% The orthogonal projection of its second argument onto the first.
\newcommand*{\proj}[2] { \operatorname{proj}\of{#1, #2} }
+% The set of all eigenvalues of its argument, which should be either a
+% matrix or a linear operator. The sigma notation was chosen instead
+% of lambda so that lambda can be reserved to denote the ordered tuple
+% (largest to smallest) of eigenvalues.
+\newcommand*{\spectrum}[1]{\sigma\of{{#1}}}
+\ifdefined\newglossaryentry
+ \newglossaryentry{spectrum}{
+ name={\ensuremath{\spectrum{L}}},
+ description={the set of all eigenvalues of $L$},
+ sort=s
+ }
+\fi
+
% The ``Automorphism group of'' operator.
\newcommand*{\Aut}[1]{ \operatorname{Aut}\of{{#1}} }