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diff --git a/mjo-linear_algebra.tex b/mjo-linear_algebra.tex
index b2ad59c..955dd3a 100644
--- a/mjo-linear_algebra.tex
+++ b/mjo-linear_algebra.tex
@@ -2,8 +2,17 @@
 % Standard operations from linear algebra.
 %
 
+% Needed for \lvert, \rVert, etc. and \operatorname.
+\usepackage{amsmath}
+
 \input{mjo-common}
 
+% Absolute value (modulis) of a scalar.
+\newcommand*{\abs}[1]{ \left\lvert {#1} \right\rvert }
+
+% Norm of a vector.
+\newcommand*{\norm}[1]{ \left\lVert {#1} \right\rVert }
+
 % The inner product between its two arguments.
 \newcommand*{\ip}[2]{ \langle {#1}, {#2} \rangle }