Hello, Codeforces! The reason why I am writing this blog is that my ACM/ICPC teammate calabash_boy is learning this technique recently(he is a master in string algorithms,btw), and he wanted me to provide some useful resources on this topic. I found that although many claim that they do know this topic well, problems concerning inclusion-exclusion principle are sometimes quite tricky and not that easy to deal with. Also, after some few investigations, the so-called "Inclusion-Exclusion principle" some people claim that they know wasn't the generalized one, and has little use when solving problems. So, what I am going to pose here, is somewhat the "Generalized Inclusion-Exclusion Principle". I'll start with the basic formula, one can choose to skip some of the text depending on your grasp with the topic.
Consider a finite set X and three subsets A, B, C, To obtain 
, we take the sum 
+ 
+ 
. Unless A, B, C are pairwise distinct, we have an overcount, since the elements of 
has been counted twice. So we subtract 
. Now the count is correct except for the elements in 
which have been added three times, but also subtracted three times. The answer is therefore
, or equivalently,
The following formula addresses the case applied to more sets.
Let A1, A2, ..., An be subsets of X. Then
This is a formula which looks familiar to many people, I'll call it The Restricted Inclusion-Exclusion Principle, it can convert the problem of calculating the size of the union of some sets into calculating the size of the intersection of some sets. It's not hard to prove the correctness of this formula, we can just check how often an element 
is counted in both sides. If 
, then it's counted once on either side. Suppose 
, and more precisely, that x is in exactly m of the sets Ai. The count on the left-hand side is 0, and on the right hand side, we have
for $m\geq 1$, thus the equality holds.
Let's see an example problem where this principle could be applied:
