- #1
Mehdi_
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Originally Posted by garrett:
using the "limit" definition for the exponential. This is an exact expression for the rotation of a vector by a bivector. In three dimensions an arbitrary bivector, B, can be written as
[tex]B = \theta b[/tex]
an amplitude, [itex]\theta[/itex] , multiplying a unit bivector encoding the orientation, [itex]bb=-1[/itex]. The exponential can then be written using Joe's expression for exponentiating a bivector:
[tex]U = e^{\frac{1}{2} B} = \cos(\frac{1}{2} \theta) + b \sin(\frac{1}{2} \theta)[/tex]
[tex]U = e^{\frac{1}{2} B} = \cos(\frac{1}{2} \theta) + b \sin(\frac{1}{2} \theta)[/tex]
we can then write:
[tex]U = e^{\frac{1}{2} \theta b} = \cos(\frac{1}{2} \theta) + b \sin(\frac{1}{2} \theta)[/tex]
And if we rely on Joe's expression, [itex]r=\frac{\theta}{2}[/itex] (rotor angle is always half the rotation):
[tex]U = e^{br} = \cos(r) + b \sin(r)[/tex]
and
[tex]\begin{align*}U=e^T & = I \left(1 - \frac{1}{2!}r^2 + \frac{1}{4!}r^4 - \frac{1}{6!}r^6 + \dots\right) + T \left(1 - \frac{1}{3!}r^2 + \frac{1}{5!}r^4 - \frac{1}{7!}r^6 + \dots \right) \\ & = I \cos(r) + \frac{1}{r} T \sin(r)\end{align*}[/tex]
we can therefore see that [itex]br = T[/itex]
then : [itex]b = \frac{T}{r}[/itex]
We have then Joe's expression :
[tex]U = e^{T} = \cos(r) + \frac{T}{r} \sin(r)[/tex]
And from [itex]bb=-1[/itex] we can deduce:
[tex]\frac{T^2}{r^2}=-I[/tex]
and therefore [itex]TT =-r^2 I[/itex] (result defined by the joe previously)
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