In mathematics, a binary operation or dyadic operation is a rule for combining two elements (called operands) to produce another element. More formally, a binary operation is an operation of arity two.
An operation of arity two that involves several sets is sometimes also called a binary operation. For example, scalar multiplication of vector spaces takes a scalar and a vector to produce a vector, and scalar product takes two vectors to produce a scalar. Such binary operations may also be called binary functions.
On the set of real numbers , is a binary operation since the sum of two real numbers is a real number.
On the set of natural numbers , is a binary operation since the sum of two natural numbers is a natural number. This is a different binary operation than the previous one since the sets are different.
On the set of matrices with real entries, is a binary operation since the sum of two such matrices is a matrix.
On the set of matrices with real entries, is a binary operation since the product of two such matrices is a matrix.
For a given set , let be the set of all functions . Define by for all , the composition of the two functions and in . Then is a binary operation since the composition of the two functions is again a function on the set (that is, a member of ).
Many binary operations of interest in both algebra and formal logic are commutative, satisfying for all elements and in , or associative, satisfying for all , , and in . Many also have identity elements and inverse elements.
The first three examples above are commutative and all of the above examples are associative.
On the set of real numbers , subtraction, that is, , is a binary operation which is not commutative since, in general, . It is also not associative, since, in general, ; for instance, but .
On the set of natural numbers , the binary operation exponentiation, , is not commutative since, (cf. Equation xy = yx), and is also not associative since . For instance, with , , and , , but . By changing the set to the set of integers , this binary operation becomes a partial binary operation since it is now undefined when and is any negative integer. For either set, this operation has a right identity (which is ) since for all in the set, which is not an identity (two sided identity) since in general.
Division (), a partial binary operation on the set of real or rational numbers, is not commutative or associative. Tetration (), as a binary operation on the natural numbers, is not commutative or associative and has no identity element.
Notation
Binary operations are often written using infix notation such as , , or (by juxtaposition with no symbol) rather than by functional notation of the form . Powers are usually also written without operator, but with the second argument as superscript.
Binary operations are sometimes written using prefix or (more frequently) postfix notation, both of which dispense with parentheses. They are also called, respectively, Polish notation and reverse Polish notation.
Binary operations as ternary relations
A binary operation on a set may be viewed as a ternary relation on , that is, the set of triples in for all and in .
Also the dot product of two vectors maps to , where is a field and is a vector space over . It depends on authors whether it is considered as a binary operation.
Magma (algebra) – Algebraic structure with a binary operation
Operator (programming) – Construct associated with a mathematical operation in computer programsPages displaying short descriptions of redirect targets
Ternary operation – Mathematical operation that combines three elements to produce another element
Fraleigh, John B. (1976), A First Course in Abstract Algebra (2nd ed.), Reading: Addison-Wesley, ISBN0-201-01984-1
Hall, Marshall Jr. (1959), The Theory of Groups, New York: Macmillan
Hardy, Darel W.; Walker, Carol L. (2002), Applied Algebra: Codes, Ciphers and Discrete Algorithms, Upper Saddle River, NJ: Prentice-Hall, ISBN0-13-067464-8
Rotman, Joseph J. (1973), The Theory of Groups: An Introduction (2nd ed.), Boston: Allyn and Bacon
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