Mathematical discoveries from program search with large language models (FunSearch)

Source: Romera-Paredes et al., Nature · 2023-12-14

Large language models (LLMs) have demonstrated tremendous capabilities in solving complex tasks, from quantitative reasoning to understanding natural language. However, LLMs sometimes suffer from confabulations (or hallucinations), which can result in them making plausible but incorrect statements1,2. This hinders the use of current large models in scientific discovery. Here we introduce FunSearch (short for searching in the function space), an evolutionary procedure based on pairing a pretrained LLM with a systematic evaluator. We demonstrate the effectiveness of this approach to surpass the best-known results in important problems, pushing the boundary of existing LLM-based approaches3. Applying FunSearch to a central problem in extremal combinatorics—the cap set problem—we discover new constructions of large cap sets going beyond the best-known ones, both in finite dimensional and asymptotic cases. This shows that it is possible to make discoveries for established open problems using LLMs. We showcase the generality of FunSearch by applying it to an algorithmic problem, online bin packing, finding new heuristics that improve on widely used baselines. In contrast to most computer search approaches, FunSearch searches for programs that describe how to solve a problem, rather than what the solution is. Beyond being an effective and scalable strategy, discovered programs tend to be more interpretable than raw solutions, enabling feedback loops between domain experts and FunSearch, and the deployment of such programs in real-world applications.

FunSearch makes discoveries in established open problems using large language models by searching for programs describing how to solve a problem, rather than what the solution is.

Many problems in mathematical sciences are ‘easy to evaluate’, despite being typically ‘hard to solve’. For example, in computer science, NP-complete optimization problems admit a polynomial-time evaluation procedure (measuring the quality of the solution), despite the widespread belief that no polynomial-time algorithms to solve such problems exist. We focus in this paper on problems admitting an efficient ‘evaluate’ function, which measures the quality of a candidate solution. Prominent examples include the maximum independent set problem and maximum constraint satisfaction problems (such as finding the ground state energy of a Hamiltonian). Our goal is to generate a ‘solve’ program, such that its outputs receive high scores from the ‘evaluate’ function (when executed on inputs of interest), and ultimately improve on the best-known solutions.

FunSearch (short for searching in the function space) combines a pretrained (frozen) LLM, whose goal is to provide creative solutions, with an evaluator, which guards against confabulations and incorrect ideas. FunSearch iterates over these two components, evolving initial low-scoring programs into high-scoring ones discovering new knowledge. Key to the success of this simple procedure is a combination of several essential ingredients. First, we sample best performing programs and feed them back into prompts for the LLM to improve on; we refer to this as best-shot prompting. Second, we start with a program in the form of a skeleton (containing boilerplate code and potentially known structure about the problem), and only evolve the part governing the critical program logic. For example, by setting a greedy program skeleton, we evolve a priority function used to make decisions at every step. Third, we maintain a large pool of diverse programs by using an island-based evolutionary method that encourages exploration and avoids local optima. Finally, leveraging the highly parallel nature of FunSearch, we scale it asynchronously, considerably broadening the scope of this approach to find new results, while keeping the overall cost of experiments low.

We show the surprising effectiveness of FunSearch on several use cases. We consider a fundamental problem in extremal combinatorics, namely, the cap set problem21,22. FunSearch demonstrates the existence of hitherto unknown constructions that go beyond existing ones, including the largest improvement in 20 years to the asymptotic lower bound. This demonstrates that it is possible to make a scientific discovery—a new piece of verifiable knowledge about a notorious scientific problem—using an LLM. Using FunSearch, we also find new algorithms for the online bin packing problem that improve on traditional ones on well-studied distributions of interest23,24, with potential applications to improving job scheduling algorithms.

Whereas most computer search techniques output directly what the solution is (for example, a list of vectors forming a cap set), FunSearch produces programs generating the solution. For structured problems, such programs tend to be more interpretable—facilitating interactions with domain experts—and concise—making it possible to scale to large instances—compared to a mere enumeration of the solution. In addition, decision procedures (such as for bin packing) described by code in a standard programming language are crucially easier to deploy compared to other types of descriptions (for example, neural networks), which typically require specialized hardware and for which verifying design specifications is notoriously hard.

The input to FunSearch is a specification of the problem in the form of an ‘evaluate’ function, which scores candidate solutions. In addition, we provide an initial program (which can be trivial) to evolve. Although in principle these are the minimum requirements, we found that performance tends to improve significantly if we write the initial ‘solve’ program in the form of a skeleton (containing boilerplate code and previous knowledge of the problem in the form of a program structure), and only use FunSearch to evolve the critical part that governs its logic. Fig. 2a shows an example in which the skeleton takes the form of a simple greedy algorithm, and the crucial part to evolve by FunSearch is the priority function that is used to make the greedy decision at every step. This delegates to FunSearch precisely the part that is usually the hardest to come up with. Whereas a fixed skeleton may constrain the space of programs that can be discovered, we find it improves overall results because it focuses the LLM resources on only evolving the critical part, instead of also using the LLM to recreate already known program structures (with more opportunities for mistakes that would render the entire program incorrect). If available, the user can optionally provide extra known information about the problem at hand, in the form of docstrings, relevant primitive functions or import packages, which FunSearch may use.

The LLM is the creative core of FunSearch, in charge of coming up with improvements to the functions presented in the prompt and sending these for evaluation. We obtain our results with a pretrained model, that is, without any fine-tuning on our problems. We use Codey, an LLM built on top of the PaLM2 model family25, which has been fine-tuned on a large corpus of code and is publicly accessible through its API26. Because FunSearch relies on sampling from an LLM extensively, an important performance-defining tradeoff is between the quality of the samples and the inference speed of the LLM. In practice, we have chosen to work with a fast-inference model (rather than slower-inference, higher-quality), and the results in the paper are obtained using a total number of samples on the order of 106. Beyond this tradeoff, we have empirically observed that the results obtained in this paper are not too sensitive to the exact choice of LLM, as long as it has been trained on a large enough corpus of code. See Supplementary Information Appendix A for a comparison to StarCoder6, a state-of-the-art open-source LLM for code.

The programs database keeps a population of correct programs, which are then sampled to create prompts. Preserving and encouraging diversity of programs in the database is crucial to enable exploration and avoid being stuck in local optima. To encourage diversity, we adopt an islands model, also known as a multiple population and multiple-deme model27,28, which is a genetic algorithm approach. Several islands, or subpopulations, are created and evolved independently. To sample from the program database, we first sample an island and then sample a program within that island, favouring higher-scoring and shorter programs (see Methods for the exact mechanism). Crucially, we let information flow between the islands by periodically discarding the programs in the worst half of the islands (corresponding to the ones whose best individuals have the lowest scores). We replace the programs in those islands with a new population, initialized by cloning one of the best individuals from the surviving islands.