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正式的表述是：同构是在数学对象之间定义的一类映射，它能揭示出在这些对象的属性或者操作之间存在的关系。若两个数学结构之间存在同构映射，那么这两个结构叫做是同构的。一般来说，如果忽略掉同构的对象的属性或操作的具体定义，单从结构上讲，同构的对象是完全等价的。

正式的表述是：同构是在数学对象之间定义的一类映射，它能揭示出在这些对象的属性或者操作之间存在的关系。若两个数学结构之间存在同构映射，那么这两个结构叫做是同构的。一般来说，如果忽略掉同构的对象的属性或操作的具体定义，单从结构上讲，同构的对象是完全等价的。

*元定理（metatheorem）A notable early citation is Quine's 1937 use of the word "metatheorem",[6] where meta- has the modern meaning of "an X about X". (Note earlier uses of "meta-economics" and even "metaphysics" do not have this doubled conceptual structure – they are about or beyond X but they do not themselves constitute an X).

*元定理（metatheorem）A notable early citation is Quine's 1937 use of the word "metatheorem",[6] where meta- has the modern meaning of "an X about X". (Note earlier uses of "meta-economics" and even "metaphysics" do not have this doubled conceptual structure – they are about or beyond X but they do not themselves constitute an X).Douglas Hofstadter, in his 1979 book Gödel, Escher, Bach (and in the sequel, Metamagical Themas), popularized this meaning of the term. The book, which deals with self-reference and strange loops, and touches on Quine and his work, was influential in many computer-related subcultures and may be responsible for the popularity of the prefix, for its use as a solo term, and for the many recent coinages which use it.[7] Hofstadter uses meta as a stand-alone word, as an adjective and as a directional preposition ("going meta," a term he coins for the old rhetorical trick of taking a debate or analysis to another level of abstraction, as when somebody says "This debate isn't going anywhere"). This book may also be responsible for the association of "meta" with strange loops, as opposed to just abstraction.[citation needed] The sentence "This sentence contains thirty-six letters," and the sentence which embeds it, are examples of "metasentences" referencing themselves in this way.

 

Douglas Hofstadter, in his 1979 book Gödel, Escher, Bach (and in the sequel, Metamagical Themas), popularized this meaning of the term. The book, which deals with self-reference and strange loops, and touches on Quine and his work, was influential in many computer-related subcultures and may be responsible for the popularity of the prefix, for its use as a solo term, and for the many recent coinages which use it.[7] Hofstadter uses meta as a stand-alone word, as an adjective and as a directional preposition ("going meta," a term he coins for the old rhetorical trick of taking a debate or analysis to another level of abstraction, as when somebody says "This debate isn't going anywhere"). This book may also be responsible for the association of "meta" with strange loops, as opposed to just abstraction.[citation needed] The sentence "This sentence contains thirty-six letters," and the sentence which embeds it, are examples of "metasentences" referencing themselves in this way.

===部分书评===

===部分书评===