The 3x6 matrix G is as follows,  

$\text{G} = [\text{V}\_\times| I\_{3\times3}]$


$\text{V}$ is a skew matrix of a vector with 3 elements about a 3D point. The dimension of $\text{V}$ is 3x3.  

$I$ is the 3x3 identity matrix.  

I think the vertical line between $\text{V}$ and $I$ is used to concatenate these two 3x3 matrices to be a 3x6 matrix.  

I would like to ask what the subscript 'x' of $\text{V}$ means.  

I could not search the information with a good keyword.


BTW, this formula came from the paper [KinectFusion: Real-Time Dense Surface Mapping and Tracking](https://www.microsoft.com/en-us/research/wp-content/uploads/2016/02/ismar2011.pdf). The 21st formula.



What does the subscript 'x' of a matrix mean?
It is the *skew-symmetric form* defined [here](https://en.wikipedia.org/wiki/Skew-symmetric_matrix#Cross_product).


