frontpage.
newsnewestaskshowjobs

Made with ♥ by @iamnishanth

Open Source @Github

fp.

Open in hackernews

Some Junk Theorems in Lean

https://github.com/James-Hanson/junk-theorems-in-lean
29•saithound•4d ago

Comments

andyjohnson0•1h ago
TIL that "junk theorems" are a thing in mathematics. Not being a mathematician myself, I found this [1] article a useful primer.

[1] https://www.cantorsparadise.com/what-are-junk-theorems-29868...

akoboldfrying•8m ago
This was helpful, thanks.
emil-lp•1h ago
I don't understand. What does this mean?

    Theorem 6. The following are equivalent: The binary expansion of 7.
silasdavis•52m ago
The following are equivalent:
bzax•15m ago
It doesn't mean anything. The point is that the language of lean, and its proof derivation system, are able to express (and prove) statements that do not correspond to any meaningful mathematics.
tux3•12m ago
This is a junk theorem, it's trying to prove something that will sound strange or meaningless but is technically allowed by the details of the foundations.

Here it's building a list with one element and saying all elements of this list are equivalent. So the following elements of the list are all equivalent to each other (there is a single element in the list)

414owen•59m ago
Wow, okay. I would imagine this makes mathematicians quite angry? I guess you're responsible for all the operations you use in your proof being well-behaved.

It sounds like subtraction over Nats needs to be split into `sub?`, and `sub!`, the former returning an option, and the latter crashing, on underflow, as is the Lean convention?

To use the default `sub`, you should need to provide a witness that the minuend is >= the subtrahend...

The version with silent underflow is still useful, it should just be called `saturatingSub`, or something, so that mathematicians using it know what they're getting themselves into...

frotaur•10m ago
I don't know much about Lean, but I attended an introductory talk at some point and I was particularly bothered by these partial function definitions. The example was sqrt, which would give 0 on the negatives.

Now, of course, if you're careful with the definitions you use, there is no problem. But in the (increasingly relevant) context of automatic theorem proving with LLMs, this seems to defeat the 'groundtruthness' of Lean!

How do you make sure that the LLM doesn't reward hack a proof using these workarounds?

Splice a Fibre

https://react-networks-lib.rackout.net/fibre
11•matt-p•50m ago•1 comments

Show HN: Ez FFmpeg – Video editing in plain English

http://npmjs.com/package/ezff
130•josharsh•4h ago•43 comments

How uv got so fast

https://nesbitt.io/2025/12/26/how-uv-got-so-fast.html
977•zdw•19h ago•330 comments

Cursed Bundler: Using go get to install Ruby Gems

https://nesbitt.io/2025/12/25/cursed-bundler-using-go-get-to-install-ruby-gems.html
23•SPBS•1h ago•3 comments

Mruby: Ruby for Embedded Systems

https://github.com/mruby/mruby
51•nateb2022•5d ago•12 comments

Intertapes – collection of found cassette tapes from different locations

https://intertapes.net/
15•wallflower•5d ago•1 comments

Faster Practical Modular Inversion

https://purplesyringa.moe/blog/faster-practical-modular-inversion/
4•todsacerdoti•6d ago•0 comments

Show HN: Mysti – Claude, Codex, and Gemini debate your code, then synthesize

https://github.com/DeepMyst/Mysti
10•bahaAbunojaim•3d ago•4 comments

Exe.dev

https://exe.dev/
268•achairapart•13h ago•139 comments

Always bet on text (2014)

https://graydon2.dreamwidth.org/193447.html
243•jesseduffield•13h ago•121 comments

Some Junk Theorems in Lean

https://github.com/James-Hanson/junk-theorems-in-lean
29•saithound•4d ago•8 comments

Langjam-Gamejam Devlog: Making a language, compiler, VM and 5 games in 52 hours

https://github.com/Syn-Nine/gar-lang/blob/main/DEVLOG.md
57•suioir•5d ago•4 comments

The battle to stop clever people betting

https://www.economist.com/christmas-specials/2025/12/18/the-battle-to-stop-clever-people-betting
8•zoenolan•11h ago•1 comments

The best things and stuff of 2025

https://blog.fogus.me/2025/12/23/the-best-things-and-stuff-of-2025.html
289•adityaathalye•3d ago•31 comments

QNX Self-Hosted Developer Desktop

https://devblog.qnx.com/qnx-self-hosted-developer-desktop-initial-release/
176•transpute•11h ago•95 comments

Package managers keep using Git as a database, it never works out

https://nesbitt.io/2025/12/24/package-managers-keep-using-git-as-a-database.html
662•birdculture•1d ago•374 comments

More dynamic cronjobs

https://george.mand.is/2025/09/more-dynamic-cronjobs/
55•0928374082•6h ago•12 comments

Publishing your work increases your luck

https://github.com/readme/guides/publishing-your-work
147•magoghm•12h ago•46 comments

Experts explore new mushroom which causes fairytale-like hallucinations

https://nhmu.utah.edu/articles/experts-explore-new-mushroom-which-causes-fairytale-hallucinations
408•astronads•19h ago•230 comments

AI Police Reports: Year in Review

https://www.eff.org/deeplinks/2025/12/ai-police-reports-year-review
148•hn_acker•3d ago•107 comments

One million (small web) screenshots

https://nry.me/posts/2025-10-09/small-web-screenshots/
113•squidhunter•4d ago•11 comments

How Lewis Carroll computed determinants (2023)

https://www.johndcook.com/blog/2023/07/10/lewis-carroll-determinants/
190•tzury•17h ago•48 comments

SIMD City: Auto-Vectorisation

https://xania.org/202512/20-simd-city
49•brewmarche•6d ago•10 comments

Show HN: Witr – Explain why a process is running on your Linux system

https://github.com/pranshuparmar/witr
353•pranshuparmar•21h ago•76 comments

Researchers develop a camera that can focus on different distances at once

https://engineering.cmu.edu/news-events/news/2025/12/19-perfect-shot.html
57•gnabgib•3d ago•18 comments

Inside the proton, the ‘most complicated thing you could possibly imagine’ (2022)

https://www.quantamagazine.org/inside-the-proton-the-most-complicated-thing-imaginable-20221019/
64•tzury•9h ago•12 comments

LearnixOS

https://www.learnix-os.com
241•gtirloni•23h ago•95 comments

Moravec's Paradox and the Robot Olympics

https://www.physicalintelligence.company/blog/olympics
68•beklein•3d ago•7 comments

Toys with the highest play-time and lowest clean-up-time

https://joannabregan.substack.com/p/toys-with-the-highest-play-time-and
402•surprisetalk•16h ago•240 comments

T-Ruby is Ruby with syntax for types

https://type-ruby.github.io/
140•thunderbong•16h ago•108 comments