HONG KONG, July 14 (Reuters) - China's financial regulators have invited some of the world's biggest investors to a rare symposium next week, three sources said, seeking to encourage foreigners to keep investing in the world's second-largest economy despite its recent weakness and rising geopolitical tensions. The meeting in Beijing next Friday will focus on the current conditions of U.S. dollar-denominated investment firms in China and the main challenges facing them, according to the sources who have direct knowledge of the matter and invitation documents reviewed by Reuters. The gathering comes at a time when global investors and banks are...