[] , [] , [^] , []Number of common tangent lines / Background of the odd number(GC World : Various Problems ,Something strange?)
The case with an odd number of lines will occur very few times.
It will only occur with a "tangent", and it is rare to be a tangent with DGS.
In summary, the next thing to think about is "are there any cases of an odd number of lines?"


Forum of Geometric Constructor by Y.Iijima