Part two (chapter two, chapter three): Introduce some important concept of extencis: matter-element, extensibility of matter-element, matter-element transformation, extension set.
第二部分（第二章、第三章）对可拓学的几个重要概念：物元、物元可拓性、物元变换以及可拓集合和评价方法进行了进一步的介绍。
参考来源 - 可拓检测技术研究Extension set pair trend make the combinations of the Extension Set and connection number more tightly. Further the use of he Extension Set and connection number will be more generalized.
这使得可拓集合与集对分析的结合更加紧密,而且可以在实际中得到更好的应用。
参考来源 - 可拓集联系数的势及其关系·2,447,543篇论文数据，部分数据来源于NoteExpress
The extension set is one of the three mainstays of extension theory.
可拓集合是可拓论的三大支柱之一。
Its theoretical pillars are the theory of matter-element and the theory of extension set.
其理论支柱是物元理论和可拓集合。
Its theory support is matter-element and extension set, and its logical cell is matter-element.
它的理论支柱是物元理论和可拓集合理论，其逻辑细胞则是物元。
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