Cycle notation for permutations?

In summary, the question is asking why the permutations (1 4 2 6)(2 3 4 5) and (1 4 5 6)(2 3) are equal. The conversation discusses the idea of dividing the cycles and investigating the composite function as well as using an algorithm to determine the result. Ultimately, the conclusion is that the two permutations are equivalent and the algorithm used makes sense in relation to composite functions. The speaker also mentions wishing for more explicit information in their book on the topic.
  • #1
Combinatus
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1

Homework Statement



Why are the permutations (1 4 2 6)(2 3 4 5) and (1 4 5 6)(2 3) equal? It seems to me as if the first pair of 4-cycles want to permute 4->2 and 4->5, yielding a contradiction, but I suspect I've misunderstood something about composite cycles.

I suspect it has something to do with dividing the cycles into [tex]\alpha[/tex] = (1 4 2 6) and [tex]\beta[/tex] = (2 3 4 5) and investigating the composite function [tex]\alpha \beta[/tex] (or [tex]\beta \alpha[/tex]?), but I can't seem to get the result right.
 
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  • #2
I think I figured it out. Go from the right cycle and move left. In the case of (1 4 2 6)(2 3 4 5), starting off with 2 in the right cycle, we get 2 -> 3, and 3 is unchanged by the left cycle. Then, 3 -> 4 and 4 -> 2 in the left cycle, so 4 -> 2. Then, 4 -> 5, and 5 is unchanged by the left cycle, and so on. We end up with (2 3)(4 5 6 1), which is equivalent to what I stated in my previous post.

So in other words, the algorithm seems to be to start with the cycle to the right, permute a "member" X in it to whatever member is one place to the right of X, and if possible, permute the result again in the cycle one step to the left, and repeat, until you can't permute it through any more cycles. Repeat for additional members in the cycle to the right. Once you're out of members in the cycle to the right, check for members in the cycle one step left of it that have not yet been permuted into anything, permute them, and so on.

I guess this "makes sense" in relation to how composite functions work; I just wish my book (Discrete Mathematics by Biggs) had been more explicit about it. Let me know if I'm wrong!
 
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FAQ: Cycle notation for permutations?

What is cycle notation for permutations?

Cycle notation is a way to represent permutations, which are rearrangements of a set of objects or elements. It uses parentheses to group elements that are moved to different positions.

How do you write a permutation in cycle notation?

To write a permutation in cycle notation, start by identifying the elements or objects that are being rearranged. Then, group the elements that are moved to the same position in parentheses, starting with the element in its original position. Repeat this process until all elements have been grouped.

What is a cycle in permutation notation?

A cycle in permutation notation is a set of elements that are moved to different positions in a permutation. It is represented by parentheses, with the first element being the one that is moved to a new position, and the last element being the one that is moved back to its original position.

How do you represent a fixed point in cycle notation?

A fixed point in cycle notation is an element that is not moved in a permutation. It is usually represented by a cycle with only one element, such as (1). This indicates that the element 1 remains in its original position.

Can cycle notation be used for all permutations?

Yes, cycle notation can be used for all permutations. It is a compact and efficient way to represent permutations, and it can be used for any permutation, regardless of its size or complexity.

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