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|Rules of inference|
|Rules of replacement|
|Rules of inference|
In mathematics, the associative property  is a property of some binary operations, which means that rearranging the parentheses in an expression will not change the result. In propositional logic, associativity is a valid rule of replacement for expressions in logical proofs.
Within an expression containing two or more occurrences in a row of the same associative operator, the order in which the operations are performed does not matter as long as the sequence of the operands is not changed. That is, (after rewriting the expression with parentheses and in infix notation if necessary) rearranging the parentheses in such an expression will not change its value. Consider the following equations:
Even though the parentheses were rearranged on each line, the values of the expressions were not altered. Since this holds true when performing addition and multiplication on any real numbers, it can be said that "addition and multiplication of real numbers are associative operations".
Associativity is not the same as commutativity, which addresses whether the order of two operands affects the result. For example, the order does not matter in the multiplication of real numbers, that is, a × b = b × a, so we say that the multiplication of real numbers is a commutative operation. However, operations such as function composition and matrix multiplication are associative, but (generally) not commutative.
However, many important and interesting operations are non-associative; some examples include subtraction, exponentiation, and the vector cross product. In contrast to the theoretical properties of real numbers, the addition of floating point numbers in computer science is not associative, and the choice of how to associate an expression can have a significant effect on rounding error.
- (x ∗ y) ∗ z = x ∗ (y ∗ z) for all x, y, z in S.
- (xy)z = x(yz) = xyz for all x, y, z in S.
The associative law can also be expressed in functional notation thus: f(f(x, y), z) = f(x, f(y, z)).
If a binary operation is associative, repeated application of the operation produces the same result regardless of how valid pairs of parentheses are inserted in the expression.  This is called the generalized associative law. For instance, a product of four elements may be written, without changing the order of the factors, in five possible ways:
If the product operation is associative, the generalized associative law says that all these formulas will yield the same result. So unless the formula with omitted parentheses already has a different meaning (see below), the parentheses can be considered unnecessary and "the" product can be written unambiguously as
As the number of elements increases, the number of possible ways to insert parentheses grows quickly, but they remain unnecessary for disambiguation.
An example where this does not work is the logical biconditional . It is associative, thus A(BC) is equivalent to (AB)C, but ABC most commonly means (AB and BC), which is not equivalent.
Some examples of associative operations include the following.
concatenation of the three strings
"world"can be computed by concatenating the first two strings (giving
"hello ") and appending the third string (
"world"), or by joining the second and third string (giving
" world") and concatenating the first string (
"hello") with the result. The two methods produce the same result; string concatenation is associative (but not commutative).
- In arithmetic, addition and multiplication of real numbers are associative; i.e.,
- Because of associativity, the grouping parentheses can be omitted without ambiguity.
- The trivial operation x ∗ y = x (that is, the result is the first argument, no matter what the second argument is) is associative but not commutative. Likewise, the trivial operation x ∘ y = y (that is, the result is the second argument, no matter what the first argument is) is associative but not commutative.
- Addition and multiplication of complex numbers and quaternions are associative. Addition of octonions is also associative, but multiplication of octonions is non-associative.
- The greatest common divisor and least common multiple functions act associatively.
- If M is some set and S denotes the set of all functions from M to M, then the operation of function composition on S is associative:
- Slightly more generally, given four sets M, N, P and Q, with h: M to N, g: N to P, and f: P to Q, then
- as before. In short, composition of maps is always associative.
- Consider a set with three elements, A, B, and C. The following operation:
× A B C A A A A B A B C C A A A
- is associative. Thus, for example, A(BC)=(AB)C = A. This operation is not commutative.
- Because matrices represent linear functions, and matrix multiplication represents function composition, one can immediately conclude that matrix multiplication is associative. 
In standard truth-functional propositional logic, association,   or associativity  are two valid rules of replacement. The rules allow one to move parentheses in logical expressions in logical proofs. The rules (using logical connectives notation) are:
Associativity is a property of some logical connectives of truth-functional propositional logic. The following logical equivalences demonstrate that associativity is a property of particular connectives. The following are truth-functional tautologies. 
Associativity of disjunction:
Associativity of conjunction:
Associativity of equivalence:
Joint denial is an example of a truth functional connective that is not associative.
A binary operation on a set S that does not satisfy the associative law is called non-associative. Symbolically,
For such an operation the order of evaluation does matter. For example:
Also note that infinite sums are not generally associative, for example:
The study of non-associative structures arises from reasons somewhat different from the mainstream of classical algebra. One area within non-associative algebra that has grown very large is that of Lie algebras. There the associative law is replaced by the Jacobi identity. Lie algebras abstract the essential nature of infinitesimal transformations, and have become ubiquitous in mathematics.
There are other specific types of non-associative structures that have been studied in depth; these tend to come from some specific applications or areas such as combinatorial mathematics. Other examples are quasigroup, quasifield, non-associative ring, non-associative algebra and commutative non-associative magmas.
In mathematics, addition and multiplication of real numbers is associative. By contrast, in computer science, the addition and multiplication of floating point numbers is not associative, as rounding errors are introduced when dissimilar-sized values are joined together. 
To illustrate this, consider a floating point representation with a 4-bit
(1.0002×20 + 1.0002×20) + 1.0002×24 = 1.0002×21 + 1.0002×24 = 1.0012×24
1.0002×20 + (1.0002×20 + 1.0002×24) = 1.0002×20 + 1.0002×24 = 1.0002×24
Even though most computers compute with a 24 or 53 bits of mantissa,  this is an important source of rounding error, and approaches such as the Kahan summation algorithm are ways to minimise the errors. It can be especially problematic in parallel computing.  
In general, parentheses must be used to indicate the order of evaluation if a non-associative operation appears more than once in an expression (unless the notation specifies the order in another way, like ). However, mathematicians agree on a particular order of evaluation for several common non-associative operations. This is simply a notational convention to avoid parentheses.
A left-associative operation is a non-associative operation that is conventionally evaluated from left to right, i.e.,
while a right-associative operation is conventionally evaluated from right to left:
Both left-associative and right-associative operations occur. Left-associative operations include the following:
- Function application:
- This notation can be motivated by the currying isomorphism.
Right-associative operations include the following:
- Exponentiation of real numbers in superscript notation:
- Exponentiation is commonly used with brackets or right-associatively because a repeated left-associative exponentiation operation is of little use. Repeated powers would mostly be rewritten with multiplication:
- Formatted correctly, the superscript inherently behaves as a set of parentheses; e.g. in the expression the addition is performed before the exponentiation despite there being no explicit parentheses wrapped around it. Thus given an expression such as , the full exponent of the base is evaluated first. However, in some contexts, especially in handwriting, the difference between , and can be hard to see. In such a case, right-associativity is usually implied.
- Using right-associative notation for these operations can be motivated by the Curry–Howard correspondence and by the currying isomorphism.
Non-associative operations for which no conventional evaluation order is defined include the following.
- Exponentiation of real numbers in infix notation: 
- Taking the cross product of three vectors:
- Taking the pairwise average of real numbers:
- Taking the relative complement of sets is not the same as . (Compare material nonimplication in logic.)
|Look up associative property in Wiktionary, the free dictionary.|
- Light's associativity test
- Telescoping series, the use of addition associativity for cancelling terms in an infinite series
- A semigroup is a set with an associative binary operation.
- Commutativity and distributivity are two other frequently discussed properties of binary operations.
- Power associativity, alternativity, flexibility and N-ary associativity are weak forms of associativity.
- Moufang identities also provide a weak form of associativity.
Hungerford, Thomas W. (1974). Algebra (1st ed.).
Springer. p. 24.
Definition 1.1 (i) a(bc) = (ab)c for all a, b, c in G.
- Durbin, John R. (1992).
Modern Algebra: an Introduction (3rd ed.). New York: Wiley. p. 78.
If are elements of a set with an associative operation, then the product is unambiguous; this is, the same element will be obtained regardless of how parentheses are inserted in the product
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- Knuth, Donald, The Art of Computer Programming, Volume 3, section 4.2.2
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- George Mark Bergman: Order of arithmetic operations
- Education Place: The Order of Operations
- Khan Academy: The Order of Operations, timestamp 5m40s
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