Dummit And Foote Solutions Chapter 4 Overleaf Apr 2026

\beginexercise[Section 4.2, Exercise 8] Let $G$ be a $p$-group acting on a finite set $A$. Prove that [ |A| \equiv |\Fix(A)| \pmodp, ] where $\Fix(A) = a \in A : g \cdot a = a \text for all g \in G$. \endexercise

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\beginsolution Let $G$ act on $G/H = gH : g \in G$ by $g \cdot (xH) = (gx)H$. \beginenumerate \item \textbfTransitivity: Take any two cosets $aH, bH \in G/H$. Choose $g = ba^-1 \in G$. Then [ g \cdot (aH) = (ba^-1a)H = bH. ] Hence, there is exactly one orbit, so the action is transitive. \item \textbfStabilizer of $1H$: [ \Stab_G(1H) = g \in G : g \cdot (1H) = 1H = g \in G : gH = H. ] But $gH = H$ if and only if $g \in H$. Therefore $\Stab_G(1H) = H$. \endenumerate \endsolution \beginexercise[Section 4