equation error fix

Author: wsehjk

chapter_preliminaries/calculus.md

@@ -333,7 +333,8 @@ by $\nabla f(\mathbf{x})$.
 The following rules come in handy 
 for differentiating multivariate functions:
 
-* For all $\mathbf{A} \in \mathbb{R}^{m \times n}$ we have $\nabla_{\mathbf{x}} \mathbf{A} \mathbf{x} = \mathbf{A}^\top$ and $\nabla_{\mathbf{x}} \mathbf{x}^\top \mathbf{A}  = \mathbf{A}$.
+* For all $\mathbf{A} \in \mathbb{R}^{m \times n}$ we have $\nabla_{\mathbf{x}} \mathbf{A} \mathbf{x} = \mathbf{A}^\top$.
+* For all $\mathbf{A} \in \mathbb{R}^{n \times m}$ we have  $\nabla_{\mathbf{x}} \mathbf{x}^\top \mathbf{A} = \mathbf{A}$.
 * For square matrices $\mathbf{A} \in \mathbb{R}^{n \times n}$ we have that $\nabla_{\mathbf{x}} \mathbf{x}^\top \mathbf{A} \mathbf{x}  = (\mathbf{A} + \mathbf{A}^\top)\mathbf{x}$ and in particular
 $\nabla_{\mathbf{x}} \|\mathbf{x} \|^2 = \nabla_{\mathbf{x}} \mathbf{x}^\top \mathbf{x} = 2\mathbf{x}$.