Interval Observer for Discrete Periodic Time-varying Descriptor Systems⁎This work was partially supported by the National Natural Science Foundation of China (No. U1813216), the Natural Science Foundation of Guangdong (No. 2020A1515010334) and the Science and Technology Research Foundation of Shenzhen (JCYJ20170817152701660 and JCYJ20170412171459177). Issue 2 (2020)