= Solution
On a product of sigma-finite measure spaces, <Tonelli's theorem> equates product and iterated integrals for a nonnegative measurable <function>, allowing infinite values. <Fubini's theorem> says that an absolutely integrable measurable <function> has integrable sections almost everywhere, their section integrals are integrable, and both iterated integrals equal the product integral. Absolute integrability may be checked first by applying Tonelli to the absolute value.
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