Topology and ferroelectricity in group-V monolayers*Project supported by the National Natural Science Foundation of China (Grant Nos. 11974307 and 61574123), Zhejiang Provincial Natural Science Foundation, China (Grant No. D19A040001), the Fundamental Research Funds for the Central Universities of China, and the 2DMOST, Shenzhen University (Grant No. 2018028). (May 2020)