# Square-law based RBF kernel

What is the Square-law based RBF kernel (SQ-RBF)? The definition in the table at the Wikipedia article Activation Function looks wrong, since it says

y =
1 - x^2/2     for |x| <= 1
2 - (2-x^2)/2 for 1 < |x| <= 2
0             for |x| > 2


but this makes it discontinuous at x = 1

That is indeed quite odd. I can't really find many other sources on SQ-RBF, except here, where the definition differs from wikipedia, but when I plot this, it does not recover the image they show. However, if you define

y =
1 - x^2/2        for |x| <= 1
0.5*(|x|-2)^2    for 1 < |x| <= 2
0                for |x| > 2


you can recover a sensible shape.