rm \times from convexity formula

posts/2018-04-17-sol.html

@@ -98,7 +98,7 @@ <h1>A crash course in second-order logic</h1>
 difficult to define without second-order quantification. Another example is a
 fairly important concept in optimization: the convexity of functions.</p>
 
-\[\forall f, x, y, \alpha: Convex(f) \iff f(\alpha \times x + (1 - \alpha) \times y) \leq \alpha \times f(x) + (1 - \alpha) \times f(y)\]
+\[\forall f, x, y, \alpha: Convex(f) \iff f(\alpha x + (1 - \alpha) y) \leq \alpha f(x) + (1 - \alpha) f(y)\]
 
 <p>with \(\alpha \in \mathbb{R} \mid 0 \leq \alpha \leq 1\), see <a href="http://www.springer.com/gp/book/9780387303031">Nocedal &amp; Wright
 (2006)</a>. What makes this second order is the quantification over \(f\), a