I am taking a class on Theory of Computation, and can't seem to find what the subscript df in this image stands for. For context, we are discussing Set Theory, specifically countably and uncountably infinite sets.
2026-04-05 14:15:07.1775398507
What does the subscript df mean in this image?
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It stands for “definition”. It means that $X$ is defined as $\{n\mid n\notin X_n\}$.