Hello,

If I had to pick one very small maths topic which has had deciding influence in an ICPC online and Regionals, Burnside Lemma it would be. However, this is a maths topic and is not generally discussed from the CP point of view.

Here is a 5 part series to get you comfortable with the topic. Do comment below for which topic do you feel requires such a series. Also, do like and subscribe to the channel to support the content.

Resources

Video Series: https://www.youtube.com/watch?v=drJN4O7tX5s&t=0s

Blogs: https://codeforces.com/blog/entry/51272

https://codeforces.com/blog/entry/62401

Problem list:

https://cses.fi/problemset/task/2210

https://cses.fi/problemset/task/2209

more in the other blogs and in the description of the Videos.