Lemma 39.6.2. Let S be a scheme. Let G be a group scheme over S. Let T be a scheme over S and let \psi : T \to G be a morphism over S. If T is flat over S, then the morphism
T \times _ S G \longrightarrow G, \quad (t, g) \longmapsto m(\psi (t), g)
is flat. In particular, if G is flat over S, then m : G \times _ S G \to G is flat.
Proof.
Consider the diagram
\xymatrix{ T \times _ S G \ar[rrr]_{(t, g) \mapsto (t, m(\psi (t), g))} & & & T \times _ S G \ar[r]_{\text{pr}} \ar[d] & G \ar[d] \\ & & & T \ar[r] & S }
The left top horizontal arrow is an isomorphism and the square is cartesian. Hence the lemma follows from Morphisms, Lemma 29.25.8.
\square
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