Lecture 6: The Double Dual and the Outer Measure of a Subset of Real Numbers

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MIT 18.102 Introduction to Functional Analysis, Spring 2021
Instructor: Dr. Casey Rodriguez

We wrap up our discussion of the Hahn-Banach theorem and move onto the basic notions of measure theory: the second major unit of this course. We define the outer measure (a first guess of “measure”) and prove some basic properties of this new tool.

License: Creative Commons BY-NC-SA

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0:26 Hahn-Banach recap
3:17 Hahn-Banach application: unit-norm in dual space
8:17 Double dual
24:35 Reflexive spaces
28:47 Lebesgue integration / outer measure
40:35 Actual start of Lebesgue integration
52:56 Outer measure
1:06:50 Outer measure additivity theorem

travischapman
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The introduction to the Lebesgue integral and measure theory was really smooth. Good stuff!

TheTacticalDood
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The norm is a real valued function, the range of the bounded linear functionals is the field of complex numbers. In your demonstration of boundedness, you are comparing the size of a real number to a complex number, which doesn't make sense. You need the magnitude of the complex number in front of the norm.

briang.valentine
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In 50:41 the [INAUDIBLE] part is "due to 'Carathéodory'".

emir
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7:35 I don't understand why does the inequality hold? Why the t has unit length?

meaningseeker
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22:30 why |f(v)|<= ||T_v|| * ||f||?

HaozheJiang