Observed arrival · 2026-10-29
Lemma, the proof checker that stops at the first bad line
A math-learning tool that checks typed or uploaded proofs, identifies where reasoning first fails, and offers a nudge instead of a finished answer.
○Open
⊠Login
$Paid
†Ads
✦Pretty
●Pro
◎Niche
◉Human
⚑Risk
ƒJS
- For
- Students practising mathematical proof
- Worth noticing
- Its sample flags the false inference that an even square root must be even, then suggests trying the contrapositive.
Field notes
The sample problem asks readers to prove that if n² is even, then n is even. Lemma marks the step from an even square to an even square root as the failure, explains that the square root may not be an integer, and suggests trying the contrapositive. The page lists problem counts across 17 subjects, from 50 problems in complex analysis and abstract algebra to 22,813 in arithmetic.
Observed signals
Read the marks
Editorial observations of this landing page, not a rating.
○OpenPublic substance visible
✦PrettyNotable craft visible
●ProPolished or operationally mature
◎NicheUnusually specific use
ƒJavaScriptBrowser-side code central
One card from the complete issue