## § Non orthogonal projections

Consider the matrix
$P = \begin{bmatrix} 1 & 1 \\ 0 & 0 \end{bmatrix}$

- $P^2 = P$, so it's a projection. It projects the vector
`[x;y]`

to thevector `[x+y;0]`

. Clearly, applying it twice will result in 0. `[x; y]`

,1. `[x+y; 0]`

, 2. `[x+y; 0]`

. - It projects the value
`(x+y)`

onto the `x`

axis,and kills the `y`

axis. - It's
*not* a projection onto the coordinate axis.