<p>Oops, you're right, thanks for the fix. My derivation of K'() for the Gaussian is wrong, too, it should be</p>
<p>K'(∥ x - x_i ∥) = exp(-∥ x - x_i ∥^2 / b^2) * -b^-2 * 2 (∥ x - x_i ∥)</p>
<p>The same reasoning still applies, though.</p>
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