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If $q$ and $r$ are quaternions, and $p$ is a point, applying $q$ then $r$ to $p$ is:

$$ (qr)p\dfrac{1}{qr} $$

What if I want to go the other way? Instead of concatenating rotations, I want to remove them.

So, I have $q$ and $m$, and I know that $qx =m$, but I don't know what $x$ is. How can I discover $x$? (How can I do quaternion divison?)

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Why the downvotes? (What etiquette am I breaching?) –  Rosarch Oct 24 '10 at 18:41
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I don't think you're breaching anything, so I upvoted to compensate. :) But you might want to write your math inside dollar signs instead of highlighting it as code, and skip the asterisks. By the way, it appears a bit strange that you seem to find $1/(qr)$ unproblematic and then right away ask how to do quaternion division... –  Hans Lundmark Oct 24 '10 at 18:51
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I feel that using $1/q$ to denote the inverse of a quaternion is very poor notation. It suggests that for two quaternions $q$ and $r$, one can rearrange the fraction $q/r = \frac{q}{r} = (1/r)q$, which is not true. Is there any actual text that uses this notation? –  Rahul Oct 25 '10 at 2:55
    
@Rahul: it looked peculiar to me too, and I don't remember any of the books I learned quaternions from use the slash or any division-related notation. Inverses, however... –  J. M. Oct 25 '10 at 3:20

4 Answers 4

In algebra, multiplication is denoted by juxtaposition, not by an asterisk. The inverse of the operation $p\mapsto qpq^{-1}$ is $p\mapsto q^{-1}pq$. The solution of $qx=m$ is $x=q^{-1}m$. Note that $$(a+bi+cj+dk)^{-1}=(a^2+b^2+c^2+d^2)^{-1}(a-bi-cj-dk).$$

All this can be found in Wikipedia.

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I would note that the situation is similar to matrix multiplication: since multiplication here is noncommutative, you have to distinguish between "premultiplying" with the inverse $p^{-1}q$ and "postmultiplying" with the inverse $qp^{-1}$.

Note also that

$$(q\cdot r)^{-1}=r^{-1}\cdot q^{-1}$$

The only other thing I can say is that what you're essentially doing is the quaternion analog of a "similarity transformation" of a matrix.

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This is described in Wikipedia under the section Conjugation, the norm, and reciprocal. You just need the reciprocal of q.

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The definition of quaternion division also applies to complex numbers and real numbers. If I write a quaternion as a scalar and a three vector like so: $q=(s,\vec{V})$, then the inverse is $q^{-1}=(s,-\vec{V})/(s^2 + V \cdot V)$. For the reals, the inverse is 1/s. I agree with other posters that writing 1/q is in error. The 1/x notation is fine so long as one's algebra commutes, which is true for the real and complex numbers which are subgroups of quaternions.

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