Building a Personal Lemma Library

AI RNG: Practical Systems That Ship

Most people do not struggle because they cannot learn new theorems. They struggle because what they learned last month is not available when they need it. A personal lemma library is the bridge between reading and reuse. It is a curated collection of small results, proof templates, and standard estimates written in your own language, indexed so you can retrieve them quickly.

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AI can accelerate building this library by extracting lemmas from notes, generating tags, and suggesting cross-links. But the library only becomes powerful when you enforce correctness and keep each entry concrete.

What a lemma library is and what it is not

A lemma library is not a list of theorems copied from a textbook. It is a set of reusable building blocks you can deploy in proofs and problem solving.

Good entries are:

  • Small enough to be used often
  • Stated with clear hypotheses and domains
  • Proven in a way you understand
  • Linked to at least one example use

Bad entries are:

  • Huge results you do not know how to apply
  • Statements without hypotheses because they “seem obvious”
  • Notes that only make sense in the context of the original chapter

The goal is portability: a lemma should still be usable when you have forgotten where you first saw it.

Use a fixed schema so every entry is searchable

The fastest way to lose a library is to let entries drift in format. Use a consistent template.

FieldWhat to writeWhy it matters
Titlea short, memorable nameretrieval and recall
Statementexact claim with hypothesescorrectness and reuse
Proof sketchthe key steps, not every linequick reactivation
Dependencieslemmas or theorems usedprevents circular confusion
Use casesat least one situation where it appliesapplication memory
Failure modewhen it does not applyprevents misuse

AI can generate the first draft of an entry from your notes, but you should always rewrite the statement and proof sketch in your own words. Ownership is part of correctness.

Build the library from your real friction points

The best seed set comes from your error ledger and your stuck moments.

Whenever you get stuck, ask:

  • What small fact would have made the next step easy?
  • Is this fact a standard lemma I should remember?
  • Can I write it as a reusable statement?

Over time, the library becomes a map of how you personally do mathematics.

Tag by technique, not only by topic

Topic tags like “analysis” and “algebra” are too broad to be useful during problem solving. Add technique tags:

  • induction
  • contradiction
  • compactness
  • epsilon-delta
  • inequality tools
  • diagonal argument
  • linear algebra estimates

AI is particularly good at suggesting technique tags based on the proof sketch. You can then standardize them so your search is consistent.

Add cross-links that preserve proof flow

A library becomes powerful when it encodes how lemmas chain.

For each entry, add:

  • prerequisites: lemmas you often use right before this one
  • follow-ons: lemmas that often come next

This turns your library into a proof navigation system. When you forget the next move, the cross-links suggest the path.

Make correctness a first-class feature

A lemma library is dangerous if it contains false statements or missing hypotheses, because it will quietly corrupt future work.

Adopt two simple safeguards:

  • Every entry gets a “hypothesis check” line that lists the conditions you must verify before applying it
  • Every entry gets at least one worked example where you apply it correctly

If a lemma is subtle, add a failure example: an object that violates one hypothesis and breaks the conclusion. This trains you to respect the boundaries.

How AI helps without diluting the library

High-value AI uses:

  • Extract candidate lemmas from your notes and identify their hypotheses
  • Suggest tags and cross-links based on proof structure
  • Generate a short quiz prompt to test recall of the lemma statement
  • Propose a minimal example where the lemma applies

Low-value AI uses:

  • Writing entire entries you do not understand
  • Producing “general statements” that are not actually true
  • Replacing proof sketches with vague confidence

Your library should feel like a toolbox you built, not a cabinet of unknown objects.

The long-term payoff

A lemma library changes how you learn. Instead of feeling like each course is a separate world, you start seeing repeated patterns:

  • The same inequality tools appear in many settings
  • The same compactness move powers different theorems
  • The same linear algebra estimate rescues different arguments

That recognition is what transforms practice into fluency.

Keep Exploring AI Systems for Engineering Outcomes

• Proof Outlines with AI: Lemmas and Dependencies
https://ai-rng.com/proof-outlines-with-ai-lemmas-and-dependencies/

• AI Proof Writing Workflow That Stays Correct
https://ai-rng.com/ai-proof-writing-workflow-that-stays-correct/

• AI for Building Counterexamples
https://ai-rng.com/ai-for-building-counterexamples/

• Formalizing Mathematics with AI Assistance
https://ai-rng.com/formalizing-mathematics-with-ai-assistance/

• Lean Workflow for Beginners Using AI
https://ai-rng.com/lean-workflow-for-beginners-using-ai/

Books by Drew Higgins