Checkerboard Pattern Using an L Shape
Date: 10/14/2002 at 10:50:57 From: chris mills Subject: Checkerboard Pattern - Using an L Shape My co-worker and I have been working on his son's homework the last day or so. Take an 8x8 checkerboard. Using the numbers 1-64, place the numbers in any order so that numbers in an L shape (start point - 1 down - 1 to the right) always add up to an odd number: 1 2 4 3 6 7 8 Using 1-64 as your number set, can you always get an L shape with the sum of the 3 digits as odd using up all the available "squares" on the board?
Date: 10/14/2002 at 12:13:56 From: Doctor Peterson Subject: Re: Checkerboard Pattern - Using an L Shape Hi, Chris. I assume your co-worker's son has been working on this too! The first thing I see is that we can ignore the specific numbers to start with, and just focus on the location of even and odd numbers. I'll represent even numbers by 0 and odd numbers by 1; then we need to fill a square with an equal number of 0's and 1's, since 1 through 64 is split evenly. Since a sum of three numbers will be odd either when one of them is odd, or when all three are odd, there are four configurations possible for each L: 1 1 0 0 1 1 0 0 1 0 0 1 I played with this for a while to see whether there are any obvious patterns. Starting at the lower left, for example, if we start with three odds, we have a choice of putting an odd number or an even number on top, and that forces all the remaining numbers in the diagonal: 1 <-- this can be 1 or 0 1 1 <-- this has to be 1, so we have three 1's 1 1 1 <-- then this has to be 1, too 0 1 0 <-- this has to be 0, so we have one 1 1 1 0 <-- and so does this So we can either continue with all odds forever (which isn't good), or put in a row of evens eventually: 0 1 0 1 1 0 1 1 1 0 1 1 1 1 0 The next row might start with an even or an odd, again: 1 <-- a 1 forces an alternating diagonal 0 0 1 0 1 1 1 0 0 1 1 1 0 1 1 1 1 1 0 0 0 <-- and so does a 0 0 1 1 0 0 1 1 0 1 1 1 1 0 0 1 1 1 1 0 1 All this playing didn't show me any obvious quick route to an answer, so I just tried building a square this way, always trying to keep it about half odd. After a couple of tries, I found a solution, which starts with three odds. To make the final answer, of course, we just have to go through and replace each 0 with an even number and each 1 with an odd number. If you have any further questions, feel free to write back. - Doctor Peterson, The Math Forum http://mathforum.org/dr.math/
Date: 10/14/2002 at 12:32:32 From: chris mills Subject: Thank you (Checkerboard Pattern - Using an L Shape) Thank you. We literally just minutes ago found a solution, though we started in the lower right corner. I'm sure there are numerous solutions but we were getting lost in our own cleverness. We finally (as you started) just eliminated numbers and went with E's and O's. What took you an hour or so took us MANY hours. Thanks for keeping our egos to a minimum :)
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