Barack Obama has started his four day trip to China with a message that the US is a partner and not an adversary.This week the US president faces delicate talks with his Chinese counterpart Hu Jintao over trade, climate change and human rights. But in an address to students in Shanghai Obama set a friendly tone: “Today we have a positive, constructive and comprehensive relationship that opens the door to partnership on the key global issues of our time; economic recovery and the development of clean energy, stopping the spread of nuclear weapons and the scourge of climate change, the promotion of peace and security in Asia and around the globe. All of these issues will be on the agenda tomorrow when I meet with President Hu,” he said. Obama is in the middle of what some analysts describe as a “charm offensive” in Asia. His nine-day trip has already taken in Japan and Singapore. From Shanghai, China’s commercial hub, he will travel to the capital Beijing, where he will hold perhaps the key meeting of his tour with President Hu. On trade, Obama will try to persuade China to allow its Yuan currency to rise, something it has been reluctant to do. In a tour spent making friends in Asia, China is the one with which Obama may have to agree to differ most.