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In mathematics, the associative property〔 〕 is a property of some binary operations. 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, rearranging the parentheses in such an expression will not change its value. Consider the following equations: : : Even though the parentheses were rearranged, 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 to be confused with commutativity, which addresses whether . Associative operations are abundant in mathematics; in fact, many algebraic structures (such as semigroups and categories) explicitly require their binary operations to be associative. However, many important and interesting operations are non-associative; some examples include subtraction, exponentiation and the vector cross product. In contrast to the theoretical counterpart, the addition of floating point numbers in computer science is not associative, and is an important source of rounding error. == Definition == Formally, a binary operation ∗ on a set ''S'' is called associative if it satisfies the associative law: :(''x'' ∗ ''y'') ∗ ''z'' = ''x'' ∗ (''y'' ∗ ''z'') for all ''x'', ''y'', ''z'' in ''S''. Here, ∗ is used to replace the symbol of the operation, which may be any symbol, and even the absence of symbol (juxtaposition) as for multiplication. :(''xy'')''z'' = ''x''(''yz'') = ''xyz'' for all ''x'', ''y'', ''z'' in ''S''. The associative law can also be expressed in functional notation thus: . 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Associative property」の詳細全文を読む スポンサード リンク
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