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Answers for “在这个复分析讲座中使用的 ML 不等式是什么?”

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The ML inequality is an estimation tool for contour integrals. It states that the absolute value of the integral of a function along a path is bounded by the supremum (maximum) of the absolute value of the function on that path, multiplied by the length of the path.

Conditions: The function is integrated along a rectifiable path.; The function is continuous on the path.

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The estimation lemma (ML inequality) is applied by bounding the absolute value of the integral of f(w)−f(a)f(w) - f(a) over the path γz\gamma_z by the supremum of ∣f(w)−f(a)∣|f(w) - f(a)| on γz\gamma_z multiplied by the length of the path γz\gamma_z. Since γz\gamma_z is the straight line segment from a to z, its length is ∣z−a∣|z - a|.

Conditions: The path γz\gamma_z is a rectifiable contour (specifically, a straight line segment).; The function f is continuous on γz\gamma_z.; The integrand is f(w)−f(a)f(w) - f(a).

Understand why

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The factors cancel because the ML inequality introduces the length of the integration path, which is ∣z−a∣|z-a| for the straight line segment from a to z. This length factor multiplies the supremum term, and it exactly cancels with the 1/∣z−a∣1/|z-a| factor that comes from the denominator of the difference quotient.

Conditions: The integration path is the straight line segment from a to z.; The difference quotient has a denominator of z−az-a, whose absolute value is ∣z−a∣|z-a|.; The ML inequality is applied to the integral over this segment.