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Difference between revisions of "1999 USAMO Problems/Problem 1"
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== Solution ==
 
== Solution ==
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for the proof lets look at the checkers board as a graph G, where the checkers are the vertices and the edges are every pair of checkers sharing a side.
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For the proof let's look at the checkers board as a graph <math>G</math>, where the checkers are the vertices and the edges are every pair of checkers sharing a side.
  
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define R as the circuit rank of G(the minimum number of edges to remove from a graph to remove all its cycles).
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Define <math>R</math> as the circuit rank of <math>G</math>(the minimum number of edges to remove from a graph to remove all its cycles).
  
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define <math>x</math> as the number of checkers placed on the board.
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Define <math>x</math> as the number of checkers placed on the board.
  
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1. from (a) for every square containing a checker there can be up to 4 distinct squares without checkers thus, <math>4\times x + x = 5\times x >= n^2</math> (this is the most naive upper bound).
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1. From (a) for every square containing a checker there can be up to 4 distinct squares without checkers; thus, <math>4\times x + x = 5\times x \ge n^2</math> (this is the most naive upper bound).
  
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2. from (b) this graph has to be connected. which means that no square which contains a checker can share 4 sides with squares that doesn't contain a checker(because it has to share at least one side with a square that  contains a checker ).
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2. From (b) this graph has to be connected. which means that no square which contains a checker can share 4 sides with squares that doesn't contain a checker (because it has to share at least one side with a square that  contains a checker).
  
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3. now it is possible to improve the upper bound. since every edge in G represents a square with a checker that shares a side with a square with another checker. the new upper bound is <math>5\times x - </math>(sum of degrees in G)<math> >= n^2</math>  
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3. Now it is possible to improve the upper bound. Since every edge in G represents a square with a checker that shares a side with a square with another checker, the new upper bound is <math>5\times x - \text{[sum of degrees in G]} \ge n^2.</math>  
  
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4. thus, <math>5\times x - (2 \times x - 2 + 2\times R) >= n^2</math>   (see lemma below).
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4. Thus, <math>5x - (2x - 2 + 2R) \ge n^2</math> (see lemma below).
  
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5. thus,  <math>3\times x + 2 - 2\times R >= n^2</math>
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5. Thus,  <math>3x + 2 - 2R \ge n^2.</math>
  
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6. thus,  <math>x >= (n^2 - 2 + 2\times R)/3</math>
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6. Thus,  <math>x \ge \frac{n^2 - 2 + 2R}3,</math> where <math>R\ge0</math> and is equal to 0 if <math>G</math> is a tree.
  
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where <math>R>=0</math> and is equal to 0 if G is a tree.
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<math>\textit{Clarification/Lemma:}</math> The sum of degrees of a connected graph <math>G = (V,E)</math> is <math>2V -2 + 2R = 2E,</math> where <math>R</math> is the circuit rank of <math>G</math>.
  
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<math>clarification/lemma:</math>
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<math>\textit{Proof.}</math> <math>2 \times V -2</math> is the sum of ranks of the spanning tree created by decircuiting the graph <math>G</math>. Since the circuit rank of <math>G</math> is <math>R</math>, <math>2R</math> is the sum of ranks removed from <math>G</math>. Thus,  <math>2V -2 + 2R = 2 E.</math> <math>\blacksquare</math>
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the sum of degrees of a connected graph <math>G = (V,E)</math> is <math>2 \times V -2 + 2\times R = 2\times E</math>
 
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where <math>R</math> is the circuit rank of <math>G</math>.
 
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<math>proof:</math>
 
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<math>2 \times V -2</math> is the sum of ranks of the spanning tree created by decircuiting the graph <math>G</math>.
 
−
 
 
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since the circuit rank of <math>G</math> is <math>R</math>, <math>2\times R</math> is the sum of ranks removed from <math>G</math>.
 
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thus,  <math>2 \times V -2 + 2\times R = 2\times E</math>
 
  
 
== See Also ==
 
== See Also ==

Revision as of 01:58, 27 November 2017

Problem

Some checkers placed on an $n\times n$ checkerboard satisfy the following conditions:

(a) every square that does not contain a checker shares a side with one that does;

(b) given any pair of squares that contain checkers, there is a sequence of squares containing checkers, starting and ending with the given squares, such that every two consecutive squares of the sequence share a side.

Prove that at least $(n^{2}-2)/3$ checkers have been placed on the board.

Solution

For the proof let's look at the checkers board as a graph $G$, where the checkers are the vertices and the edges are every pair of checkers sharing a side.

Define $R$ as the circuit rank of $G$(the minimum number of edges to remove from a graph to remove all its cycles).

Define $x$ as the number of checkers placed on the board.

1. From (a) for every square containing a checker there can be up to 4 distinct squares without checkers; thus, $4\times x + x = 5\times x \ge n^2$ (this is the most naive upper bound).

2. From (b) this graph has to be connected. which means that no square which contains a checker can share 4 sides with squares that doesn't contain a checker (because it has to share at least one side with a square that contains a checker).

3. Now it is possible to improve the upper bound. Since every edge in G represents a square with a checker that shares a side with a square with another checker, the new upper bound is $5\times x - \text{[sum of degrees in G]} \ge n^2.$

4. Thus, $5x - (2x - 2 + 2R) \ge n^2$ (see lemma below).

5. Thus, $3x + 2 - 2R \ge n^2.$

6. Thus, $x \ge \frac{n^2 - 2 + 2R}3,$ where $R\ge0$ and is equal to 0 if $G$ is a tree.

$\textit{Clarification/Lemma:}$ The sum of degrees of a connected graph $G = (V,E)$ is $2V -2 + 2R = 2E,$ where $R$ is the circuit rank of $G$.

$\textit{Proof.}$ $2 \times V -2$ is the sum of ranks of the spanning tree created by decircuiting the graph $G$. Since the circuit rank of $G$ is $R$, $2R$ is the sum of ranks removed from $G$. Thus, $2V -2 + 2R = 2 E.$ $\blacksquare$

See Also

1999 USAMO (Problems • Resources)
Preceded by
First Question 		Followed by
Problem 2
1 • 2 • 3 • 4 • 5 • 6
All USAMO Problems and Solutions

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