1 Correction to: Peking Mathematical Journal https://doi.org/10.1007/s42543-018-0001-7

Example 4.2 as well as Lemma 3.31 and Lemma 2.51 have been incorrectly referenced by error. Furthermore, the multi-letter variables such as gr, div, spec have been set in italic instead in roman. The original article has been corrected.