# Thread: Geometric action of an arbitrary orthogonal 3x3 matrix with determinant -1

1. ## Geometric action of an arbitrary orthogonal 3x3 matrix with determinant -1

Hi,
I have a question about describing geometrically the action of an arbitrary orthogonal 3x3 matrix with determinant -1. I would like to know if my proposed solutions are satisfactory, or if they lack justification. I have two alternate solutions, but have little confidence in their validity. Any help would be greatly appreciated!!

Solution 1: The orthogonal 3 x 3 matrix with determinant 1 is an improper rotation, meaning it is a reflection combined with a proper rotation. In another sense, an improper rotation is an indirect isometry, which is an affine transformation with an orthogonal matrix with a determinant 1.

Solution 2:
A rotation about the origin, followed by inversion through the origin (i.e. (x,y,z)-->(-x,-y,-z) ). Note that a "left-handed object" turns into a "right handed object", so "handedness is reversed" but otherwise it is just like a rotation.

2. ## Re: Geometric action of an arbitrary orthogonal 3x3 matrix with determinant -1

Hey abscissa.

I think the first solution is good. Basically you can decompose your transformation into two matrices one which is det +1 (rotation) and one which is -1 (reflection matrix).

The other reason why I think its good IMO is because it specifically talks about the orientation aspect which is the critical difference between +1 and -1 orthogonal matrices (in terms of the determinant).