Research on "Boundary Thickening" Strategy under the Protection and Renewal of the Old Urban District——Taking Yulin District of Chengdu as an ExampleLu Lin, Wu Lingli, Li Kunyang, Yang Mengzhu, Yan Hui and Liu FeiE3S Web Conf., 143 (2020) 01028DOI: https://doi.org/10.1051/e3sconf/202014301028