PLEASE NOTE THE UNUSUAL TIME AND LOCATION

A 2-complex X has nonpositive immersions if for every immersion Y to X with Y compact and connected, either euler(Y)\leq 0 or pi_1Y=1. In the first part of my talk, I will give a survey on nonpositive immersions and some variants definitions. And discuss the relationships with various properties of X and subgroup properties of pi_1X.