Table 1 Properties of lower approximation operations (Zhu and Wang 2012, 2007; Zhang and Luo 2013; Qin et al. 2007)
| Â | \( \underline{C}_1\) | \( \underline{C}_6\) | \( \underline{C}_7\) | \( \underline{C}_8\) | \( \underline{C}_{10}\) |
|---|---|---|---|---|---|
1L | \(\checkmark \) | \(\checkmark \) | \(\checkmark \) | \(\checkmark \) | \(\checkmark \) |
2L | \(\checkmark \) | \(\checkmark \) | \(\checkmark \) | \(\checkmark \) | \(\checkmark \) |
3L | \(\checkmark \) | \(\checkmark \) | \(\checkmark \) | \(\checkmark \) | \(\checkmark \) |
4L | – | \(\checkmark \) | \(\checkmark \) | \(\checkmark \) | \(\checkmark \) |
5L | \(\checkmark \) | \(\checkmark \) | \(\checkmark \) | – | – |
6L | \(\checkmark \) | \(\checkmark \) | \(\checkmark \) | \(\checkmark \) | \(\checkmark \) |
7L | – | – | – | – | – |