In a card game, each card is associated with a numerical value from 1 to 100, with each card beating less, with one exception: 1 beats 100. The player knows that 100 cards with different values lie in front of him. The dealer who knows the order of these cards can tell the player which card beats the other for any pair of cards he draws. Prove that the dealer can make one hundred such messages, so that after that the player can accurately determine the value of each card.
Problem
Source: All-Russia 2018 Grade 9 P7
Tags: combinatorics
02.05.2018 17:21
After considering this problem hard, I finally found the following. Let $a_i$ be the card written $i$. The dealer write the following one hundred messages; $a_1$ beats $a_{100}$, $a_3$ beats $a_1$, $a_{100}$ beats $a_3$, $a_3$ beats $a_2$, $a_4$ beats $a_3$, $\ldots$, $a_{99}$ beats $a_{98}$. Then, we show that only $a_i=i$ for any $i=1,2,\ldots,100$ is possible. Since the first three messages are "trilemma", two of $a_1, a_{100}, a_3$ contains 1 and 100. But $a_3$ is beaten by two others and beats two others, it cannot be 1 or 100. Thus, $a_1=1, a_{100}=100$. From other 97 messages, $a_2, a_3, \ldots, a_{99}$ are sorted order and none of them are 1 or 100, we must have $a_2=2, a_3=3, \ldots, a_{99}=99$.
17.07.2018 11:59
There is no way where $n=3$. Actually, 3 information is sufficient to find exact numbers where $n=4$ : $a_{2}$ beats $a_{1}$, $a_{3}$, and $a_{4}$ beats $a_{3}$. If $a_{2}$ is $4$, $a_{4}=1$ beats someone except $4$ which is contradiction. So $a_{2}$ must be $3$. It leads to conclusion that $a_{1}=1, a_{2}=3, a_{3}=2, a_{4}=4$. Unfortunately I found the ineffective way where $n=5$, i can't help but trying to using inductive ways.(if $n=5$, $a_{1}<a_{2}<a_{3}<a_{4}>a_{1}$, $a_{3}>a_{5}$) Let's suppose induction holds where $4<k<n$, It says that we could find sub-graph containing $k$ edges which have only one way to label $k$ vertices. in case of $k=n$, we erase one number except $1,n$. By inductive hypothesis, there is sub-graph meeting the condition. After you erase a number, let's see direct of edges between erasing number and the remaining in every case. If you get sub-graph, you can find a unique di-graph by comparing $k$ with some number of di-graph. Actually, you always get unique di-graph by erasing $2$ because that number is the smallest except $1$.
24.08.2020 05:39
We write $i>j$ iff $i$ beats $j$ CLAIM. The only solution to the system \begin{align} a_{100}&<a_{1}\\ a_{3}&<a_{100}\\ a_1&<a_3\\ a_2<a_3&<a_4<...<a_{99} \end{align}is $a_i=i$ for every $1\leq i\leq 100$. Proof. Firstly if $2\leq a_1,a_3,a_{100}\leq {99}$ then from $a_1>a_{100}$ and $a_{100}>a_3$ we have $a_3>a_1$. Therefore at least one of $a_1,a_3,a_{100}$ is in the set $\{1,100\}$. It is easy to see that the only possibility is that one of $(a_{100},a_1),(a_3,a_{100}),(a_1,a_3)$ is equal to $(100,1)$. Therefore, we have $2\leq a_2,a_4,...,a_{99}\leq 99$. Now suppose on the contrary that $a_3=1$ then $a_3>a_2$ where $a_2\neq 100$ which is impossible, similarly $a_3\neq 100$. Hence we must have $(a_{100},a_1)=(100,1)$. Now from $2\leq a_2,a_3,a_4,...,a_{99}\leq 99$ and $(4)$ we easily obtain $a_i=i$ for every $1\leq i\leq 100$$\blacksquare$ Since the dealre knows the order of the card, he can offer the message which consists of the above system and we are done.