MHB Solving for $Mf \notin L^1(\mathbb{R})$ with $f \in L^1 (\mathbb{R})$

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mathmari
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Hello! :o

How can we find a $f \in L^1 (\mathbb{R})$ such taht $Mf \notin L^1 ( \mathbb{R})$, where $Mf$ is the maximum function ?? (Wondering)
 
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What is the maximum function?
 
A sphere as topological manifold can be defined by gluing together the boundary of two disk. Basically one starts assigning each disk the subspace topology from ##\mathbb R^2## and then taking the quotient topology obtained by gluing their boundaries. Starting from the above definition of 2-sphere as topological manifold, shows that it is homeomorphic to the "embedded" sphere understood as subset of ##\mathbb R^3## in the subspace topology.
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