I've been (slowly) working my way through a book on geometric algebra and have found one part particularly confusing.
I can understand the equation $e_1e_2=\exp(e_1e_2 \pi/2)$
Where the substiution $e_1e_2=i$ has been made ($e_1$ and $e_2$ are orthongonal unit vectors and $i^2=-1$)
But I cannot understand understand the following ($R_\phi$,$R_\theta$ are rotors):
If $R_\phi=\exp(-e_1e_2\phi/2$) then:
How are the rotors removed from the exponent?