Mathematician Zhang Yitang, who joined Sun Yat-sen University in Guangzhou from California in June, has called the wave of Chinese researchers exiting the US a “positive trend”.

Zhang, known for his prime number research, told Phoenix TV, a partially state-owned broadcaster, in a programme aired on Monday that he had been thinking about returning to the mainland “in light of recent international political issues that have strained China-US relations”.

The Shanghai native, 70, is now a full professor at Sun Yat-sen’s newly established Institute of Advanced Studies Hong Kong. He left the University of California, Santa Barbara, where he taught mathematics, to work and live in the Greater Bay Area.

“Many Chinese scholars and professors in the US have already returned, and many others are still considering it,” Zhang said. “I think this is a positive trend.”

“My field is not greatly affected, but for researchers in computers, chips or anything related to the military industry, they need to be especially careful,” he warned. “The US is very strict about these areas now.”

Zhang said that one advantage of studying mathematics, especially theoretical mathematics, was not having to be tied to a particular place.

“For those in experimental science or experimental physics, without a laboratory or equipment, everything is off the table,” he added.

“The [Donald] Trump administration recently cut funding for many laboratory research programmes, which has left those in fields like biology in a tough situation.”

Zhang said he decided to join Sun Yat-sen in March after receiving several invitations from universities across China.

“The university planned for my return very meticulously. They waited until I was on the plane and had flown out of US airspace before announcing my appointment.

“When I landed in Guangzhou, a customs officer came on board to greet us. I was really touched,” he recalled. “It felt like the country valued and respected me highly.”

Zhang said he could continue his research in China, adding that he did “not want to be like a yellow fallen leaf, settling on the ground as if it were the end”.

The number theorist achieved acclaim in 2013 when he solved one of mathematics’ oldest problems: the twin primes conjecture.

The conjecture proposes that there are infinitely many pairs of prime numbers that differ by two. Zhang’s proof was the first to show that infinitely many prime pairs exist with a gap of less than 70 million.

His proof was significant because it established a finite upper limit for the gaps between infinitely many prime pairs, opening a path to further narrowing that distance, potentially all the way down to two.

Zhang has also made notable progress in solving the Landau–Siegel zeros conjecture, a key problem linked to the Riemann hypothesis, an unsolved problem in mathematics that concerns the distribution of prime numbers.

“At that time, a conference held in the western US to discuss this conjecture was attended by experts from around the world,” Zhang said of his 2013 proof. “I was not there. Their conclusion was that the problem was impossible to solve.”

“When I learned why they could not solve it, I realised that this might be my strength. In the end, I managed to solve it. In academia, it is best to maintain a sense of freshness and think about areas that others have not considered,” he added.

“Science is becoming increasingly difficult, and the time and cycles needed are growing longer. I do not worry about age – whether you are 30 or 40 – you just need to keep moving forward.”

After graduating from Purdue University with a PhD in 1991, Zhang struggled to find an academic position in the United States without a recommendation letter and worked as an accountant in a fast-food restaurant for seven years.

“Around 1991, when the Soviet Union collapsed, the Americans, quite cleverly, recruited many of the Soviet Union’s renowned mathematicians and physicists to the US,” he said. “Finding a job in the US was incredibly difficult for PhD graduates in those few years.”

“I did not give up because I felt that I could continue to research mathematics, which can be done anywhere,” Zhang added.

“Forty years later, I am back to my starting point, working in and serving my country.”