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[Merged by Bors] - feat(analysis/fourier): convergence of Fourier series #17913

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@loefflerd loefflerd commented Dec 12, 2022

This PR adds a straightforward but useful criterion for convergence of Fourier series: for a continuous periodic function f, if the sequence of Fourier coefficients of f is absolutely summable, then the Fourier series converges uniformly, and hence pointwise everywhere, to f. (Note that it is obvious in this case that the Fourier series converges uniformly to something, the difficult part is that the limit is actually f.)


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@loefflerd loefflerd marked this pull request as ready for review December 12, 2022 20:44
@loefflerd loefflerd requested a review from a team as a code owner December 12, 2022 20:44
@loefflerd loefflerd added the awaiting-review The author would like community review of the PR label Dec 12, 2022
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Looks good. Given how many times you use (fourier_series.repr (to_Lp 2 haar_add_circle ℂ f) i for f a continuous function (and I expect it to show up more and more in the future), would it make sense to introduce f.fourier_coeff i for this?

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@sgouezel sgouezel added awaiting-author A reviewer has asked the author a question or requested changes t-analysis Analysis (normed *, calculus) and removed awaiting-review The author would like community review of the PR labels Dec 16, 2022
loefflerd and others added 2 commits December 16, 2022 14:17
Co-authored-by: sgouezel <sebastien.gouezel@univ-rennes1.fr>
@loefflerd loefflerd 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 Dec 17, 2022
@mathlib-dependent-issues-bot mathlib-dependent-issues-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 Jan 7, 2023
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@loefflerd loefflerd marked this pull request as draft January 7, 2023 22:15
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Marking this as draft because in the time it took for the prerequisites to get merged, I realised that the changes to "continuous_map.eval_clm" could be done in a different (bettter) way.

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... and the alternative approach doesn't work. Sorry for the noise.

@loefflerd loefflerd marked this pull request as ready for review January 7, 2023 22:50
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bors d+
Thanks!

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bors bot commented Jan 8, 2023

✌️ loefflerd can now approve this pull request. To approve and merge a pull request, simply reply with bors r+. More detailed instructions are available here.

@leanprover-community-bot-assistant leanprover-community-bot-assistant added delegated The PR author may merge after reviewing final suggestions. and removed awaiting-review The author would like community review of the PR labels Jan 8, 2023
loefflerd and others added 3 commits January 8, 2023 10:19
Co-authored-by: sgouezel <sebastien.gouezel@univ-rennes1.fr>
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Thanks for the review!

bors r+

bors bot pushed a commit that referenced this pull request Jan 8, 2023
This PR adds a straightforward but useful criterion for convergence of Fourier series: for a continuous periodic function `f`, if the sequence of Fourier coefficients of `f` is absolutely summable, then the Fourier series converges uniformly, and hence pointwise everywhere, to `f`. (Note that it is obvious in this case that the Fourier series converges uniformly to _something_, the difficult part is that the limit is actually `f`.)



Co-authored-by: Junyan Xu <junyanxu.math@gmail.com>
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bors bot commented Jan 8, 2023

Pull request successfully merged into master.

Build succeeded:

@bors bors bot changed the title feat(analysis/fourier): convergence of Fourier series [Merged by Bors] - feat(analysis/fourier): convergence of Fourier series Jan 8, 2023
@bors bors bot closed this Jan 8, 2023
@bors bors bot deleted the to_Lp_injective branch January 8, 2023 14:24
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6 participants