Generally, it is common that cited papers are earlier than citing papers. But we found three different cases, with more undiscovered. In this letter, we attempted to explain the reasons. However, negative time lag between citing and cited papers may mislead us when we study the characteristics of science.

中文翻译：

---

Citing 早于 Cited？

一般来说，被引论文早于施引论文是很常见的。但是我们发现了三种不同的情况，还有更多未被发现。在这封信中，我们试图解释原因。然而，当我们研究科学的特征时，引用和被引论文之间的负时间差可能会误导我们。