in one of my questions a 2x2 matrix is mentioned that is , it doesnt tell me what it is but somewhere else in my notes it mentions that . can someone please tell me (quickly) what this matrix is?

in one of my questions a 2x2 matrix is mentioned that is , it doesnt tell me what it is but somewhere else in my notes it mentions that . can someone please tell me (quickly) what this matrix is?

Haha, there are quite a few matrices with . Can you give us more context.

In general, a linear transformation with negative determinant changes orientation, but as Drexel28 comments, we need more context. For example if with and orthogonal we have a symmetry with respect to a line.