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[Merged by Bors] - refactor(measure_theory/simple_func_dense): generalize approximation results from L^1 to L^p #8114

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@hrmacbeth hrmacbeth commented Jun 29, 2021

Simple functions are dense in L^p. The argument for 1 ≤ p < ∞ is exactly the same as for p = 1, which was already in mathlib: construct a suitable sequence of pointwise approximations and apply the Dominated Convergence Theorem. This PR refactors to provide the general-p result.

The argument for p = ∞ requires finite-dimensionality of E and a different approximating sequence, so is left for another PR.


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@hrmacbeth hrmacbeth added the awaiting-review The author would like community review of the PR label Jun 29, 2021
@github-actions github-actions bot added the blocked-by-other-PR This PR depends on another PR which is still in the queue. A bot manages this label via PR comment. label Jun 29, 2021
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Thanks for this work!
I see that you use snorm' in several places. Would it be very inconvenient to use snorm instead?
My motivation for the change is that the results on Lp functions use norm, which is defined as snorm, which is snorm' in most cases, which is an integral. That is a lot of different names for the same thing and I was thinking about minimizing the uses of snorm', making it into a tool used to build snorm but not used outside of that role.

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Sure, that sounds like a good rule to have! I'll make the change.

@github-actions github-actions bot removed the blocked-by-other-PR This PR depends on another PR which is still in the queue. A bot manages this label via PR comment. label Jun 29, 2021
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🎉 Great news! Looks like all the dependencies have been resolved:

💡 To add or remove a dependency please update this issue/PR description.

Brought to you by Dependent Issues (:robot: ). Happy coding!

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@RemyDegenne I switched to snorm! It requires a different few new helper lemmas in measure_theory/lp_space, so could you please make sure to review that part again?

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Looks good to me!
I would change a few lemma names, though.

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Co-authored-by: Rémy Degenne <remydegenne@gmail.com>
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LGTM, thanks!

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@sgouezel sgouezel added awaiting-author A reviewer has asked the author a question or requested changes and removed awaiting-review The author would like community review of the PR labels Jul 2, 2021
@hrmacbeth hrmacbeth added awaiting-review The author would like community review of the PR and removed awaiting-author A reviewer has asked the author a question or requested changes labels Jul 2, 2021
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sgouezel commented Jul 3, 2021

bors r+
Thanks!

@github-actions github-actions bot added ready-to-merge All that is left is for bors to build and merge this PR. (Remember you need to say `bors r+`.) and removed awaiting-review The author would like community review of the PR labels Jul 3, 2021
bors bot pushed a commit that referenced this pull request Jul 3, 2021
…results from L^1 to L^p (#8114)

Simple functions are dense in L^p.  The argument for `1 ≤ p < ∞` is exactly the same as for `p = 1`, which was already in mathlib:  construct a suitable sequence of pointwise approximations and apply the Dominated Convergence Theorem.  This PR refactors to provide the general-`p` result.

The argument for `p = ∞` requires finite-dimensionality of `E` and a different approximating sequence, so is left for another PR.
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bors bot commented Jul 3, 2021

Pull request successfully merged into master.

Build succeeded:

@bors bors bot changed the title refactor(measure_theory/simple_func_dense): generalize approximation results from L^1 to L^p [Merged by Bors] - refactor(measure_theory/simple_func_dense): generalize approximation results from L^1 to L^p Jul 3, 2021
@bors bors bot closed this Jul 3, 2021
@bors bors bot deleted the simple_dense_lp branch July 3, 2021 13:39
bors bot pushed a commit that referenced this pull request Jul 4, 2021
…construction of embedding of L1 simple funcs (#8185)

At the moment, there is a low-level construction in `measure_theory/simple_func_dense` for the approximation of an element of L1 by simple functions, and this is generalized to a more abstract version (with a normed space `L1.simple_func` and a dense linear embedding of this into `L1`) in `measure_theory/bochner_integration`.  #8114 generalized the low-level construction, and I am thinking of rewriting the more abstract version to apply to `Lp`, too.

But since this will all be more generally useful than in the construction of the Bochner integral, and since the Bochner integral file is very long, I propose moving all this material into `measure_theory/simple_func_dense`.  This PR shows what it would look like.  There are no mathematical changes.
b-mehta pushed a commit that referenced this pull request Jul 6, 2021
…results from L^1 to L^p (#8114)

Simple functions are dense in L^p.  The argument for `1 ≤ p < ∞` is exactly the same as for `p = 1`, which was already in mathlib:  construct a suitable sequence of pointwise approximations and apply the Dominated Convergence Theorem.  This PR refactors to provide the general-`p` result.

The argument for `p = ∞` requires finite-dimensionality of `E` and a different approximating sequence, so is left for another PR.
b-mehta pushed a commit that referenced this pull request Jul 6, 2021
…construction of embedding of L1 simple funcs (#8185)

At the moment, there is a low-level construction in `measure_theory/simple_func_dense` for the approximation of an element of L1 by simple functions, and this is generalized to a more abstract version (with a normed space `L1.simple_func` and a dense linear embedding of this into `L1`) in `measure_theory/bochner_integration`.  #8114 generalized the low-level construction, and I am thinking of rewriting the more abstract version to apply to `Lp`, too.

But since this will all be more generally useful than in the construction of the Bochner integral, and since the Bochner integral file is very long, I propose moving all this material into `measure_theory/simple_func_dense`.  This PR shows what it would look like.  There are no mathematical changes.
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