The Tiananmen Square protests (from the 15th of april 1989 to the 4th of june 1989) are the cause of this censorship in the Chinese Web last years.
The number 64 : for the 4th of june.
The number 89 : for the year.
The number 535 : for the 35th day of may (5th month), subtle alusion commonly used to refer to these events pro Democracy.
The censorship doesn't allow any research or publication with these numbers.
I put here just a small part of information about these historic events, and add the link for more.
The Tiananmen Square protests
The Tiananmen Square protests of 1989, commonly known as the June Fourth Incident or '89 Democracy Movement in Chinese, were student-led popular demonstrations in Beijing which took place in the spring of 1989 and received broad support from city residents, exposing deep splits within China's political leadership.
The protests were forcibly suppressed by hardline leaders who ordered the military to enforce martial law in the country's capital.
The crackdown that initiated on June 3–4 became known as the Tiananmen Square Massacre or the June 4 Massacre as troops with assault rifles and tanks inflicted casualties on unarmed civilians trying to block the military's advance towards Tiananmen Square in the heart of Beijing, which student and other demonstrators had occupied for seven weeks.
The scale of military mobilization and the resulting bloodshed were unprecedented in the history of Beijing, a city with a rich tradition of popular protests in the 20th century.
The Chinese government condemned the protests as a "counter-revolutionary riot", and has prohibited all forms of discussion or remembrance of the events since.
Due to the lack of information from China, many aspects of the events remain unknown or unconfirmed. Estimates of the death toll range from a few hundred to a few thousand.
The protests were triggered in April 1989 by the death of former Communist Party General Secretary Hu Yaobang, a liberal reformer who was deposed after losing a power struggle with hardliners over the direction of political and economic reforms.
University students marched and gathered in Tiananmen Square to mourn.
Hu had also voiced grievances against inflation, limited career prospects, and corruption of the party elite.
The protesters called for government accountability, freedom of the press, freedom of speech, and the restoration of workers' control over industry.
At the height of the protests, about a million people assembled in the Square.
Most of them were university students in Beijing.
The government initially took a conciliatory stance toward the protesters. The student-led hunger strike galvanized support for the demonstrators around the country and the protests spread to 400 cities by mid-May. Ultimately, China's paramount leader Deng Xiaoping and other party elders resolved to use force.
Party authorities declared martial law on May 20, and mobilized as many as 300,000 troops to Beijing.
In the aftermath of the crackdown, the government conducted widespread arrests of protesters and their supporters, cracked down on other protests around China, expelled foreign journalists and strictly controlled coverage of the events in the domestic press.
The police and internal security forces were strengthened. Officials deemed sympathetic to the protests were demoted or purged. Zhao Ziyang was ousted in a party leadership reshuffle and replaced with Jiang Zemin.
Political reforms were largely halted and economic reforms did not resume until Deng Xiaoping's 1992 southern tour.
The Chinese government was widely condemned internationally for the use of force against the protesters. Western governments imposed economic sanctions and arms embargoes.