Square-law based RBF kernel
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| &lt;= 1
    0.5*(|x|-2)^2    for 1 &lt; |x| &lt;= 2
    0                for |x| &gt; 2


you can recover a sensible shape.