Two points 
and 
are symmetric with respect to a circle or straight line 
if all circles and straight lines passing through 
and 
are orthogonal to 
. Möbius transformations
preserve symmetry. Let a straight line be given by a point 
and a unit vector 
, then
 |
where 
is the complex conjugate. Let a circle
be given by center 
and radius 
, then
 |
