I am an enthusiast of Duality in linear programming and convex optimization.
There are 12 videos I have made in Mandarin and uploaded on a Chinese video-sharing website, Bilibili:
​All these videos are about intuitions, tricks, and applications of Duality, and without mathematical theories. The original plan is to make the 13th video about Max-Flow and Min-Cut, but I'm too lazy to update. I will try to reproduce the same videos in English and keep updating when I have a detailed plan of the next 12 videos in future.​
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Btw, my ID on Bilibili is the Chinese characters of 0.878, which pays tribute to the best-known approximation bound for the Max-Cut problem, using Goemans-Williamson's algorithm.
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If you know any interesting paper or proof applying Duality in the key steps, please let me know. I will list some brilliant cases here afterwards.