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After the success of Polymath1 and the launching of Polymath3 and Polymath4, Tim Gowers wrote a blog post “Possible future Polymath projects” for planning the next polymath project on his blog. The post mentioned 9 possible projects. (Four of them later turned to polymath projects.) Following the post and separate posts describing some of the proposed projects, a few polls were taken and a problem – the Erdős discrepancy problem, was selected for the next project polymath5. In Combinatorics and more I reviewed some of the proposed projects from 2009, and in the same post I briefly and sometimes vaguely discussed the 2021 list, that I plan to present and discuss in detail in the next couple of months.
