https://baike.baidu.com/item/%E4%B8%AD%E5%BF%83%E7%A7%AF/18898265?fr=aladdin
https://groupprops.subwiki.org/wiki/Internal_central_product
https://groupprops.subwiki.org/wiki/External_central_product
https://wuli.wiki/online/GrpPrd.html
http://www.doc88.com/p-171809260889.html
https://baike.baidu.com/item/%E5%8D%8A%E7%9B%B4%E7%A7%AF/3477548?fr=aladdin
https://mp.weixin.qq.com/s?__biz=MzA4NjMxMTgxNw==&mid=2448122071&idx=1&sn=a3621fc2764ca71bb7e5fb95e1a637c9&chksm=8bd55631bca2df2728808445e762135c7daf91d6ea4f25d98457d948f8cfe21852daadccbcd2&mpshare=1&scene=1&srcid=0417t6t4WFbOfgSaphaA8TjB&sharer_sharetime=1618623484131&sharer_shareid=7efeeeb14e777fa5f7b805d52926ebd8&exportkey=AdrY7bJxhJmQ9h3S0XMh94I%3D&pass_ticket=vl88crqeUkpTDCOE3j6zc%2BNffqzudVxhhtBTjUB8P9H1LvI2fcsSXmjKBbwkzbQO&wx_header=0#rd
https://www.doc88.com/p-9116601827993.html