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Official problem 3 · A sharp integral bound under a zero-mean constraint
- Source locator
- Part M, Section 1, official problem 3, paper PDF page 2
- Review note
- Independently derived; extremising sequence and admissibility conditions checked; official-key match · 2026-07-28
Problem summary
Find the sharp weighted-integral supremum for bounded continuous functions whose ordinary integral vanishes.
Core idea
Use the zero-mean condition to subtract one half from the decreasing weight, bound the centred integral by its absolute value, and approach equality with continuous sign approximations.
Derivation
- 1Subtracting one half from the weight leaves the objective unchanged because the integral of the function is zero.
- 2The centred weight changes sign at two, and the pointwise bound gives an absolute-value integral.
- 3Evaluating the two elementary integrals gives log(4/3).
- 4A symmetric continuous transition near the sign-change point has exact zero mean and converges to the extremising sign pattern.
Final answer
The supremum is log(4/3).





