Multiplying permutation cycles

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Homework Help Overview

The discussion revolves around computing the product of permutation cycles in the symmetric group \( S_9 \), specifically the cycles (145)(37)(682). Participants are exploring the representation of permutations and the concept of disjoint cycles.

Discussion Character

  • Exploratory, Conceptual clarification, Problem interpretation

Approaches and Questions Raised

  • The original poster attempts to compute the product of the cycles step by step but expresses confusion about the resulting cycles and the notation used. Some participants question the clarity of the problem statement regarding what constitutes a "single permutation."

Discussion Status

Participants are actively engaging with the problem, with some providing clarifications about the notation and the nature of the cycles. There is an acknowledgment of the original poster's confusion, and a few hints have been offered to guide the understanding of the notation and the expected format of the answer.

Contextual Notes

There is a mention of the theorem regarding permutations being expressed as disjoint cycles, and the original poster is grappling with the implications of this theorem in their computation. The discussion also touches on the preference for cycle notation versus matrix notation for representing permutations.

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Homework Statement


Compute each of the following products in ##S_9## (Write your answer as a single permutation).

a) (145)(37)(682)

Homework Equations


Theorem: Every permutation is either the identity, a single cycle, or the product of disjoint cycles.

The Attempt at a Solution



I start with (145)(37)(682).

Going from the rightmost cycle to the left, starting with 6.. 6 goes to 8, 8 goes to 8, then 8 goes to 8. So 6 goes to 8.

So i have (68) so far.

Now 8 goes to 2, 2 goes to 2, 2 goes to 2. So 8 goes to 2.

Now i have (682).

Now 2 goes to 6, 6 goes to 6, 6 goes to 6. So 2 goes to 6.

Now i have (6826) and I'm going to have a loop like (682682682...). I'm not sure where to go from here. If I just skip to 7, like (6827), then 2 goes to 7.. but 2 needs to go to 6 somehow. Also, by the theorem, its true that a permutation cannot be expressed as a single cycle and the product of disjoint cycles. Its a product of disjoint cycles..so is what I'm doing impossible?
 
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Your original product is a product of disjoint cycles. I believe the problem statement parenthesis should be "your answer should only contain disjoint cycles". It is unclear what "single permutation" means.
 
Orodruin said:
Your original product is a product of disjoint cycles. I believe the problem statement parenthesis should be "your answer should only contain disjoint cycles". It is unclear what "single permutation" means.
I asked here first to see if I could get a hint before looking at the answer key. It seems "single permutation" means the form:

##
\begin{pmatrix}
1 & 2 & 3 & 4 & 5 & 6 \\
1 & 2 & 3 & 4 & 5 & 6 \\
\end{pmatrix}
##

I can take it from here, sorry for the weird question, I was trying to avoid the answer key on the very first question..
 
Ok, that makes sense. I just don't like that notation as it takes more space and is less transparent than cycle notation, but as a first exercise to get the hang of what it means I guess it is fine.
 

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