19
19（十九）是18搭20之间个自然数。
- 第8个素数。前一个为17、下一个为23。
- 第13个十进制个等数位数。前一个为17、下一个为21。
- 第4个中心三角形数、第3个中心六边形数、第3个八面体数。
- 钾个原子序数。
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