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A bit of light googling turns up several sources asserting that the model structure on simplicial commutative algebras over a ring is left proper (for example, 2.9 in Charles Rezk's paper Every homotopy theory of simplicial algebras admits a proper model). Does a proof of this fact occur anywhere in the literature?

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I don't know where it "appears", though of course it is not hard to prove; it amounts to the fact that polynomial algebras are flat. –  Charles Rezk Sep 27 '12 at 2:48

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