists alongside non-algebraic structure.

Algebraic structures in different disciplines

Some algebraic structures find uses in disciplines outside of abstract algebra. The following is meant to demonstrate some specific applications in other fields.

In Physics:

References

• Garrett Birkhoff, 1967. Lattice Theory, 3rd ed, AMS Colloquium Publications Vol. 25. American Mathematical Society.
• ———, and Saunders MacLane, 1999 (1967). Algebra, 2nd ed. New York: Chelsea.
• George Boolos and Richard Jeffrey, 1980. Computability and Logic, 2nd ed. Cambridge Univ. Press.
• Dummit, David S., and Foote, Richard M., 2004. Abstract Algebra, 3rd ed. John Wiley and Sons.
• Grätzer, George, 1978. Universal Algebra, 2nd ed. Springer.
• David K. Lewis, 1991. Part of Classes. Blackwell.
• Michel, Anthony N., and Herget, Charles J., 1993 (1981). Applied Algebra and Functional Analysis. Dover.
• Potter, Michael, 2004. Set Theory and its Philosophy, 2nd ed. Oxford Univ. Press.
• Smorynski, Craig, 1991. Logical Number Theory I. Springer-Verlag.

A monograph available free online:

• Burris, Stanley N., and H.P. Sankappanavar, H. P., 1981. A Course in Universal Algebra. Springer-Verlag. ISBN 3-540-90578-2.