Binary operation

More precisely, a binary operation on a set ${\displaystyle S}$ is a mapping of the elements of the Cartesian product ${\displaystyle S\times S}$ to ${\displaystyle S}$:[1][2][3]

${\displaystyle \,f\colon S\times S\rightarrow S.}$

Because the result of performing the operation on a pair of elements of ${\displaystyle S}$ is again an element of ${\displaystyle S}$, the operation is called a closed (or internal) binary operation on ${\displaystyle S}$ (or sometimes expressed as having the property of closure).[4]

If ${\displaystyle f}$ is not a function but a partial function, then ${\displaystyle f}$ is called a partial binary operation. For instance, division of real numbers is a partial binary operation, because one can't divide by zero: ${\displaystyle {\frac {a}{0}}}$ is undefined for every real number ${\displaystyle a}$. In both model theory and classical universal algebra, binary operations are required to be defined on all elements of ${\displaystyle S\times S}$. However, partial algebras[5] generalize universal algebras to allow partial operations.

Sometimes, especially in computer science, the term binary operation is used for any binary function.

Typical examples of binary operations are the addition (${\displaystyle +}$) and multiplication (${\displaystyle \times }$) of numbers and matrices as well as composition of functions on a single set. For instance,

• On the set of real numbers ${\displaystyle \mathbb {R} }$, ${\displaystyle f(a,b)=a+b}$ is a binary operation since the sum of two real numbers is a real number.
• On the set of natural numbers ${\displaystyle \mathbb {N} }$, ${\displaystyle f(a,b)=a+b}$ 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 ${\displaystyle M(2,\mathbb {R} )}$ of ${\displaystyle 2\times 2}$ matrices with real entries, ${\displaystyle f(A,B)=A+B}$ is a binary operation since the sum of two such matrices is a ${\displaystyle 2\times 2}$ matrix.
• On the set ${\displaystyle M(2,\mathbb {R} )}$ of ${\displaystyle 2\times 2}$ matrices with real entries, ${\displaystyle f(A,B)=AB}$ is a binary operation since the product of two such matrices is a ${\displaystyle 2\times 2}$ matrix.
• For a given set ${\displaystyle C}$, let ${\displaystyle S}$ be the set of all functions ${\displaystyle h\colon C\rightarrow C}$. Define ${\displaystyle f\colon S\times S\rightarrow S}$ by ${\displaystyle f(h_{1},h_{2})(c)=(h_{1}\circ h_{2})(c)=h_{1}(h_{2}(c))}$ for all ${\displaystyle c\in C}$, the composition of the two functions ${\displaystyle h_{1}}$ and ${\displaystyle h_{2}}$ in ${\displaystyle S}$. Then ${\displaystyle f}$ is a binary operation since the composition of the two functions is again a function on the set ${\displaystyle C}$ (that is, a member of ${\displaystyle S}$).

Many binary operations of interest in both algebra and formal logic are commutative, satisfying ${\displaystyle f(a,b)=f(b,a)}$ for all elements ${\displaystyle a}$ and ${\displaystyle b}$ in ${\displaystyle S}$, or associative, satisfying ${\displaystyle f(f(a,b),c)=f(a,f(b,c))}$ for all ${\displaystyle a}$, ${\displaystyle b}$, and ${\displaystyle c}$ in ${\displaystyle S}$. 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 ${\displaystyle \mathbb {R} }$, subtraction, that is, ${\displaystyle f(a,b)=a-b}$, is a binary operation which is not commutative since, in general, ${\displaystyle a-b\neq b-a}$. It is also not associative, since, in general, ${\displaystyle a-(b-c)\neq (a-b)-c}$; for instance, ${\displaystyle 1-(2-3)=2}$ but ${\displaystyle (1-2)-3=-4}$.

On the set of natural numbers ${\displaystyle \mathbb {N} }$, the binary operation exponentiation, ${\displaystyle f(a,b)=a^{b}}$, is not commutative since, ${\displaystyle a^{b}\neq b^{a}}$ (cf. Equation x^y = y^x), and is also not associative since ${\displaystyle f(f(a,b),c)\neq f(a,f(b,c))}$. For instance, with ${\displaystyle a=2}$, ${\displaystyle b=3}$, and ${\displaystyle c=2}$, ${\displaystyle f(2^{3},2)=f(8,2)=8^{2}=64}$, but ${\displaystyle f(2,3^{2})=f(2,9)=2^{9}=512}$. By changing the set ${\displaystyle \mathbb {N} }$ to the set of integers ${\displaystyle \mathbb {Z} }$, this binary operation becomes a partial binary operation since it is now undefined when ${\displaystyle a=0}$ and ${\displaystyle b}$ is any negative integer. For either set, this operation has a right identity (which is ${\displaystyle 1}$) since ${\displaystyle f(a,1)=a}$ for all ${\displaystyle a}$ in the set, which is not an identity (two sided identity) since ${\displaystyle f(1,b)\neq b}$ in general.

Division (${\displaystyle \div }$), a partial binary operation on the set of real or rational numbers, is not commutative or associative. Tetration (${\displaystyle \uparrow \uparrow }$), as a binary operation on the natural numbers, is not commutative or associative and has no identity element.

Binary operations are often written using infix notation such as ${\displaystyle a\ast b}$, ${\displaystyle a+b}$, ${\displaystyle a\cdot b}$ or (by juxtaposition with no symbol) ${\displaystyle ab}$ rather than by functional notation of the form ${\displaystyle f(a,b)}$. 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 ${\displaystyle \ast ab}$ and reverse Polish notation ${\displaystyle ab\ast }$.

A binary operation ${\displaystyle f}$ on a set ${\displaystyle S}$ may be viewed as a ternary relation on ${\displaystyle S}$, that is, the set of triples ${\displaystyle (a,b,f(a,b))}$ in ${\displaystyle S\times S\times S}$ for all ${\displaystyle a}$ and ${\displaystyle b}$ in ${\displaystyle S}$.

An external binary operation is a binary function from ${\displaystyle K\times S}$ to ${\displaystyle S}$. This differs from a binary operation on a set in the sense in that ${\displaystyle K}$ need not be ${\displaystyle S}$; its elements come from outside.

An example of an external binary operation is scalar multiplication in linear algebra. Here ${\displaystyle K}$ is a field and ${\displaystyle S}$ is a vector space over that field.

Some external binary operations may alternatively be viewed as an action of ${\displaystyle K}$ on ${\displaystyle S}$. This requires the existence of an associative multiplication in ${\displaystyle K}$, and a compatibility rule of the form ${\displaystyle a(bs)=(ab)s}$, where ${\displaystyle a,b\in K}$ and ${\displaystyle s\in S}$ (here, both the external operation and the multiplication in ${\displaystyle K}$ are denoted by juxtaposition).

The dot product of two vectors maps ${\displaystyle S\times S}$ to ${\displaystyle K}$, where ${\displaystyle K}$ is a field and ${\displaystyle S}$ is a vector space over ${\displaystyle K}$. It depends on authors whether it is considered as a binary operation.

• Category:Properties of binary operations
• Iterated binary operation
• Operator (programming)
• Ternary operation
• Truth table#Binary operations
• Unary operation
• Magma (algebra), a set equipped with a binary operation.

• Fraleigh, John B. (1976), A First Course in Abstract Algebra (2nd ed.), Reading: Addison-Wesley, ISBN 0-201-01984-1
• 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, ISBN 0-13-067464-8
• Rotman, Joseph J. (1973), The Theory of Groups: An Introduction (2nd ed.), Boston: Allyn and Bacon

• Weisstein, Eric W. "Binary Operation". MathWorld.