small tweak to burnside

Author: UlrikBuchholtz

actions.tex

@@ -2351,10 +2351,10 @@ \section{The lemma that is not Burnside's}
   \label{lem:burnside}
   Let $G$ be a finite group and let $X:\BG\to\Set$ be a finite $G$-set.
   Define $X^g = \setof{x:X(\sh_G)}{g\cdot x = x}$ for any $g:\USymG$.
-  Then $X^g$, $\sum_{g:\USymG} X^g$ and the set of orbits $X/G$
+  Then each $X^g$, the sum type $\sum_{g:\USymG} X^g$, and the set of orbits $X/G$
   are finite sets, and we have
   \[
-    \Card(\sum_{g:\USymG} X^g) = \Card(X/G) \times \Card(G).
+    \Card\Bigl(\sum_{g:\USymG} X^g\Bigr) = \Card(X/G) \times \Card(G).
   \]
 \end{lemma}
 \begin{proof}