Last month I wrote about building a math AI and mentioned that I think we need to optimize the “library search” problem. In this post I’d like to explain what library search is, why it’s important, and what we can do about it.


A few years ago I got excited about the IMO Grand Challenge. I thought it would be interesting to look for a team of smart people working on it and see if I could help out. One group of people was focused on using the Lean theorem prover, and one place it seemed like I could help out was formalizing IMO problems in Lean, to use as training data. I formalized some of them, for example here and here, but it seemed like progress was so slow, the whole strategy was unsound. If it takes you more than a day to code review a single point of training data, it’s going to be hard to train an AI. So, I gave up on this approach. But, I did learn a lot about the annoying parts of formalizing proofs.

Lean has many different tactics for solving proofs. One general operation that you do all the time is just “applying a theorem”.

For example, let’s say you have two existing theorems. Foo implies bar, and bar implies baz. Now you want to prove that foo implies baz. In Lean this proof looks like:

theorem foo_implies_baz : foo → baz :=
  intro _,
  apply bar_implies_baz,
  apply foo_implies_bar,

In particular, you need to know the names of the existing theorems you want to use, foo_implies_bar and bar_implies_baz. “library search” is the name of a Lean tactic that tries to figure this out for you, but is slow and fails a lot. (The tactic is now renamed exact?, but it’ll always be library_search! in my heart.)

Why This Is Hard For Humans

It’s hard for humans to remember all these theorem names.

This might be unintuitive. For a programmer, it doesn’t seem unreasonable that you have to know the name of a function you’re calling. How could it be any other way?

For a mathematician, a way of manipulating a formula doesn’t typically have a name. Imagine you’re doing long division. You can remember the whole process, but you don’t give a name to every step of the way. There are too many different steps, only slightly different from each other.

It becomes harder as the size of the library increases. It’s like the category of jokes where one mathematician calls a problem “trivial” but another mathematician is stumped by it.

Take a look at these theorems and it’ll make more sense why it’s hard. There is just an explosion of basic facts about numbers, all of which a reasonably competent mathematician would just say, “this is trivially true”.

For example, really do click that link above. Then take a look at eq_mul_of_div_eq_left. In English, “if a is a multiple of b, and a divided by b rounded down is c, then c times b is a.”

For enlightenment, go ahead and read the definitions of div_eq_iff_eq_mul_left, add_mod_eq_add_mod_left, and the dozens of others of similarly named theorems.

In programming, a library with 400 rarely-used functions would be unusable. In mathematics, though, it’s fine, in a sense. As long as everything is true, and a human looking at it thinks it’s obviously true, then the only problem is when you have to know these names.

Why This Is Hard For Computers

It’s hard for humans to memorize a whole dictionary, too. For computers, it’s easy. So why isn’t this lookup easy for computers? Why isn’t library search nicely solved in existing proof assistants?

I do think this is solvable. The basic problem is the inexact nature of the lookup. Theorems have variables in them. When you know that a < b implies a + c < b + c, this theorem is true for any values of a, b, and c. As you have more facts available, and more theorems in your library, this gets slower. In the most general case, you have to do an exponential matching process.

That said, people have found some reasonable data structures for doing this sort of lookup. The discrimination tree and fingerprint indexing from the E theorem prover are good examples. And I think a lot of it is the sort of performance low-hanging fruit where if you specifically design data structures and profile for this, you’ll be able to do better than tacking a lookup structure on after the fact.

Will LLMs just solve this automatically, with scale, by putting the entire library inside their context window? It’s possible. Maybe there’s a good argument against this. I’m honestly not sure how to predict the future path of LLM skills. At some point I have to fall back on my thesis of “scaling LLMs will not solve all outstanding problems in everything”, if only by a sort of Pascal’s reasoning.

The Ideal Proof

A formalized proof should not have to be constantly citing the names of theorems. For either a human or an AI, rather than writing and validating this proof:

theorem foo_implies_baz : foo → baz :=
  intro _,
  apply bar_implies_baz,
  apply foo_implies_bar,

you should be able to write the proof the way a human writes a proof in prose, by just stating true things that follow from the premises without giving an explicit rationale for them.

theorem foo_implies_baz: foo -> baz {

In 99% of cases, the compiler should just be automatically handling the library search for you. I think natural exceptions would be fancy tactics like induction. In most human-written proofs, when you’re using induction, you do say, “now I’m using induction”.

You can think of this like a mini proof search. It’s just that you are searching just a few steps. One retrieval from the library, plus a bit of normalizing between logically equivalent expressions. It’s like a base case that the proof search needs to handle before it can expand to more complicated cases.


I think automatically handling library search will make proofs easier for both humans and AIs. You shouldn’t need to use the names of theorems.

There’s a related, somewhat harder problem of “rewrite search”. Originally I was going to write about that, too, but this post is getting long, so I’ll cut off here. Thanks for reading! Next month I’ll write about the rewrite search problem.