Drake Equation
Frank Drake’s 1961 factorization of the expected number of communicating civilizations in the Milky Way as a product of star-formation rate, planet fraction, habitable-fraction, life-emergence probability, intelligence emergence, communicative emergence, and civilization lifetime. Iconic in the SETI literature for emphasizing that the answer hinges on parameters whose values span many orders of magnitude.
The structure inspired the venus-life-equation (Izenberg et al. 2021) and is cited as such in src-venus-panspermia-earth-origin-2026-04.