Dear friends and colleagues, on Monday, 28 August 2017, Professor Xiao Zhang (Chinese Academy of Sciences, China) is giving a talk in the PDE Seminar on "Some mathematical problems in gravitational waves" An abstract is available at http://www.maths.usyd.edu.au/u/PDESeminar/abstracts17/xzhang.xhtml The talk is from 2 pm - 3 pm in Carslaw 829 (Access Grid Room) on the Camperdown Campus at the University of Sydney. Everybody is invited to attend the talk, take part in the discussion afterward in Carslaw Common Room 727 on level 7, and to join us for lunch at 12:30 pm. Kind regards, Daniel D and Daniel H P.S: If you know suitable speakers or want to give a talk yourself, please let us know.