When Ai Weiwei was asked by the Public Art Fund about doing a citywide project, he was nervous. “I approached it very cautiously because it is a city I had once lived in and love,” he says. “It is like presenting yourself to an old lover; to give to the city what I never had a chance to achieve when I was there.” Come October, the project, “Good Fences Make Good Neighbors,” will, if all goes according to plan, be impossible to miss. As will, more than likely, its point, since the dissident Chinese artist has become openly concerned in his work with the rise of nationalism and the plight of migrant populations (the title is from the Robert Frost poem, “Mending Wall.”) But it’s also a homecoming of sorts for Weiwei, who lived in the East Village in the 1980s and for a while considered staying here. There will be large pieces — some quite large — riffing off the idea of a fence or barrier or border, in prominent places like nestled inside the Washington Square Park or as a low net-like (you can sit on it!) piece demarking the space around the Unisphere in Queens. His sponsor in this endeavor, the Public Art Fund, is celebrating its 40th year this year, and its director, Nicholas Baume, says: “I thought when we wanted to do something big for the 40th we also wanted to something which would be about New York City, really activate the city as a platform.” He first met with Weiwei in 2009 to discuss collaborating. But their plans were put on hold when the Chinese government put Weiwei under house arrest, and it wasn’t until he got his passport back and was able to set up his studio in Berlin that they were able to discuss a project further, in 2015.