Four years later, in April 2001, he stunned the world with a simple graphical criterion that generalizes the front door, the back door, and all doors we could think of at the time. I recall presenting Tian’s criterion at a Santa Fe conference. One by one, leaders in the research community stared at my poster and shook their heads in disbelief. How could such a simple criterion work for all diagrams? Tian (now a professor at Iowa State University) came to our lab with a style of thinking that was foreign to us then, in the 1990s. Our conversations were always loaded with wild metaphors and
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