81
81是80搭82之间个自然数。
- 第58个合数，正约数有1、3、9、27搭81。前一个为80、下一个为82。
- 第62个亏数，真约数和为40，亏度为41。前一个为79、下一个为82。
- 第9个平方数，为9个平方。前一个为64、下一个为100。
- 铊个原子序数。
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