We give a brief survey of virtual knot invariants derived from chord parity or chord index. These invariants have grown into an area in its own right due to rapid developing in the last decade. Several similar invariants of flat virtual knots and free knots are also discussed.

中文翻译：

---

从和弦奇偶校验到和弦索引

我们对从弦奇偶性或弦指数派生的虚拟结不变量进行了简要调查。由于过去十年的快速发展，这些不变量本身已经发展成为一个领域。还讨论了平面虚拟结和自由结的几个类似的不变量。