In recent years, he had turned to study the thought of President Xi Jinping, including his view on traditional culture, as well as the Thought on Socialism with Chinese Characteristics for a New Era that was brought up last October.

Zheng started tutoring doctoral students at age 60, after he quit his job as vice-president of the university. In the past 30 years, some of his 49 doctoral students have taken up leadership roles within the government or become doctoral supervisors themselves.

In his last days, he displayed relief on hearing that Qiu Xiao, his last full-time doctoral student, had got the highest score in a double-blind peer review of his doctoral thesis.

His student, Kuang Cuizhi, said that Zheng had given him a safe and peaceful learning environment for the doctoral entrance examination, while another former student, Liu Shihua, recalled that Zheng had refused to recruit a leading cadre in order to enroll him, who was keen to study but from an ordinary family.

Zheng had also been donating to help the sick parents of some of his poor students.

Hu Hua, late founder of the discipline of the CPC history, described Zheng as a productive author.

The older he became, the more Zheng treasured his time. He often got up at 4 am, reading and composing his articles. At 6 am, he usually went out for a walk, with academics continuously in his mind.

Once he came up with a new idea, he would call his students at once, tell them about it in case he forgot and would ask to discuss it with them in detail later.

Meanwhile, he would give several lectures of two or three hours every year to the younger students, in high spirits and with a thundering voice.

Zhou Qiuyu, an undergraduate, said that she was surprised and moved when Zheng stood up at the end of a speech in 2016 and bowed to the students for their patience of listening to such a lengthy lecture.

Most of the time, however, Zheng was too busy with academia to show his emotions. The only time he broke down in tears was in May 2016, when his wife－who, for 17 years, had volunteered to teach pupils from poor families for free－passed away.

According to Zheng Xiaoliang, one of his sons, there were no last words to the family.

Yet, Zheng Derong did leave some for his students, asking them to stick to their faith. "Stay true to our founding mission," he said.

Contact the writer at fangaiqing@chinadaily.com.cn