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<p>This above definition can be generalized to real-valued functions f defined on subsets of <b>R</b><sup>n</sup> using a weaker version of the <a href="page.php?w=directional_derivative">directional derivative</a>. Let <b>a</b> be an interior point of the domain of f. Then f is called semi-differentiable at the point <b>a</b> if for every direction <b>u</b>&nbsp;&isin;&nbsp;<b>R</b><sup>n</sup> the limit</p>

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<p>exists as a real number, with h&nbsp;&isin;&nbsp;<b>R</b>.</p>

<p>Semi-differentiability is thus weaker than <a href="page.php?w=Gateaux_derivative">Gateaux differentiability</a>,</p><p>
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