HONG KONG (AP) — Shares of CXMT, China's largest memory chipmaker, soared Monday after they began trading in Shanghai in mainland China’s biggest initial public stock offering in recent years. CXMT's shares surged 472% and were trading up 462% by early afternoon. It has become the most valuable company listed on a mainland Chinese exchange, with a market capitalization of about 3. 3 trillion yuan (more than $487 billion). But its estimated market value is still smaller than those of South...