Thread: projection onto a convex set

Hi,
How can i derive a projection onto this set $\displaystyle \|x - a \| \leq r^2$.

we'll need a little more information than that. what set do the variables belong to? real numbers? integers? complex numbers? vectors? and what exactly are you trying to project? another set? a sphere? an entire space? does the projection have to be linear, or distance-preserving, or continuous?