6.Counting

Basic Principles

基本原理 $|S\cup T|=|S|+|T|-|S\cap T|$

$|S\times T|=|S|\times|T|$

Eg1. $|A|=m,|B|=n,f:A\to B$

$n^m$ functions 给A中每一个元素选一个对应的

$n(n-1)(n-1)\dots(n-m+1)$ one-to-one functions($m\leq n$). 从B中选m个元素(有顺序),对应于A

$n! S(m,n)$ onto functions($m \geq n$)

$S(m,n)$ is Stirling number of the second kind. the number of ways of dividing A into n subsets. We can use inclusion-exclusion principle

\[ n!S(m,n)=\sum_{k=0}^n (-1)^kC_n^k (n-k)^m \]

减去不是满射的，第k项表示我们在$B$中钦定$k$个不被映射到的,其他的$n-k$个随便被A映射

Pigeonhole principle(!)

Eg1. In a party of 2 or more people,there are 2 people with the same number of friends in the party. You can't be your own friend and friendship is mutual

Pigeons: n people;

Pigeonholes: Possible number of friends:0,1,2...n-1. But 0 and n-1 can't coexist.(n-1 means one knows everyone in the party, because friendship is mutual, then everyone must have a friend). so only n-1 possible number of friends

The generalized pigeonhole principle: $n$ elements in $k$ boxes, there is at least one box containing at least $\lceil n/k\rceil$ elements

关键是构造$n,k,\lceil n/k\rceil$中的两个。注意$n$的计算, 如$k=2,\lceil n/k\rceil=3$, 则n最小可为5

Eg2. Show that among any n + 1 positive integers not exceeding 2n there must be an integer that divides one of the other integers

$a_i=2^{k_i}q_i$, $q_i$ is odd. pigeons:$q_1,q_2 \dots q_{n+1}$ pigeonholes: n odd possitive integers less than 2n. So two of $q_1,q_2 \dots q_{n+1}$ must be equal.

Eg3. During a month with 30 days, a baseball team plays at least one game a day, but no more than 45 games. Show that there must be a period of some number of consecutive days during which the team must play exactly 14 games.

对于连续区间的问题，考虑构造前缀和。Let $s_i$ be the sum of games played from the 1st day to i th day. We need to prove that $\existssss i,j(i<j),s_j-s_i=14$ . Let $s_0=0$

Pigeons: $s_0,s_1,s_2\dots s_{30}, s_0+14,s_1+14,s_2+14,\dots s_{30}+14$. (62 Elements) 书上的写法好像没有考虑到$s_0$?

Pegeonholes: values $0 \leq s_i,s_i+14 \leq 45+14=59$ (60 elements)

So there must be equal numbers in $s_1,s_2\dots s_{30}, s_1+14,s_2+14,\dots s_{30}+14$

法一：加入$s_0$

法二: 分情况(如果刚好一一对应,那么一定有一个)（如果没有，根据鸽笼原理得出)

参考

Eg4. Every sequence of $n^2 + 1$ distinct real numbers contains a subsequence of length n + 1 that is either strictly increasing or strictly decreasing

$(i_k,d_k)$ denotes the length of increasing and decreasing sequence start with $a_k$.
Suppose that $\max \{i_i,d_k\}\leq n$,with $n^2$ pairs and $n^2+1$ elements, there must exist two equal pairs. $i_s=i_t,d_s=d_t$. if $a_s>a_t$, then $a_s,a_t \dots$ forms a decreasing sequence of length $i_t+1$ starting with $i_s$.

For every integer n,there there is a multiple of n whose decimal expansion contains only 0s and 1s

For every integer n, consider n+1 integers$1,11,111\dots,111111(n+1 \ 1s)$

Divide them by n, there are at most n different reminders. so at least two of these numbers have the same reminder.

Suppose that $a\equiv b \pmod n,a<b$ then $b-a$ is a multiple of n and its decimal expansion contains only 0s and 1s.(etc.11111-11=11100) ref

Mutual friend and enemies . Ramsey number.

Ramsey number

https://math.mit.edu/~apost/courses/18.204_2018/ramsey-numbers.pdf

The minimum number of people at a party such that for all cases, there are either m mutual friends or n mutual enemies (people=vertices, relation=edges, color the edge in two color,the least number of vertices that a graph must have so that in any red-blue coloring, there exists either a red $K_s$ or a blue $K_t$($K_n$代表完全图))

$R(2,n)=n$

$R(3,3)=6, R(3,4)=9,R(4,4)=18,R(3,5)=14$

证明思路: - 先证明$R(s,t)>n-1$, 只需要找一个$n-1$个点的,既没有$K_s$也没有$K_t$的图 - 再证明$R(s,t)=n$. 一般要选一个点$v$出来，考虑剩下的和$v$是friends 还是enemies

$R(s,t)\leq R(s,t-1)+R(s-1,t)$

if $R(s,t-1),R(s-1,t)$ is even, $R(s,t)\leq R(s,t-1)+R(s-1,t)$

proof: see pdf

etc $R(3,4)\leq R(3,3)+R(2,4)-1=9$

choose $A$, in the rest 8 people

$\geq 6$ friends ,$R(3,3)=6$, so 3 mutual friends
$\geq 4$ enemies$R(2,4)=4$, so 4 mutual enemies
$5$ friends+3 enemies(impossible for every vertice,because 5*9=45,but each friends is count twice, the result should be even) so there exist at least one vertex that satisfy case 1,2.

Permutations and Combinations

Combinatorial Proofs

证明组合恒等式的时候，可以构造组合的例子。注意格式!

用两种不同的方法计数同一个集合(double counting proof)
证明两个集合之间存在双射(Bijective proof)

Pascal's Identity.

\[\binom{n+1}{k}=\binom{n}{k-1}+\binom{n}{k} \]

Proof:（Double Counting). $A=\{x,a_1,\dots a_n\}$ The left side is the number of subsets of size k from set A. For the right side, the subsets of size k

contains $x$
not contain $x$

Vandemonde's Identity:

\[ \binom{m+n}{r}=\sum_{k=0}^r\binom{m}{r-k}\binom{n}{k} \]

Proof:（Double Counting)

Corrollary $\binom{2n}{r}=\sum \binom{n}{r}^2$

\[ \binom{n+1}{r+1}=\sum_{j=r}^n\binom{j}{r} \]

Proof: Left side: r+1 ones in a 01 sequence of length n+1.

Right side: j+1 is the position of the last 1-bit in the sequence

Generalized Permutations

r-circle

$P(n,r)/r$ . r people sit in a round table of n chairs.

如果把顺时针/逆时针的看做一种，那么方法数是$\begin{cases} \frac{P(n,r)}{2r} ,r \geq 3\\ \frac{P(n,r)}{r},r \leq 2\end{cases}$. 注意只有1或2个人的时候

r-combination

$C(n+r-1,r-1)$ (插板法)

$x_1+x_2+\dots x_r=n$ 非负整数解个数

变形:

$x_1+x_2+\dots x_r=n,x_i\geq 2$. Let $y_i=x_i+2$
$x_1+x_2+\dots x_r \leq n$. Let $x_1+x_2+\dots x_r+x_{r+1}=n,x_{r+1}\geq 0$.
$x_i\leq k$. 减去$x_i \geq k+1$的情况

Distributing objects into boxes

distinguishable boxes. distinguishable objects.

$$ \frac{n!}{n_1!n_2!\dots n_k!} (\text{the kth boxes have }n_k\text{ objects})

$$

indistinguishable boxes. distinguishable objects.

(The second kind of Strling number: n objects in m same boxes, no empty boxes)

\[ \boxed{S(n,m)=\frac{1}{m!}\sum_{k=0}^m (-1)^kC_m^k (m-k)^n} \]

\[ S(r,2)=\frac{2^n-2}{2}\\ S(r,r-1)=C(r,2)\\ S(r+1,n)=S(r,n-1)+nS(r,n) \]

If we allow boxes to be empty, then the number will be $\sum_{k=1}^m S(n,k)$ 如果题目没有特别说明，默认是可以为空的，要对斯特林数求和!

distinguishable boxes. indistinguishable objects. (r-combination) $C(n+r-1,r-1)$
indistinguishable boxes. indistinguishable objects. (brute force)

(HW) How many ways are there for a horse race with 4 horses to finish if ties(平局) are possible.

no tie $4!$
2 tie(aabb+aabc) $C(4,2)\text{(choose 2 horses to be the first two)}+C(4,2)\times 3!$
3 tie(aaab) $C(4,3)\times 2$
4 tie $1$

Algorithms generating permutations and combinations

next-permutation

124653-> 125346