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Let $\begin{equation*} p = \begin{pmatrix} 1 & 2 & 3 & 4 & 5 & 6 & 7 & 8 \\ 1 & 8 & 4 & 7 & 3 & 6 & 2 & 5 \\ \end{pmatrix} \end{equation*}$

I would like to write $p$ as a product of transpositions. To do this, I first write out $p$ as a product of disjoint cycles as follows:

$p = (6)\circ(2, 8, 5, 3, 4, 7)\circ(1)$

or, in simpler terms, given the fact that we can drop all single element brackets:

$p = (2, 8, 5, 3, 4, 7)$

I know this is correct because when I calculate the above permutation, I end up with the given $p$.

My challenge now comes in when I expand on this intermediary step and write out the product of transpositions, i.e. splitting these cycles to length 2. According to @amWhy's answer on this thread, I use the quoted method with an example given: $Method 1: τ=(1,3,4,6,7,9)=(1,9)(1,7)(1,6)(1,4)(1,3)$

Using this method on my example gives this answer: $p = (2,7)\circ(2,4)\circ(2,3)\circ(2,5)\circ(2,8)$

However checking this answer by multiplying out this product doesnt give me the original $p$ in the question.

Where did I go wrong?

Dean P
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    You must be multiplying out the transpositions wrongly - maybe you are reading them in the wrong order. What do you get when you multiply them? – Matthew Towers Mar 17 '20 at 10:03
  • Multiplying out my product of transpositions gives $\begin{equation} p = \begin{pmatrix} 1 & 2 & 3 & 4 & 5 & 6 & 7 & 8 \ 1 & 7 & 5 & 3 & 8 & 6 & 4 & 2 \ \end{pmatrix} \end{equation}$ I always work from right to left when multiplying the product – Dean P Mar 17 '20 at 10:08
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    Hey Matthew, you're right. I wasn't multiplying correctly. On a high level I followed right to left but each operand was still multiplied incorrectly. What I accidentally did was this: $A\circ B \circ C \circ D \circ E$ as a first step I started going right to left and calculated $D \circ E$ but I swapped my order of evaluating that product. In other words in every step from going right to left I evaluated each product incorrectly. Thanks, good spot – Dean P Mar 17 '20 at 10:22

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