[Tutorial] Inclusion-Exclusion Principle, Part 1.

Правка en13, от Roundgod, 2019-01-18 21:06:41

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". Most of the describing text are from the graduate text book Graduate Text in Mathematics 238, A Course in Enumeration, and the problems are those that I encountered in real problem set, so if possible, I'll add a link to the real problem so that you can solve it by yourself. 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 and three subsets , To obtain , we take the sum + + . Unless 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.

The Restricted Inclusion-Exclusion Principle. Let be subsets of . 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 is in exactly of the sets . The count on the left-hand side is , and on the right hand side, we have

for , thus the equality holds.

Example 1. Let's see an example problem Co-prime where this principle could be applied: Given , you need to compute the number of integers in the interval such that is coprime with , that is, . There are testcases. Constraints: , .

Solution

The standard interpretation leads to the principle of inclusion-exclusion. Suppose we are given a set , called the universe, and a set of properties that the elements of may or may not process. Here we can define the properties as we like, such as , , or even . Let Ai be the subset of elements that enjoy property (and possibly others). Then is the number of elements that process none of the properties. Clearly, is the set of elements that possess the properties (and maybe others). Using the notation

we arrive at the inclusion-exclusion principle.

Inclusion-Exclusion Principle. Let be a set, and a set of properties. Then

The formula becomes even simpler when depends only on the size . We can then write for , and call a homogeneous set of properties, and in this case also depends only on the cardinality of . Hence for homogeneous properties, we have

This is the very essence of Inclusion-Exclusion Principle . Please make sure you understand every notation before you proceed. One can figure out, by letting , we arrive at the restricted inclusion-exclusion principle.

Example 2. This problem Character Encoding requires you to compute the number of solutions to the equation , satisfying that , modulo . Constraints: . Hint: the number of non-negative integer solutions of is given by .

Solution

Example 3. Well, this one is a well-known problem. K-Inversion Permutations. The statement is neat and simple. Given N, K, you need to output the number of permutations of length N with K inversions, taken modulo . Constraint: .

Solution

История

 
 
 
 
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en27 Английский Roundgod 2022-01-07 09:58:51 9
en26 Английский Roundgod 2020-11-01 10:27:44 2
en25 Английский Roundgod 2020-10-13 21:06:15 4 Tiny change: 'irwise distinct, we have' -> 'irwise disjoint, we have'
en24 Английский Roundgod 2019-03-23 21:22:54 8 Tiny change: '-j,j)+g(i-1,j-1)-g(i-j-N,j-1)$. An' -> '-j,j)+g(i-j,j-1)-g(i-(N+1),j-1)$. An'
en23 Английский Roundgod 2019-03-22 11:04:49 13 Tiny change: 'g(i-1,j-1)$. An' -> 'g(i-1,j-1)-g(i-j-N,j-1)$. An'
en22 Английский Roundgod 2019-03-22 10:30:16 2 Tiny change: 'ts_{i\geq 1}(-1)^{i}g' -> 'ts_{i\geq 0}(-1)^{i}g'
en21 Английский Roundgod 2019-03-22 10:27:48 2 Tiny change: '-j,j)+g(i-j,j-1)$. An' -> '-j,j)+g(i-1,j-1)$. An'
en20 Английский Roundgod 2019-01-19 16:24:31 2 Tiny change: '\n$$N_{\subseteq T}:=' -> '\n$$N_{\supseteq T}:='
en19 Английский Roundgod 2019-01-19 10:10:02 2 Tiny change: '1}\cdot f(i)$$\n\nwhe' -> '1}\cdot f(s)$$\n\nwhe'
en18 Английский Roundgod 2019-01-18 23:20:27 5
en17 Английский Roundgod 2019-01-18 22:39:48 1 Tiny change: ' for the $\relax i$th eleme' -> ' for the $i$th eleme'
en16 Английский Roundgod 2019-01-18 21:44:53 7 Tiny change: '}"$. Let $A_i$ be th' -> '}"$. Let $\relax A_i$ be th'
en15 Английский Roundgod 2019-01-18 21:34:57 697 (published)
en14 Английский Roundgod 2019-01-18 21:27:56 1521
en13 Английский Roundgod 2019-01-18 21:06:41 613
en12 Английский Roundgod 2019-01-18 20:59:52 2119 Tiny change: 'ution is $O(n)$, due' -> 'ution is $\reO(n)$, due'
en11 Английский Roundgod 2019-01-18 20:23:49 1028 Tiny change: 'hen write $N_{\sups' -> 'hen write \n $N_{\sups'
en10 Английский Roundgod 2019-01-18 20:01:13 2 Tiny change: '\leq x_{i}\< m$? Cons' -> '\leq x_{i} < m$? Cons'
en9 Английский Roundgod 2019-01-18 20:00:40 1594 Tiny change: 'clusion.\n**Princi' -> 'clusion.\n\n**Princi'
en8 Английский Roundgod 2019-01-18 19:36:24 1162
en7 Английский Roundgod 2019-01-18 19:16:44 1245 Tiny change: 'm}=0$$ for $m\geq 1$,' -> 'm}=0$$ for$m\geq 1$,'
en6 Английский Roundgod 2019-01-18 18:49:31 699
en5 Английский Roundgod 2019-01-18 18:41:06 583
en4 Английский Roundgod 2019-01-18 18:21:38 58
en3 Английский Roundgod 2019-01-18 18:17:55 472
en2 Английский Roundgod 2019-01-18 18:14:24 423
en1 Английский Roundgod 2019-01-18 18:09:36 831 Initial revision (saved to drafts)