Author 
Topic: Knight on an infinite chessboard (Read 6501 times) 

NickH
Senior Riddler
Gender:
Posts: 341


Knight on an infinite chessboard
« on: Apr 15^{th}, 2003, 12:01pm » 
Quote Modify

Consider a knight on an infinite chessboard. How many squares can it reach after precisely n moves?


IP Logged 
Nick's Mathematical Puzzles



towr
wu::riddles Moderator Uberpuzzler
Some people are average, some are just mean.
Gender:
Posts: 13595


Re: Knight on an infinite chessboard
« Reply #1 on: Apr 15^{th}, 2003, 1:09pm » 
Quote Modify

it might be 2^(2n+1)+n1 but it's probably not.. (I only tried n=0,1,2 so far )

« Last Edit: Apr 15^{th}, 2003, 1:11pm by towr » 
IP Logged 
Wikipedia, Google, Mathworld, Integer sequence DB



NickH
Senior Riddler
Gender:
Posts: 341


Re: Knight on an infinite chessboard
« Reply #2 on: Apr 15^{th}, 2003, 2:08pm » 
Quote Modify

I would say f(0) = 1. After no moves, it can only be on one square  the square it starts from. The knight can move back to a square it has moved from.


IP Logged 
Nick's Mathematical Puzzles



cho
Guest


Re: Knight on an infinite chessboard
« Reply #3 on: Apr 15^{th}, 2003, 2:22pm » 
Quote Modify
Remove

It takes a few rounds to stabilize. It starts 1,8,33,76,129,196. From that point on each round grows by a number 14 greater than the previous. I'm not sure how to turn the whole sequence into one equation.


IP Logged 



ThudnBlunder
wu::riddles Moderator Uberpuzzler
The dewdrop slides into the shining Sea
Gender:
Posts: 4489


Re: Knight on an infinite chessboard
« Reply #4 on: Apr 15^{th}, 2003, 4:20pm » 
Quote Modify

Quote:It takes a few rounds to stabilize. It starts 1,8,33,76,129,196. From that point on each round grows by a number 14 greater than the previous. I'm not sure how to turn the whole sequence into one equation. 
 I'm not sure what you mean there, cho. Do you mean f(n)  f(n1) = 14n + some constant ? OR Do you mean f(n)  f(n1) = 14 ? OR Perhaps you mean something else? Why 14? How did you get your sequence?

« Last Edit: Apr 16^{th}, 2003, 4:56am by ThudnBlunder » 
IP Logged 
THE MEEK SHALL INHERIT THE EARTH.....................................................................er, if that's all right with the rest of you.



cho
Guest


Re: Knight on an infinite chessboard
« Reply #5 on: Apr 15^{th}, 2003, 6:13pm » 
Quote Modify
Remove

Okay, I'm going to show my utter lack of math skills here. By instability I mean that moves 1 and 2 don't fit the pattern. If they did the sequence would be 1,12,37,76,129,196,277,372,481... Notice that the difference between adjacent values is 11,25,39,53,67... There's the 14. The pattern is obvious, but my last math class was a basic algebra class in high school 30 years ago. How I got my sequence was by counting (very little math required). I took a piece of graph paper and marked out the possible locations for one quadrant and multiplied by 4: 0 3212343456567878 214323454567678 8 3232343456567878 232343454567678 8 3434345456567878 434345456567678 8 5454545656767878 454545656767878 8 56565656767878 8 6565656767878 8 767676767878 8 67676767878 8 7878787878 8 878787878 8 8 8 8 8 8 8 8 8 8 8


IP Logged 



cho
Guest


Re: Knight on an infinite chessboard
« Reply #6 on: Apr 15^{th}, 2003, 6:20pm » 
Quote Modify
Remove

I'm also assuming the question seeks the number of squares you could be sitting on on the Nth move, and not all the squares it has touched so far (only the same colored squares as the original square after an even number of moves, eg)


IP Logged 



Chronos
Full Member
Gender:
Posts: 288


Re: Knight on an infinite chessboard
« Reply #7 on: Apr 17^{th}, 2003, 4:51pm » 
Quote Modify

Quote:Personally, I think the function is more likely to be linear than exponential. 
 After a given number of moves, a knight can (approximately) reach any square within a "circle" of radius proportional to the number of moves. The number of squares in that "circle" is approximately the area of the circle. So it makes sense that, asymptotically, at least, the function would be quadratic. Which is exactly what cho found, so this post is rather redundant now...


IP Logged 



Netman
Guest


Re: Knight on an infinite chessboard
« Reply #8 on: Apr 23^{rd}, 2003, 6:59pm » 
Quote Modify
Remove

For the version where the knight cannot revisit squares... after wasting 12 CPU minutes bruteforcing the problem, I came to the realization that the only square that restriction eliminates is the starting square, all other squares can be reached via some sequence of moves. So the equation: f(n) = 7n^{2} + 4n + 1, if n > 2 holds for all odd n, and for even n, just subtract one for the starting square. Chris


IP Logged 



ThudnBlunder
wu::riddles Moderator Uberpuzzler
The dewdrop slides into the shining Sea
Gender:
Posts: 4489


Re: Knight on an infinite chessboard
« Reply #9 on: Apr 24^{th}, 2003, 10:36am » 
Quote Modify

But the original square is already included. That is, f(0) = 1. Let's generalize the question: Consider a knight moving on an infinite chessboard. How many squares can it arrive at after precisely n moves given that: 1) it can double back and revisit squares? 2) it cannot double back and revisit squares? 3) we also count every square that it has visited? Also, 4) how many for n or less moves? 1) is the original question. You are suggesting that 1) and 2) are the same sets, which seems doubtful to me. Have you written a program? However, 3) and 4) would appear to be identical. [i][/i]

« Last Edit: Apr 25^{th}, 2003, 5:34am by ThudnBlunder » 
IP Logged 
THE MEEK SHALL INHERIT THE EARTH.....................................................................er, if that's all right with the rest of you.



cho
Guest


Re: Knight on an infinite chessboard
« Reply #10 on: Apr 24^{th}, 2003, 11:27am » 
Quote Modify
Remove

In #2, of course, you can never end up back at square one because it's been visited, but any other square that can be reached can be reached without using a square twice. For example: Move 1 could put you at 2,1 from the start. Move 3 could put you there by going 2,1; 4,0; 2,1. For any square you want to reach 2 or 4 or 6 or 8 moves after the move that would most quickly put it there, you just move 1 or 2 or 3 or 4 moves beyond it and come back on a parallel route. Your #3 is just each number added to the previous term in the series. Even I can do that math: 14x^{2}6x+5.


IP Logged 



