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It is trivially true but maybe worth noting that the converse is also true - if there exists such an f, then Y is σ(X)-measurable.

This and the question asked are theorem 20.1(ii) in Billingsley's Probability and Measure, 3rd edition.

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It is trivially true but maybe worth noting that the converse is also true - if there exists such an f, then Y is σ(X)-measurable.