incl-excl / divisibility:
1) https://cses.fi/problemset/task/2417 -- coprime pairs 
2) https://codeforces.com/contest/1900/problem/D -- small gcd
3) https://atcoder.jp/contests/agc038/tasks/agc038_c -- lcms
4) https://arena.olimpiici.com/#/catalog/765/problem/101987
5) https://arena.olimpiici.com/#/catalog/713/problem/101812

incl-excl:
1) https://arena.olimpiici.com/#/catalog/801/problem/102117
!) https://arena.olimpiici.com/#/catalog/883/problem/102438

permutations:
1) https://arena.olimpiici.com/#/catalog/608/problem/101518
2) https://arena.olimpiici.com/#/catalog/318/problem/100890
3) https://codeforces.com/contest/612/problem/E
4) https://kilonova.ro/problems/3952

additional:
https://infos.infosbg.com/files/Schools/2022/Sofia/common/7G_combinatorics/combinatorics.pdf
