What does this equation mean?

Semi-endogenous growth theory, the framework Jones developed and that his 2022 Annual Review of Economics piece surveys, formalizes this into a relationship between population growth and the long-run growth rate of ideas [ 4 ] . Population enters the idea-production function as the stock of people available to search the technological fitness landscape for improvements, and because ideas are non-rival — one firm’s discovery of a better transistor layout does not use up the possibility for another firm to discover a different one — expanding that search population raises the rate of discovery roughly in proportion to its own growth, discounted by how much duplicated effort dilutes the return to any one searcher and by how much each new discovery makes the next one to find. The canonical form of the relationship is $g_A = \frac{\lambda n}{1-\phi}$. where n is the growth rate of the population of researchers, $\phi$ measures how much easier or harder each new idea makes the next one to find (values below 1 are required for the model’s growth path to be stable at all — the discipline’s own name for this is diminishing returns to knowledge accumulation), and $\lambda$ measures how much duplicated research effort — different teams stumbling onto the same idea — dilutes each additional researcher’s marginal contribution. The formula says something that should be intuitive from the fitness-landscape framing: if the population of people searching for new technology stops growing, n falls toward zero, and the long-run growth rate of ideas falls with it, one for one, unless something else changes $\lambda$ or $\phi$ .