purely imaginary

real and non negative

on the unit circle

equal in real and imaginary parts.

$$i, -i$$

$$\eqalign{ & \cos \left( { - {\pi \over 4}} \right) + \sin \left( { - {\pi \over 4}} \right), \cr & \cos \left( {{{3\pi } \over 4}} \right) + i\,\sin \left( {{{3\pi } \over 4}} \right) \cr} $$

$$\eqalign{ & \cos \left( {{\pi \over 4}} \right) + i\sin \left( {{{3\pi } \over 4}} \right), \cr & \cos \left( {{{3\pi } \over 4}} \right) + i\sin \left( {{\pi \over 4}} \right) \cr} $$

$$\eqalign{ & \cos \left( {{{3\pi } \over 4}} \right) + i\sin \left( { - {{3\pi } \over 4}} \right), \cr & \cos \left( { - {{3\pi } \over 4}} \right) + i\sin \left( {{{3\pi } \over 4}} \right) \cr} $$

a^{n - 1}

aⁿ

naⁿ

na^{n - 1}