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Help on query formulation
If $F(x) = \int _{x}^{2x} f(t) \, \mathrm{dt}$ is constant, must $f(t)=c/t$? (English)
Coll. Math. J. 36, No. 3, 199-204 (2005).
There are examples showing that the answer is "no".
Classification: I55