Pigeonhole principle
EXT : Pigeonhole Principle
Let q 1 , q 2 , ⋯ q n q_1,q_2,\cdots q_n q 1 , q 2 , ⋯ q n be positive integer, put − n + 1 + ∑ i = 1 n q i -n + 1 + \sum_{i=1}^{n} q_i − n + 1 + i = 1 ∑ n q i items into n n n containers, either the first contains at least q 1 q_1 q 1 items, or the second contains at least q 2 q_2 q 2 items, …, or the n n n th contains at least q n q_n q n items.
中國剩餘定理(CRT)
m 1 , m 2 , ⋯ , m r ∈ N m_1,m_2, \cdots , m_r \in \mathbb{N} m 1 , m 2 , ⋯ , m r ∈ N are pairwise coprime , so for all a 1 , a 2 , ⋯ , a r a_1 , a_2 , \cdots ,a_r a 1 , a 2 , ⋯ , a r we can find a x x x s.t.
x = a i ( m o d m i ) ∀ i ∈ { 1 , 2 , ⋯ , r } x = a_i ( \bmod m_i ) \quad \forall i \in \{1,2 ,\cdots, r\} x = a i ( m o d m i ) ∀ i ∈ { 1 , 2 , ⋯ , r }
let M = ∏ i = 1 r m i M = \prod _{i=1}^{r} m_i M = i = 1 ∏ r m i , t i t_i t i is the inverse element for M m i ( m o d m i ) {M \over m_i} (\bmod m_i) m i M ( m o d m i ) .
x = ∑ i = 1 r a i M m i t i x = \sum_{i=1}^{r} a_i {M \over m_i} t_i x = i = 1 ∑ r a i m i M t i is a solution
Ramsey 定理
Ramsey定理實際上是鴿巢原理的加強形式的擴展。
問題的引入
K 6 → K 3 , K 3 K_6 \rightarrow K_3,K_3 K 6 → K 3 , K 3 : K 6 K_6 K 6 中僅有紅藍兩種顏色的邊,一定存在一個紅色的K 3 K_3 K 3 或者藍色的K 3 K_3 K 3 。
Ramsey 定理
若存在最小整數p p p 使得K p → K m , K n K_p \rightarrow K_m,K_n K p → K m , K n ,記做p = r ( m , n ) p = r(m,n) p = r ( m , n ) 爲Ramsey數,這樣的數一定存在。
Ramsey數的結論
r ( 2 , n ) = r ( n , 2 ) = n r(2,n) = r(n,2) = n r ( 2 , n ) = r ( n , 2 ) = n
r ( m , n ) ≤ r ( m − 1 , n ) + r ( m , n − 1 ) r(m,n) \le r(m-1,n) + r(m,n-1) r ( m , n ) ≤ r ( m − 1 , n ) + r ( m , n − 1 )
r ( 3 , 4 ) = 9 r(3,4) = 9 r ( 3 , 4 ) = 9
r(3,4)= 9的證明
Ramsey 定理的推廣形式
滿足條件K p → K n 1 , K n 2 , ⋯ , K n l K_p \rightarrow K_{n_1} , K_{n_2} , \cdots, K_{n_l} K p → K n 1 , K n 2 , ⋯ , K n l 的最小整數稱爲r ( n 1 , n 2 , ⋯ , n l ) r(n_1,n_2,\cdots , n_l) r ( n 1 , n 2 , ⋯ , n l )
r(3,3,3) = 17
Ramsey 更一般的形式
給定一正整數t t t ,及q 1 , q 2 , ⋯ q k ≥ t q_1,q_2,\cdots q_k \ge t q 1 , q 2 , ⋯ q k ≥ t ,存在一個整數p p p ,將其中每一個t t t 元素子集指定爲k k k 中顏色c 1 , c 2 , ⋯ , c k c_1,c_2,\cdots,c_k c 1 , c 2 , ⋯ , c k 中的一種,滿足:
存在q 1 q_1 q 1 個元素,所有t t t 子集都被染成指定顏色c 1 c_1 c 1
… …
存在q k q_k q k 個元素,所有t t t 子集都被染成指定顏色c k c_k c k
則r t ( q 1 , ⋯ , q k ) r_t(q_1,\cdots,q_k) r t ( q 1 , ⋯ , q k ) 爲最小的p p p
特例
The Strong form of Pigeonhole Principle :r 1 ( q 1 , q 2 , ⋯ , q k ) = q 1 + q 2 + ⋯ + q k + n − 1 r_1(q_1,q_2,\cdots,q_k) = q_1 + q_2 + \cdots + q_k + n - 1 r 1 ( q 1 , q 2 , ⋯ , q k ) = q 1 + q 2 + ⋯ + q k + n − 1