Projection of vector on $(x_1,x_3)$-subspace If I have a three-dim. vector
$$
(x_1,x_2,x_3)\in\mathbb{R}^3,
$$
what then is its projection onto the $(x_1,x_3)$-subspace?
I am not sure.

The picture represents the idea:

Red point is the three dimensional position $(x_1,x_2,x_3)$ and yellow one is the projection onto the two dimensional subspace (you mention) and has the specifications $(x_1,0,x_3)$.  
