Proving the Existence/Non-Existence of a Grid Tour

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Homework Help Overview

The discussion revolves around proving the existence or non-existence of a tour on a grid defined by its height \( p \) and width \( q \). A tour is characterized as a valid walk that visits each square exactly once and returns to the starting point. The participants are exploring conditions under which such a tour can exist, particularly focusing on cases where either \( p \) or \( q \) is even versus when both are odd.

Discussion Character

  • Exploratory, Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants discuss coloring the grid like a chessboard to analyze the implications of step parity on the colors of the squares. They explore how the number of steps taken affects the color of the square visited and question the conditions necessary for completing a tour.

Discussion Status

Some participants have provided guidance on understanding the relationship between the parity of steps and the colors of the squares. There is an ongoing exploration of how to formally prove the necessity of even parity for completing a tour, with some participants expressing uncertainty about their reasoning and seeking further clarification.

Contextual Notes

Participants are working within the constraints of the problem statement, which specifies conditions for \( p \) and \( q \) and the nature of valid walks on the grid. There is an acknowledgment that examples have been used to support reasoning but a desire for a more formal proof remains.

AUCTA
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Homework Statement



Consider a grid with height p >= 2 and width q >= 2, so there are pq squares in the grid. A valid walk on the grid is a walk that starts on one square and subsequently moves to adjacent squares (you cannot move diagonally). Define a tour to be a valid walk on the grid that touches each and every square exactly once and begins and ends on the same square. First show that if either p or q is even then there exists a tour. Prove that if p and q are odd, there does not exist a tour.

Homework Equations



No idea.

The Attempt at a Solution



No induction. I have no idea how to go about this proof. I would love some guidance on how to get started.
 
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Hello AUCTA,

I cannot help you guarantee a tour if p or q are even but I can definitely help you show that a tour is non existent for both p and q being odd.

Firstly, color your grid like a chess board i.e. alternative black and white. Once you have done that, think about how many steps must you take on a tour?
 
Since a tour is all squares, then it is pq.
 
Yes that is correct. Now what happens to the color of the square as you take each step?
 
The colors change. From black to white.
 
Correct again, So how is the color of the square you are on (after the taking the first step) depend on the number of steps you take?
 
0, 2, 4, 6, ... (all the even number of steps) have the same color and all the odds have the same color.
 
Okay, Think about it like this: From the first square (assuming black), what would be the color of the square you are on after
a)even number of steps and
b)Odd number of steps

Once you figure that out, what should be the parity of the number of steps you must take to complete a tour?
 
How do I calculate parity?
 
  • #10
Parity (in this case) means whether the number is even or odd.
 
  • #11
Well the parity must be even for a tour to be able to be completed.
 
  • #12
AUCTA said:
Well the parity must be even for a tour to be able to be completed.

Bingo. You have the desired result!
What does this tell about p and q?
 
  • #13
Then they must be both even. But my question is, how can I proof that the parity must be even. I just said it because I did a few examples and it made sense. But how do you actually know it must be even?
 
  • #14
AUCTA said:
Then they must be both even. But my question is, how can I proof that the parity must be even. I just said it because I did a few examples and it made sense. But how do you actually know it must be even?

It is sufficient if one is even.

As for the parity; If you start on a black square, you must end on a black square right? So what will be the parity of the number of steps?

(go back to post#8, first question for a hint)
 
  • #15
Ah it makes sense now. Thanks a lot Sunil, you have helped me greatly.
 
  • #16
You are welcome.

Did you get any lead on proving the other part of the question?
(draw some grids and see if there can be a general method to tour )
 

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