Inclusion-exclusion formula or exclusion-exclusion principle and examples

Lecture



The formula of inclusions-exceptions (or the principle of inclusions-exceptions ) is a combinatorial formula that allows you to determine the power of the union of a finite number of finite sets, which in the general case can intersect with each other. In probability theory, an analogue of the inclusion-exclusion principle is known as the Poincaré formula.

Inclusion-exclusion formula or exclusion-exclusion principle and examples

The case of two sets

For example, in the case of two sets Inclusion-exclusion formula or exclusion-exclusion principle and examples the formula of inclusions-exceptions is:

Inclusion-exclusion formula or exclusion-exclusion principle and examples

In total Inclusion-exclusion formula or exclusion-exclusion principle and examples intersection elements Inclusion-exclusion formula or exclusion-exclusion principle and examples taken into account twice, and to compensate for this, we subtract Inclusion-exclusion formula or exclusion-exclusion principle and examples from the right side of the formula. The validity of this reasoning is visible from the Euler-Venn diagram for the two sets shown in the figure on the right.

In the same way and in the case Inclusion-exclusion formula or exclusion-exclusion principle and examples sets the process of finding the number of elements of the union Inclusion-exclusion formula or exclusion-exclusion principle and examples consists in including everything, then excluding the superfluous, then including the erroneously excluded, and so on, that is, in alternate inclusion and exclusion. Hence the name of the formula.

Wording

The formula of inclusions-exceptions can be formulated in different forms.

In terms of sets

Let be Inclusion-exclusion formula or exclusion-exclusion principle and examples - finite sets. The formula of inclusions-exceptions states:

Inclusion-exclusion formula or exclusion-exclusion principle and examples

With Inclusion-exclusion formula or exclusion-exclusion principle and examples we obtain the formula for the two sets given above.

In terms of properties

The principle of inclusions-exclusions is often given in the following alternative formulation [2]. Let given a finite set Inclusion-exclusion formula or exclusion-exclusion principle and examples . Then the formula is:

Inclusion-exclusion formula or exclusion-exclusion principle and examples

The formulation of the principle of inclusions-exceptions in terms of sets is equivalent to a formulation in terms of properties. Indeed, if the sets Inclusion-exclusion formula or exclusion-exclusion principle and examples ), and the inclusion-exception formula can be rewritten as:

Inclusion-exclusion formula or exclusion-exclusion principle and examples

If now instead of "element Inclusion-exclusion formula or exclusion-exclusion principle and examples , Then we get the formulation of the principle of inclusions-exceptions in terms of properties, and vice versa.

Denote by Inclusion-exclusion formula or exclusion-exclusion principle and examples Then the formula of inclusions-exceptions can be rewritten in the following closed form ( English ) .

Inclusion-exclusion formula or exclusion-exclusion principle and examples

Evidence

There are several proofs of the formula for inclusion inclusions.

Proof by induction

The formula for inclusions-exclusions can be proved by induction [1] [3].

With Inclusion-exclusion formula or exclusion-exclusion principle and examples the inclusion-exclusion formula is trivial:

Inclusion-exclusion formula or exclusion-exclusion principle and examples

Let the formula be true for Inclusion-exclusion formula or exclusion-exclusion principle and examples .

Let each element of the set Inclusion-exclusion formula or exclusion-exclusion principle and examples :

Inclusion-exclusion formula or exclusion-exclusion principle and examples

Now apply the formula for the properties Inclusion-exclusion formula or exclusion-exclusion principle and examples :

Inclusion-exclusion formula or exclusion-exclusion principle and examples

Finally, apply the formula for one property Inclusion-exclusion formula or exclusion-exclusion principle and examples :

Inclusion-exclusion formula or exclusion-exclusion principle and examples

Combining the three formulas, we obtain the formula of inclusions-exceptions for Inclusion-exclusion formula or exclusion-exclusion principle and examples . Q.E.D. ■

Combinatorial evidence

Consider an arbitrary element Inclusion-exclusion formula or exclusion-exclusion principle and examples [2].

If the item Inclusion-exclusion formula or exclusion-exclusion principle and examples ).

Let element Inclusion-exclusion formula or exclusion-exclusion principle and examples in the right side is equal to

Inclusion-exclusion formula or exclusion-exclusion principle and examples

With Inclusion-exclusion formula or exclusion-exclusion principle and examples combination numbers are zero. The remaining sum due to the binomial theorem is equal to

Inclusion-exclusion formula or exclusion-exclusion principle and examples

Thus, the right-hand part of the formula of inclusions-exclusions takes into account each element that does not have the specified properties exactly once, and each element that has at least one of the properties zero times. Therefore, it is equal to the number of elements that do not possess any of the properties Inclusion-exclusion formula or exclusion-exclusion principle and examples . Q.E.D. ■

Proof through indicator functions

Let be Inclusion-exclusion formula or exclusion-exclusion principle and examples ).

Indicator function of their additions Inclusion-exclusion formula or exclusion-exclusion principle and examples equals

Inclusion-exclusion formula or exclusion-exclusion principle and examples

and the indicator function of the intersection of additions:

Inclusion-exclusion formula or exclusion-exclusion principle and examples

Opening the brackets in the right part and again using the fact that the indicator function of intersection of sets is equal to the product of their indicator functions, we get:

Inclusion-exclusion formula or exclusion-exclusion principle and examples

This ratio is one of the forms of the principle of inclusions-exceptions. It expresses a logical identity [1] and is true for arbitrary sets Inclusion-exclusion formula or exclusion-exclusion principle and examples its power is equal to

Inclusion-exclusion formula or exclusion-exclusion principle and examples

we obtain the formulation of the principle of inclusions-exceptions in terms of cardinalities of sets (or in terms of properties). ■

Application

Riot problem

Riot problem

A classic example of the use of the inclusion-exclusion formula is the problem of unrest [2]. Required to find the number of permutations Inclusion-exclusion formula or exclusion-exclusion principle and examples . Such permutations are called unrest.

Let be Inclusion-exclusion formula or exclusion-exclusion principle and examples unrest:

Inclusion-exclusion formula or exclusion-exclusion principle and examples

This ratio can be converted to the form

Inclusion-exclusion formula or exclusion-exclusion principle and examples

It is easy to see that the expression in brackets is a partial sum of a series Inclusion-exclusion formula or exclusion-exclusion principle and examples permutations:

Inclusion-exclusion formula or exclusion-exclusion principle and examples

Euler function calculation

Another example of using the inclusion-exception formula is finding an explicit expression for the Euler function. Inclusion-exclusion formula or exclusion-exclusion principle and examples .

Let the canonical decomposition of a number Inclusion-exclusion formula or exclusion-exclusion principle and examples on prime factors looks like

Inclusion-exclusion formula or exclusion-exclusion principle and examples

Number Inclusion-exclusion formula or exclusion-exclusion principle and examples .

amount Inclusion-exclusion formula or exclusion-exclusion principle and examples .

According to the formula of inclusions-exceptions we find

Inclusion-exclusion formula or exclusion-exclusion principle and examples

This formula is converted to the form:

Inclusion-exclusion formula or exclusion-exclusion principle and examples

Variations and generalizations

Inclusion / Exclusion Principle for Probabilities

Let be Inclusion-exclusion formula or exclusion-exclusion principle and examples [five]

Inclusion-exclusion formula or exclusion-exclusion principle and examples

This formula expresses the principle of inclusions-exceptions for probabilities. It can be obtained from the principle of inclusions-exceptions in the form of indicator functions:

Inclusion-exclusion formula or exclusion-exclusion principle and examples

Let be Inclusion-exclusion formula or exclusion-exclusion principle and examples , we obtain the inclusion-exclusion formula for probabilities.

The principle of inclusions-exclusions in spaces with a measure

Let be Inclusion-exclusion formula or exclusion-exclusion principle and examples the inclusion-exception formula is:

Inclusion-exclusion formula or exclusion-exclusion principle and examples

Obviously, the principle of inclusions-exceptions for probabilities and for the powers of finite sets are special cases of this formula. In the first case, the measure is, naturally, a probability measure in the corresponding probability space: Inclusion-exclusion formula or exclusion-exclusion principle and examples .

The principle of inclusions-exclusions for spaces with a measure can also be derived, as for the indicated special cases, from the identity for indicator functions:

Inclusion-exclusion formula or exclusion-exclusion principle and examples

Let be Inclusion-exclusion formula or exclusion-exclusion principle and examples , and we obtain the formula of inclusions-exceptions for the measure.

The identity of the highs and lows

The formula of inclusions-exceptions can be considered as a special case of the identity of maxima and minima:

Inclusion-exclusion formula or exclusion-exclusion principle and examples

This relationship holds true for arbitrary numbers. Inclusion-exclusion formula or exclusion-exclusion principle and examples , then we obtain the ratio for the indicator functions of the sets:

Inclusion-exclusion formula or exclusion-exclusion principle and examples

Mobius appeal

Let be Inclusion-exclusion formula or exclusion-exclusion principle and examples as follows:

Inclusion-exclusion formula or exclusion-exclusion principle and examples

Then the following inversion formula takes place [6] [7]:

Inclusion-exclusion formula or exclusion-exclusion principle and examples

This statement is a special case of the general Mobius inversion formula for the incidence algebra ( English ) of the set Inclusion-exclusion formula or exclusion-exclusion principle and examples .

Let us show how the inclusion-exclusion principle for finite sets follows from this formula. Let a family of subsets be given Inclusion-exclusion formula or exclusion-exclusion principle and examples . Mathematically, it can be written like this:

Inclusion-exclusion formula or exclusion-exclusion principle and examples

Then the function Inclusion-exclusion formula or exclusion-exclusion principle and examples defined by the formula

Inclusion-exclusion formula or exclusion-exclusion principle and examples

gives the number of elements, each of which is included in all the sets Inclusion-exclusion formula or exclusion-exclusion principle and examples and perhaps even more. I.e

Inclusion-exclusion formula or exclusion-exclusion principle and examples

Note further that Inclusion-exclusion formula or exclusion-exclusion principle and examples - the number of elements that do not possess any of the properties:

Inclusion-exclusion formula or exclusion-exclusion principle and examples

Taking into account the comments made, we write down the Möbius treatment formula:

Inclusion-exclusion formula or exclusion-exclusion principle and examples

This is exactly the inclusion-exception formula for finite sets, only it does not group the terms related to the same values. Inclusion-exclusion formula or exclusion-exclusion principle and examples .

Story

For the first time, the inclusion-exclusion formula was published by the Portuguese mathematician Daniel da Silva ( English ) in 1854 [1]. But back in 1713, Nikolay Bernoulli ( Eng. ) Used this method to solve the problem of Montmore ( Eng. ), Known as the problem of meetings (Fr. Le problème des rencontres ) [8], a special case of which is the problem of unrest. Also, the formula of inclusions-exclusions is associated with the names of the French mathematician Abraham de Muavre [ source not specified 1933 days ] and the English mathematician Joseph Sylvestre [9].

see also

  • Grassman Formula

The principle of inclusions-exceptions

The principle of inclusions-exceptions is an important combinatorial technique that allows you to calculate the size of any sets, or calculate the probability of complex events.

Formulations of the principle of inclusions-exceptions

Verbal wording

The principle of inclusions-exceptions is as follows:

To calculate the size of the union of several sets, we must sum up the sizes of these sets separately , then subtract the sizes of all pairwise intersections of these sets, add back the sizes of the intersections of all possible triples of sets, subtract the sizes of the intersections of fours , and so on until the intersection of all sets.

Formulation in terms of sets

In mathematical form, the above verbal formulation is as follows:

Inclusion-exclusion formula or exclusion-exclusion principle and examples

It can be written more compactly, through the sum over subsets. Denote by Inclusion-exclusion formula or exclusion-exclusion principle and examples . Then the principle of inclusions-exceptions takes the form:

Inclusion-exclusion formula or exclusion-exclusion principle and examples

This formula is attributed to Moivre (Abraham de Moivre).

Formulation with Venn Diagrams

Let the chart marked three figures Inclusion-exclusion formula or exclusion-exclusion principle and examples :

Inclusion-exclusion formula or exclusion-exclusion principle and examples

Then the area of ​​the union Inclusion-exclusion formula or exclusion-exclusion principle and examples :

Inclusion-exclusion formula or exclusion-exclusion principle and examples

Similarly, this is generalized to join. Inclusion-exclusion formula or exclusion-exclusion principle and examples figures.

Formulation in terms of probability theory

If a Inclusion-exclusion formula or exclusion-exclusion principle and examples - their probabilities, the probability of their unification (i.e., that at least one of these events occurs) is equal to:

Inclusion-exclusion formula or exclusion-exclusion principle and examples

This sum can also be written as a sum over subsets of the set Inclusion-exclusion formula or exclusion-exclusion principle and examples :

Inclusion-exclusion formula or exclusion-exclusion principle and examples

Proof of the principle of inclusions-exceptions

For the proof, it is convenient to use a mathematical formulation in terms of set theory:

Inclusion-exclusion formula or exclusion-exclusion principle and examples

Where Inclusion-exclusion formula or exclusion-exclusion principle and examples th

We need to prove that any element contained in at least one of the sets Inclusion-exclusion formula or exclusion-exclusion principle and examples , can not be taken into account, since they are absent in the right part of the formula)

Consider an arbitrary element Inclusion-exclusion formula or exclusion-exclusion principle and examples . We show that it will be considered a formula exactly once.

Notice, that:

  • in those terms in which Inclusion-exclusion formula or exclusion-exclusion principle and examples times with a plus sign;

  • in those terms in which Inclusion-exclusion formula or exclusion-exclusion principle and examples ;

  • в тех слагаемых, у которых Inclusion-exclusion formula or exclusion-exclusion principle and examples раз, со знаком плюс;

  • Inclusion-exclusion formula or exclusion-exclusion principle and examples

  • в тех слагаемых, у которых Inclusion-exclusion formula or exclusion-exclusion principle and examples ;

  • в тех слагаемых, у которых Inclusion-exclusion formula or exclusion-exclusion principle and examples учтётся ноль раз.

Таким образом, нам надо посчитать такую сумму биномиальных коэффициентов:

Inclusion-exclusion formula or exclusion-exclusion principle and examples

Проще всего посчитать эту сумму, сравнив её с разложением в бином Ньютона выражения Inclusion-exclusion formula or exclusion-exclusion principle and examples :

Inclusion-exclusion formula or exclusion-exclusion principle and examples

Видно, что при Inclusion-exclusion formula or exclusion-exclusion principle and examples , что и требовалось доказать.

Применения при решении задач

Принцип включений-исключений сложно хорошо понять без изучения примеров его применений.

Сначала мы рассмотрим три простые задачи "на бумажке", иллюстрирующие применение принципа, затем рассмотрим более практические задачи, которые трудно решить без использования принципа включений-исключений.

Особо следует отметить задачу "поиск числа путей", поскольку в ней демонстрируется, что принцип включений-исключений может иногда приводить к полиномиальным решениям, а не обязательно экспоненциальным.

Простая задачка о перестановках

Сколько есть перестановок чисел от Inclusion-exclusion formula or exclusion-exclusion principle and examples ?

Посчитаем число "плохих" перестановок, т.е. таких, у которых первый элемент Inclusion-exclusion formula or exclusion-exclusion principle and examples .

Обозначим через Inclusion-exclusion formula or exclusion-exclusion principle and examples

Inclusion-exclusion formula or exclusion-exclusion principle and examples

Проведя несложные комбинаторные вычисления, получаем, что это равно:

Inclusion-exclusion formula or exclusion-exclusion principle and examples

Отнимая это число от общего числа перестановок Inclusion-exclusion formula or exclusion-exclusion principle and examples , мы получим ответ.

Простая задачка о (0,1,2)-последовательностях

Сколько существует последовательностей длины Inclusion-exclusion formula or exclusion-exclusion principle and examples , причём каждое число встречается хотя бы раз?

Снова перейдём к обратной задаче, т.е. будем считать число последовательностей, в которых не присутствует хотя бы одно из чисел.

Обозначим через Inclusion-exclusion formula or exclusion-exclusion principle and examples

Inclusion-exclusion formula or exclusion-exclusion principle and examples

Размеры каждого из Inclusion-exclusion formula or exclusion-exclusion principle and examples (поскольку доступных цифр вообще не остаётся).

Вспоминая, что мы решали обратную задачу, получаем итоговый ответ :

Inclusion-exclusion formula or exclusion-exclusion principle and examples

Количество целочисленных решений уравнения

Дано уравнение:

Inclusion-exclusion formula or exclusion-exclusion principle and examples

где все Inclusion-exclusion formula or exclusion-exclusion principle and examples ).

Требуется посчитать число решений этого уравнения.

Забудем сначала про ограничение Inclusion-exclusion formula or exclusion-exclusion principle and examples местам:

Inclusion-exclusion formula or exclusion-exclusion principle and examples

Посчитаем теперь по формуле включений-исключений число "плохих" решений, т.е. таких решений уравнения, в которых один или более Inclusion-exclusion formula or exclusion-exclusion principle and examples .

Обозначим через Inclusion-exclusion formula or exclusion-exclusion principle and examples элементов исключены из рассмотрения и точно принадлежат первой группе. In this way:

Inclusion-exclusion formula or exclusion-exclusion principle and examples

Аналогично, мощность пересечения двух множеств Inclusion-exclusion formula or exclusion-exclusion principle and examples равна числу:

Inclusion-exclusion formula or exclusion-exclusion principle and examples

Мощность каждого пересечения трёх и более множеств равна нулю, поскольку Inclusion-exclusion formula or exclusion-exclusion principle and examples .

Объединяя всё это в формулу включений-исключений и учитывая, что мы решали обратную задачу, окончательно получаем ответ :

Inclusion-exclusion formula or exclusion-exclusion principle and examples

Количество взаимно простых чисел в заданном отрезке

Пусть даны числа Inclusion-exclusion formula or exclusion-exclusion principle and examples .

Сразу перейдём к обратной задаче — посчитаем количество не взаимно простых чисел.

Рассмотрим все простые делители числа Inclusion-exclusion formula or exclusion-exclusion principle and examples ).

Сколько чисел в отрезке Inclusion-exclusion formula or exclusion-exclusion principle and examples ? Их количество равно:

Inclusion-exclusion formula or exclusion-exclusion principle and examples

Однако если мы просто просуммируем эти числа, то получим неправильный ответ — некоторые числа будут просуммированы несколько раз (те, которые делятся сразу на несколько Inclusion-exclusion formula or exclusion-exclusion principle and examples ). Поэтому надо воспользоваться формулой включений-исключений.

Например, можно за Inclusion-exclusion formula or exclusion-exclusion principle and examples -ых, посчитать их произведение, и прибавить или вычесть в формуле включений-исключений очередное слагаемое.

Итоговая реализация для подсчёта количества взаимно простых чисел:

int solve (int n, int r) {
	vector p;
	for (int i=2; i*i<=n; ++i)
		if (n % i == 0) {
			p.push_back (i);
			while (n % i == 0)
				n /= i;
		 }
	if (n > 1)
		p.push_back (n);
 
	int sum = 0;
	for (int msk=1; msk<(1< 

Асимптотика решения составляет Inclusion-exclusion formula or exclusion-exclusion principle and examples .

Количество чисел в заданном отрезке, кратных хотя бы одному из заданных чисел

Даны Inclusion-exclusion formula or exclusion-exclusion principle and examples .

Алгоритм решения практически совпадает с предыдущей задачей — делаем формулу включений-исключений над числами Inclusion-exclusion formula or exclusion-exclusion principle and examples (иными словами, делящихся на их наименьшее общее кратное).

Таким образом, решение сводится к тому, чтобы за Inclusion-exclusion formula or exclusion-exclusion principle and examples операций найти их наименьшее общее кратное, и прибавить или вычесть из ответа очередное значение.

Количество строк, удовлетворяющих заданному числу паттернов

Given Inclusion-exclusion formula or exclusion-exclusion principle and examples паттернам.

Заметим вначале, что мы можем легко посчитать число строк , удовлетворяющих сразу всем указанным паттернам. Для этого надо просто "пересечь" эти паттерны: посмотреть на первый символ (во всех ли паттернах на первой позиции стоит вопрос, или не во всех — тогда первый символ определён однозначно), на второй символ, и т.д.

Научимся теперь решать первый вариант задачи : когда искомые строки должны удовлетворять ровно Inclusion-exclusion formula or exclusion-exclusion principle and examples паттернам.

Для этого переберём и зафксируем конкретное подмножество Inclusion-exclusion formula or exclusion-exclusion principle and examples , и либо прибавляем к текущему ответу, либо отнимаем от него количество строк, подходящих под текущее множество:

Inclusion-exclusion formula or exclusion-exclusion principle and examples

Where Inclusion-exclusion formula or exclusion-exclusion principle and examples .

Если мы просуммируем Inclusion-exclusion formula or exclusion-exclusion principle and examples , то получим ответ:

Inclusion-exclusion formula or exclusion-exclusion principle and examples

Однако тем самым мы получили решение за время порядка Inclusion-exclusion formula or exclusion-exclusion principle and examples .

Решение можно ускорить, заметив, что в разных Inclusion-exclusion formula or exclusion-exclusion principle and examples .

Перевернём формулу включений-исключений и будем вести суммирование по Inclusion-exclusion formula or exclusion-exclusion principle and examples :

Inclusion-exclusion formula or exclusion-exclusion principle and examples

Решение получилось с асимптотикой Inclusion-exclusion formula or exclusion-exclusion principle and examples .

Перейдём теперь ко второму варианту задачи : когда искомые строки должны удовлетворять как минимум Inclusion-exclusion formula or exclusion-exclusion principle and examples паттернам.

Понятно, мы можем просто воспользоваться решением первого варианта задачи и просуммировать ответы от Inclusion-exclusion formula or exclusion-exclusion principle and examples .

Таким образом, в итоговой формуле перед Inclusion-exclusion formula or exclusion-exclusion principle and examples будет стоять другой коэффициент: не один биномиальный коэффициент с каким-то знаком, а их сумма:

Inclusion-exclusion formula or exclusion-exclusion principle and examples

Заглянув в Грэхема (Грэхем, Кнут, Паташник. "Конкретная математика" [1998] ), мы видим такую известную формулу для биномиальных коэффициентов:

Inclusion-exclusion formula or exclusion-exclusion principle and examples

Применяя её здесь, получаем, что вся эта сумма биномиальных коэффициентов сворачивается в:

Inclusion-exclusion formula or exclusion-exclusion principle and examples

Таким образом, для этого варианта задачи мы также получили решение с асимптотикой Inclusion-exclusion formula or exclusion-exclusion principle and examples :

Inclusion-exclusion formula or exclusion-exclusion principle and examples

Количество путей

Есть поле Inclusion-exclusion formula or exclusion-exclusion principle and examples , избежав все препятствия. Требуется посчитать число путей, которыми он может это сделать.

Предполагаем, что размеры Inclusion-exclusion formula or exclusion-exclusion principle and examples ).

Для решения сразу в целях удобства отсортируем препятствия в том порядке, в каком мы можем их обойти: т.е., например, по координате Inclusion-exclusion formula or exclusion-exclusion principle and examples .

Также сразу научимся решать задачу без препятствий: т.е. научимся считать число способов дойти от одной клетки до другой. Если по одной координате нам надо пройти Inclusion-exclusion formula or exclusion-exclusion principle and examples клеток, то из несложной комбинаторики мы получаем такую формулу через биномиальные коэффициенты:

Inclusion-exclusion formula or exclusion-exclusion principle and examples

Теперь чтобы посчитать число способов дойти от одной клетки до другой, избежав всех препятствий, можно воспользоваться формулой включений-исключений : посчитаем число способов дойти, наступив хотя бы на одно препятствие.

Для этого можно, например, перебрать подмножество тех препятствий, на которые мы точно наступим, посчитать число способов сделать это (просто перемножив число способов дойти от стартовой клетки до первого из выбранных препятствий, от первого препятствия до второго, и так далее), и затем прибавить или отнять это число от ответа, в соответствии со стандартной формулой включений-исключений.

Однако это снова будет неполиномиальное решение — за асимптотику Inclusion-exclusion formula or exclusion-exclusion principle and examples . Покажем, как получить полиномиальное решение .

Решать будем динамическим программированием : научимся вычислять числа Inclusion-exclusion formula or exclusion-exclusion principle and examples точки, поскольку к препятствиям добавляются стартовая и конечная клетки.

Если мы на секунду забудем про все препятствия и просто посчитаем число путей из клетки Inclusion-exclusion formula or exclusion-exclusion principle and examples

Таким образом, значение Inclusion-exclusion formula or exclusion-exclusion principle and examples .

Число взаимно простых четвёрок

Given Inclusion-exclusion formula or exclusion-exclusion principle and examples . Требуется посчитать количество способов выбрать из них четыре числа так, что их совокупный наибольший общий делитель равен единице.

Будем решать обратную задачу — посчитаем число "плохих" четвёрок, т.е. таких четвёрок, в которых все числа делятся на число Inclusion-exclusion formula or exclusion-exclusion principle and examples .

Воспользуемся формулой включений-исключений, суммируя количество четвёрок, делящихся на делитель Inclusion-exclusion formula or exclusion-exclusion principle and examples (но, возможно, делящихся и на больший делитель):

Inclusion-exclusion formula or exclusion-exclusion principle and examples

Where Inclusion-exclusion formula or exclusion-exclusion principle and examples .

Чтобы посчитать функцию Inclusion-exclusion formula or exclusion-exclusion principle and examples , и биномиальным коэффициентом посчитать число способов выбрать из них четвёрку.

Таким образом, с помощью формулы включений-исключений мы суммируем количество четвёрок, делящихся на простые числа, затем отнимаем число четвёрок, делящихся на произведение двух простых, прибавляем четвёрки, делящиеся на три простых, и т.д.

Число гармонических троек

Дано число Inclusion-exclusion formula or exclusion-exclusion principle and examples , что они являются гармоническими тройками, т.е.:

  • либо Inclusion-exclusion formula or exclusion-exclusion principle and examples ,
  • либо Inclusion-exclusion formula or exclusion-exclusion principle and examples .

Во-первых, сразу перейдём к обратной задаче — т.е. посчитаем число негармонических троек.

Во-вторых, заметим, что в любой негармонической тройке ровно два её числа находятся в такой ситуации, что это число взаимно просто с одним числом тройки и не взаимно просто с другим числом тройки.

Thus, the number of non-harmonic triples is equal to the sum of all the numbers from the Inclusion-exclusion formula or exclusion-exclusion principle and examplesproducts of the number of coprime with the current number of numbers and the number of non-coprime numbers.

Now all that is left for us to solve the problem is to learn how to count for each number in the segment Inclusion-exclusion formula or exclusion-exclusion principle and examples, and then look through all sorts of products of prime numbers from factorization.

Therefore, we need a faster solution that counts the answers for all the numbers in the segment Inclusion-exclusion formula or exclusion-exclusion principle and examples right away

For this you can implement this modification of the sieve of Eratosthenes :

Definition:
The inclusion-exclusion formula is a combinatorial formula expressing the power of combining finite sets in terms of the powers and powers of all their possible intersections.

Inclusion-exclusion formula or exclusion-exclusion principle and examples

The case for two sets

For the case of two sets Inclusion-exclusion formula or exclusion-exclusion principle and examples the inclusion-exclusion formula is as follows:

Inclusion-exclusion formula or exclusion-exclusion principle and examples

Due to the fact that Inclusion-exclusion formula or exclusion-exclusion principle and examples taken into account twice, then we reduce the current value of the sum by the intersection power so that each element is counted exactly once. For clarity, we use the Euler – Venn diagram for the two sets shown in the figure to the right.

For the case with a large number of considered sets Inclusion-exclusion formula or exclusion-exclusion principle and examples the process of finding the number of elements of the union consists of alternately including erroneously excluded and exceptions erroneously included. Hence the name of the formula.

We formulate and prove a theorem for finding the cardinality of the union of an arbitrary number of sets.

Theorem :

Let be Inclusion-exclusion formula or exclusion-exclusion principle and examples .

Evidence:
Inclusion-exclusion formula or exclusion-exclusion principle and examples

We present two diverse proofs of the theorem.

I. Combinatorial proof of the theorem.

Consider some element Inclusion-exclusion formula or exclusion-exclusion principle and examples in the right side of the formula.

Inclusion-exclusion formula or exclusion-exclusion principle and examples

Prove that Inclusion-exclusion formula or exclusion-exclusion principle and examples

Due to the fact that Inclusion-exclusion formula or exclusion-exclusion principle and examples then the equality is proven.

In this way, Inclusion-exclusion formula or exclusion-exclusion principle and examples , that is, each element is counted in the right part of the formula exactly once, then the theorem is proved.

Ii. Proof of the theorem by induction.

Let be Inclusion-exclusion formula or exclusion-exclusion principle and examples - base of induction.

Suppose for Inclusion-exclusion formula or exclusion-exclusion principle and examples

Let be Inclusion-exclusion formula or exclusion-exclusion principle and examples .

On the assumption of induction, we have that Inclusion-exclusion formula or exclusion-exclusion principle and examples

Also, since the formula is true for Inclusion-exclusion formula or exclusion-exclusion principle and examples :

It's obvious that Inclusion-exclusion formula or exclusion-exclusion principle and examples

Based on the assumption of induction and equality Inclusion-exclusion formula or exclusion-exclusion principle and examples

Substitute the obtained values ​​in Inclusion-exclusion formula or exclusion-exclusion principle and examples :

Inclusion-exclusion formula or exclusion-exclusion principle and examples

Prove that Inclusion-exclusion formula or exclusion-exclusion principle and examples

Equality is fair because all sets Inclusion-exclusion formula or exclusion-exclusion principle and examples can be divided into two groups:

  1. Inclusion-exclusion formula or exclusion-exclusion principle and examples .
  2. Inclusion-exclusion formula or exclusion-exclusion principle and examples .

As can be seen from the equality, the first and third terms are “responsible” for the second group, and the second term is for the first group. So equality is true and Inclusion-exclusion formula or exclusion-exclusion principle and examples .

So for Inclusion-exclusion formula or exclusion-exclusion principle and examples we proved that equality is true. Hence, the induction step is correct, that is, the theorem is proved.
Inclusion-exclusion formula or exclusion-exclusion principle and examples

Disorder

Definition:
Derangement is a permutation of numbers from Inclusion-exclusion formula or exclusion-exclusion principle and examples in which no element stands in its place.
Theorem :

Number of order disorder Inclusion-exclusion formula or exclusion-exclusion principle and examples

Evidence:
Inclusion-exclusion formula or exclusion-exclusion principle and examples

We use the principle of inclusion-exclusion: denote by Inclusion-exclusion formula or exclusion-exclusion principle and examples th element is in place. Then, by the inclusion-exclusion formula, we have:

Inclusion-exclusion formula or exclusion-exclusion principle and examples .

Inclusion-exclusion formula or exclusion-exclusion principle and examples place

In this way Inclusion-exclusion formula or exclusion-exclusion principle and examples , that is, the number of riots sought.

Inclusion-exclusion formula or exclusion-exclusion principle and examples

Will consider Inclusion-exclusion formula or exclusion-exclusion principle and examples . Thus, we obtain that:

Inclusion-exclusion formula or exclusion-exclusion principle and examples

Substituting the corresponding values ​​of cardinalities into the inclusion-exclusion formula, we get:

Inclusion-exclusion formula or exclusion-exclusion principle and examples

Revealing Inclusion-exclusion formula or exclusion-exclusion principle and examples .
Inclusion-exclusion formula or exclusion-exclusion principle and examples
  • First, we need to find all the numbers in the segment Inclusion-exclusion formula or exclusion-exclusion principle and examples , in which factoring simple no part of twice. In addition, for the formula of inclusions-exceptions, we need to know how many primes contain the factorization of each such number.

    For this we need to have arrays Inclusion-exclusion formula or exclusion-exclusion principle and examples or not.

    After that, during the sieve of Eratosthenes, when processing the next prime number, we will go through all the numbers that are a multiple of the current number and increase Inclusion-exclusion formula or exclusion-exclusion principle and examples .

  • Secondly, we need to calculate the answer for all numbers from Inclusion-exclusion formula or exclusion-exclusion principle and examples - The number of numbers that are not mutually simple with the data.

    To do this, we recall how the inclusion-exception formula works - here, in fact, we implement it, but with inverted logic: we sort of iterate over the term and look at which inclusion-exception formulas for which numbers this term is included.

    So let us have a number Inclusion-exclusion formula or exclusion-exclusion principle and examples is odd, it is necessary to add, otherwise subtract.

Implementation :

 int n;
 bool good [MAXN];
 int deg [MAXN], cnt [MAXN];
 
 long long solve () {
	 memset (good, 1, sizeof good);
	 memset (deg, 0, sizeof deg);
	 memset (cnt, 0, sizeof cnt);
 
	 long long ans_bad = 0;
	 for (int i = 2; i <= n; ++ i) {
		 if (good [i]) {
			 if (deg [i] == 0) deg [i] = 1;
			 for (int j = 1; i * j <= n; ++ j) {
				 if (j> 1 && deg [i] == 1)
					 if (j% i == 0)
						 good [i * j] = false;
					 else
						 ++ deg [i * j];
				 cnt [i * j] + = (n / i) * (deg [i]% 2 == 1? +1: -1);
			 }
		 }
		 ans_bad + = (cnt [i] - 1) * 1ll * (n-1 - cnt [i]);
	 }
 
	 return (n-1) * 1ll * (n-2) * (n-3) / 6 - ans_bad / 2;
 } 

The asymptotics of such a solution is Inclusion-exclusion formula or exclusion-exclusion principle and examples nested loop iterations.

The number of permutations without fixed points

Let us prove that the number of length permutations Inclusion-exclusion formula or exclusion-exclusion principle and examples without fixed points is equal to the following number:

Inclusion-exclusion formula or exclusion-exclusion principle and examples

and approximately equal to the number:

Inclusion-exclusion formula or exclusion-exclusion principle and examples

(moreover, if you round this expression to the nearest integer, you get exactly the number of permutations without fixed points)

Denote by Inclusion-exclusion formula or exclusion-exclusion principle and examples ).

We now use the inclusion-exception formula to calculate the number of permutations with at least one fixed point. To do this, we need to learn how to count the sizes of intersection sets. Inclusion-exclusion formula or exclusion-exclusion principle and examples , they look like this:

Inclusion-exclusion formula or exclusion-exclusion principle and examples

because if we know that the number of fixed points is Inclusion-exclusion formula or exclusion-exclusion principle and examples items can stand anywhere.

Substituting this into the inclusion-exception formula and considering that the number of ways to choose a subset of the size Inclusion-exclusion formula or exclusion-exclusion principle and examples , we get the formula for the number of permutations with at least one fixed point:

Inclusion-exclusion formula or exclusion-exclusion principle and examples

Then the number of permutations without fixed points is:

Inclusion-exclusion formula or exclusion-exclusion principle and examples

Simplifying this expression, we obtain an exact and approximate expression for the number of permutations without fixed points :

Inclusion-exclusion formula or exclusion-exclusion principle and examples

(since the amount in parentheses is the first Inclusion-exclusion formula or exclusion-exclusion principle and examples )

In conclusion, it is worth noting that the problem is solved in a similar way, when it is required that there are no fixed points among the first m permutation elements (and not among all the elements we just solved). The formula will be such as the exact formula given above, but the sum in it will go to k , and not to n .

Examples with a detailed solution.

Problem 1. Of 100 schoolchildren, 42 know English, 30 German speak French, 28 speak English and German, 10 speak English and French, 8 speak German and French, 8 speak English and French and French. How many students do not know a single language?

Decision. I way.

Denote by A - the set of schoolchildren who know English; N - many schoolchildren who know German; F - many schoolchildren who know French.

Then n (A) = 42, n (N) = 30, n (F) = 28, n (A ∩ N) = 5,

n (A ∩ F) = 10, n (N F) = 8, n (A N F) = 3.

Using the formula for inclusions and exceptions, we find the number of students who know at least one of the listed foreign languages.

n (A ∪ N ∪ F) = n (A) + n (N) + n (F) =

= n (A ∩ N) - n (A ∩ F) - n (N F) + n (A N F) =

= 42 + 30 + 28 - 5 - 10 - 8 + 3 = 80.

Consequently, they do not know a single foreign language:

100 - 80 = 20 schoolchildren.

II way.

The same problem can be solved using the Euler-Venn diagram (Fig. 1).

Inclusion-exclusion formula or exclusion-exclusion principle and examples

Since 3 students know 3 languages, English and German know 5 - 3 = 2, English and French - 10 - 3 = 7,

German and French - 8 - 3 = 5 students.

Only English know 42 - (2 + 3 + 7) = 30,

German only - 30 - (2 + 3 + 5) = 20,

French only - 28 - (3 + 5 + 7) = 13 schoolchildren.

Not a single language does not know 100 - (2 + 3 + 5 + 7 + 13 + 20 + 30) = 20 schoolchildren.

Problem 2. How many two-digit numbers are not divided either by 2, or 3, or 5, or 11?

Decision. Denote: A - the set of two-digit numbers, divisible by 2;

B is the set of two-digit numbers divisible by 3;

C is the set of two-digit numbers divisible by 5;

D is the set of two-digit numbers divisible by 11.

n (A ∪ B ∪ C ∪ D) is the number of two-digit numbers divisible by at least one of the numbers 2; 3; five; eleven.

n (A ∪ B ∪ C ∪ D) = n (A) + n (B) + n (C) + n (D) -

- n (A ∩ B) - n (A ∩ C) - n (A ∩ D) - n (B C) -

- n (B ∩ D) - n (C ∩ D) + n (A ∩ B ∩ C) + n (A B ∩ D) +

+ n (A ∩ C ∩ D) + n (B ∩ C ∩ D) - n (A ∩ B ∩ C ∩ D).

n (A) = 45, n (B) = 30, n (C) = 18, n (D) = 9,

n (A ∩ B) = 15, n (A ∩ C) = 9, n (A ∩ D) = 4, n (B C) = 6,

n (B ∩ D) = 3, n (C ∩ D) = 1, n (A ∩ B C) = 3,

n (A ∩ B ∩ D) = 1, n (A ∩ C ∩ D) = n (B C ∩ D) = n (A ∩ B ∩ C ∩ D) = 0.

So, n (A ∪ B ∪ C ∪ D) = 45 + 30 +18 + 9 - 15 - 9 - 4 - 6 - 3 - 1 + 3 + 1 = 68.

Since there are a total of 90 two-digit numbers, there are numbers that are not divisible by any of the given numbers:

90 - 68 = 22.

Problem 3. From n students it’s known to be fond of a students, b programming, mathematics c, sports and programming d, sports and mathematics e, programming and mathematics f, sports, mathematics and programming g students. How many students are interested only in programming? How many students are interested only in mathematics? How many students are not fond of anything?

Option

n

a

b

c

d

e

f

g

14

70

32

21

23

eight

12

four

3

Decision. Let A be a lot of students who are interested in sports,

B - programming, C - mathematics.

Then | A | = 32, | B | = 21, | C | = 23, | A ∩ B | = 8, | A ∩ C | = 12, | B ∩ C | = 4 | A ∩ B ∩ C | = 3

| (A ∩ B) ∪ (B ∩ C) | = | A ∩ B | + | B ∩ C | - | A ∩ B ∩ C | = 8 + 4 - 3 = 9

Inclusion-exclusion formula or exclusion-exclusion principle and examples

created: 2014-10-30
updated: 2023-07-01
134215



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Discrete Math. Set theory. Graph theory. Combinatorics.

Terms: Discrete Math. Set theory. Graph theory. Combinatorics.