Chess knights are weird. If you look at every chess other piece, their movements are more-or-less straightforward. Rooks can move up, down, left or right. Bishops move along the diagonals. Queens and kings can do either. Pawns are a little weird, but in general they always have to move forward.
Knights are… different. No piece besides the knight can jump over another piece in its path. Furthermore, all other pieces are restricted to moving up, down, left, right or diagonally. With their “L”-shaped movement, knights move in a direction that’s somewhere between the 8 compass points in a way that no other piece can. It’s because of these unique features that knights have such an interesting and important role in chess; knights are one of the first pieces that can be moved (since they can jump over the pieces in front of them), and their unique movement pattern makes it easy for them to attack multiple pieces at once.
Still, the movement of knights feels somewhat arbitrary. Knights travel by taking two steps in one direction and then another step perpendicularly. But why shouldn’t a knight take three steps, then another perpendicular one? Or four and two? There are a number of other possibilities that can be chosen while maintaining the “L”-shaped spirit of the knight. The knight is one of the oldest and most consistent pieces in history, but it also feels like the one with which we can do the most experimentation. Let’s see what happens to the knight if we change things up a bit.
The Generalized Knight
The idea is this: we replace the normal two-then-one pattern knights have with an a-then-b pattern, where a and b can be any whole number. From now on, when we refer to an (a,b)-knight, we’re referring to a knight that takes a steps in any direction and then b steps in a perpendicular direction. Using this definition, the original knight is a (2,1)-knight.
It also turns out that an (a,b)-knight has the same movement as a (b,a)-knight. By convention, we’ll list the bigger number first.
The regular 8-by-8 board will be too small for most of these knights to move around in. We’ll adjust the board size based on the knight we look at to account for this.
A regular knight can eventually reach any square from any other square on any rectangular board, given enough steps. Therefore, it’s more interesting to look at what a knight can’t do than what it can. The knights we’ll look at will explore every possible path they can take from their starting position. We’ll take a look at the squares that they can’t reach and try to understand why.
Above, we have a (4,2)-knight and a (9,6)-knight. Some of the rows and columns remain unvisited even after the knights have finished moving. The knights simply can’t reach them from their starting positions. This happens because the numbers associated with the knights share a common factor: 4 and 2 are both multiples of 2, while 9 and 6 are both multiples of 3.
Why should this matter? Think of the knight’s position as a coordinate on a grid. When the knight moves, the coordinates have to change by an amount equal to the knight’s numbers. If the knight’s numbers have a factor in common, we are always changing our coordinates by some multiple of that factor.
Let’s say we pick (4, 5) as our starting position. 4 is one above a multiple of 3, so we can write it as 1 + 3(1). Similarly, we can write 5 as 2 + 3(1). We’re breaking 4 and 5 up into their quotient (1) and remainder (1 and 2, respectively) when they’re divided by 3. If we want to add any multiple of 3 to these numbers, only the quotient will change. Because our knight’s numbers are both multiples of 3, our x coordinate will always be one above a multiple of 3; never two above a multiple of 3, nor a multiple of 3 itself.
Mathematicians have special terminology for being “one above a multiple of 3”. They would say that 4 is congruent to 1, modulo 3. Basically, this means that 1 and 4 have the same remainder when divided by 3. This is also true of 7, 10, and an infinite amount of other numbers. It’s not very convenient write out an infinite amount of numbers, so there’s a catch-all term for when you want to talk about all the numbers with this property: the set of all numbers one greater than a multiple of 3 is the congruence class of 1, modulo 3. Because of the remainder property we saw above, our coordinates will always stay in the same congruence class (modulo 3) that they started in. This is why our knight can only reach every third row and column.
The same general idea happens when the knight’s numbers share any factor other than 1. If a knight’s numbers share a factor, then we will always be adding a multiple of that factor to our coordinates, trapping us in the same congruence class forever.
Although the (2,1)-knight had no problems reaching every square, the (3,1)-knight can’t reach half of them. Since 3 and 1 have no factors in common, this is a different problem than we ran into earlier. If we look at some more examples, we might get more insight into why this is happening.
Take a look at the ones where the knight is able to reach every square. In this chart, they’re organized in a checkerboard pattern not unlike the checkerboard pattern made by the (3,1)-knight. This all has to do with the parity of the knight.
Parity is just another way of saying if something is even or odd. Normally, we deal with parity of numbers: 4 has even parity while 7 has odd parity. It turns out pairs of numbers have parity as well: you just take the parity of the two numbers added together. For example, (2,1) is odd since 3 is odd, while (3,1) is even since 4 is even.
In the figure above, each knight that is able to reach every square has odd parity. Why would that be?
It turns out a lot of the same properties of single-number parities apply to parities of pairs as well. For example, when you add an even number to another number, the parity will stay the same. If you add an odd number, the parity will swap!
This is a clue as to why knights with odd parity can reach more squares than even-parity knights. Odd parity knights swap the parity of their coordinate with each move, so we can reach both even and odd squares regardless of where we start. With even knights, on the other hand, we are stuck on the same parity as we started with. This is why the (3,1)-knight can only reach half the squares that the (2,1)-knight can.
Everything we’ve talked about to this point would hold true regardless of the board size. But in some situations, we get interesting patterns if the knight is just a little too big for the board. In these cases, squares in the middle become impossible to reach because the knight can’t maneuver tightly enough in the small spaces.
Besides noting that these patterns remind me of the bouncing DVD logo, I don’t have any insight for predicting when patterns like these will occur. If you have any ideas, let me know!
Reaching every square
Generalizing knights thus far has been unsuccessful. Because many of our generalized knights are unable to reach some of the squares, they have been rendered much less useful on the chessboard. Only (a,b)-knights that meet a very specific set of criteria can reach every square:
- a and b must not share any factors
- a + b must be odd
- a and b can’t be too big relative to the size of the board
However, it turns out a small tweak to the board itself will allow any of the limited knights we’ve discussed to reach any square.
When in doubt, take the modulus
The modulus operation is another name for taking the remainder of one number divided by another. We’ve already talked a little bit about the modulus when we looked at knights that shared common factors. In that context, we saw how knights with common factors would miss some rows and columns because they were restricted to the same congruence class; the coordinates always had the same remainder when divided by the common factor.
The most common place you’d find the modulus outside of a math classroom is on a wall clock. If it’s 10 o’clock and you wait 3 hours, it’ll be 1 o’clock. With clocks, we take the modulus with respect to 12. After 12, the hours wrap back around to 1. Effectively, the modulus makes numbers wrap around to 0 in a cyclical manner.
We can also use the modulus in the context of the knight’s coordinates. Instead of preventing the knight from moving past the board’s boundary, what if we allowed the knight to wrap around to the other side of the board? To do so, we could first add the knight’s coordinates, like before. To get the knight’s final position, we can take the modulus of the resulting coordinates with respect to the board’s size to ensure that the new coordinates are within the board’s boundary. This has the effect of making the knight wrap around the board as it moves.
It’s not immediately clear how the modulus will help us solve the problems we had with the knight. First, let’s answer a simpler question from the 1D case: what happens if we repeatedly add 3 hours on a regular wall clock? It turns out we’ll only see 4 different times. On the other hand, if our wall clock had 11 hours instead of 12, we would eventually see all the hours.
The difference between the two cases is that 12 and 3 share a common factor but 11 and 3 don’t. This is similar to the common factor problem we saw earlier: sharing factors causes some numbers to be skipped. However, if we make sure that the number we take the modulus with doesn’t share factors with our knight, we should be able to reach every square. By picking a prime number, like 11, we can guarantee that no knight will have a shared factor with it.
So long as the prime we pick is odd, we’ll also solve our parity problem. Adding two even numbers can result in an odd number if the numbers we add are large enough to cause a wrap-around. Suppose we pick 7 for our modulus. Normally, 4 + 4 = 8, which is even as expected. However, 8 is congruent to 1 modulo 7, and 1 is an odd number. If we add 4 and 4 modulo 7, we actually get an odd result by adding even numbers. Before, we saw issues with even knights because there was no way of changing parities by adding even numbers. With the modulus, we now have a way of getting an odd number by adding two evens.
Finally, our spatial constraint problem should be solved as well, albeit for a less interesting reason: the modulus removes the boundaries at the edge of the board by allowing knights to wrap around to the other side. With no spatial constraints, the knight is able to reach every square on the board.
It turns out the secret to unlocking the potential of a generalized knight is to extend our idea of the board as well. What we’re left with doesn’t very closely resemble chess, but it does lead to some pretty pictures.
- Tom7’s chess: some awesome chess-related explorations that initially inspired me to write this post
- Generalized knight’s tours on rectangular chessboards: more fun with generalized knights
- Cryptonomicon, by Neal Stephenson: excellent book that, among many other things, contains descriptions of modular arithmetic as it applies to cryptography