Skew-Symmetric Matrix §

A skew-symmetric matrix is a square matrix whose transpose equals its negative, i,e.

This is used a lot in Robotics. Not to be confused with Orthogonal Matrix, where the transpose is equal to its inverse.

We use the operator to write as a skew-symmetric matrix.

Example of use (from Visual SLAM book) §

The Cross Product can be written like this:

\mathbf{e}_1 & \mathbf{e}_2 & \mathbf{e}_3 \\ {{a_1}}&{{a_2}}&{{a_3}}\\ {{b_1}}&{{b_2}}&{{b_3}} \end{array}} \right\| = \left[ \begin{array}{l} {a_2}{b_3} - {a_3}{b_2}\\ {a_3}{b_1} - {a_1}{b_3}\\ {a_1}{b_2} - {a_2}{b_1} \end{array} \right] = \left[ {\begin{array}{*{20}{c}} 0&{ - {a_3}}&{{a_2}}\\ {{a_3}}&0&{-{a_1}}\\ {-{a_2}}&{{a_1}}&0 \end{array}} \right] \mathbf{b} \buildrel \Delta \over = { \mathbf{a}^ \wedge } \mathbf{b} $$The result of the outer product is a vector whose direction is perpendicular to the two vectors, and the length is \left | \mathbf{a} \right | \left | \mathbf{b} \right | \sin \langle { \mathbf {a}, \mathbf {b}} \rangle, which is also the area of the quadrilateral of the two vectors.$$$$\mathbf{a}^\wedge = \left[ {\begin{array}{*{20}{c}} 0&{-{a_3}}&{{a_2}}\\ {{a_3}}&0&{ - {a_1}}\\ { - {a_2}}&{{a_1}}&0 \end{array}} \right]$$