Matrices of SL $${(4, \mathbb{R})}$$ ( 4 , R ) that are the Product of Two Skew-Symmetric Matrices

Abstract

The Jordan forms of matrices that are the product of two skew-symmetric matrices over a field of characteristic \({\neq 2}\) have been a research topic in linear algebra since the early twentieth century. For such a matrix, its Jordan form is not necessarily real, nor does the matrix similarity transformation change the matrix into the Jordan form. In 3-D oriented projective geometry, orientation-preserving projective transformations are matrices of \({SL(4, \mathbb{R})}\), and those matrices of \({SL(4, \mathbb{R})}\) that are the product of two skew-symmetric matrices are the generators of the group \({SL(4, \mathbb{R})}\). The canonical forms of orientation-preserving projective transformations under the group action of \({SL(4, \mathbb{R})}\)-similarity transformations, called \({SL(4, \mathbb{R})}\)-Jordan forms, are more useful in geometric applications than complex-valued Jordan forms. In this paper, we find all the \({SL(4, \mathbb{R})}\)-Jordan forms of the matrices of \({SL(4, \mathbb{R})}\) that are the product of two skew-symmetric matrices, and divide them into six classes, so that each class has an unambiguous geometric interpretation in 3-D oriented projective geometry. We then consider the lifts of these transformations to SO(3, 3) by extending the action of \({SL(4, \mathbb{R})}\) from points to lines in space, so that in the vector space \({\mathbb{R}^{3, 3}}\) spanned by the Plucker coordinates of lines these projective transformations become special orthogonal transformations, and the six classes are lifted to six different rotations in 2-D planes of \({\mathbb{R}^{3, 3}}\).