If $f:X \to \mathbb{R}$ is summable function, then the set $$A=\{x \in X : f(x) \neq 0\}$$ must be countable.
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If $f:X \to \mathbb{R}$ is summable function, then the set $$A=\{x \in X : f(x) \neq 0\}$$ must be countable.