数列{a_n}极限是A的描述性定义为:当项数(自然数)n无限增大时,a无限地趋近于A。这里有两个“无限”,前者表明对数列项由小到大观察其变化,后者若出现,则说明极限存在。第二个“无限”只要求趋近程度为无限地趋近,而对趋近方式未做任何限制。a可以从A的左侧趋近于A,也可以从A的右侧趋近,再者可以同时从左右两 The {a_n} limit is the descriptive definition of A as: a goes infinitely to A when the number of terms (natural number) n increases indefinitely. Here are two “infinity”, the former shows that the number of items from small to large observation of its change, the latter if it appears, then the limit exists. The second “infinity” simply requires that the degree of approach be infinitely close, without any restriction on the approach to approach. a can approach A from the left side of A, and can approach from the right side of A,