Where e Hides

The number e turns up far beyond banking. The fraction of radioactive atoms left after one half-life follows an e-based curve. Population growth, the rate a drug clears your bloodstream, and even the shape of a chain hanging between two posts (a catenary) all involve e.

The pattern was first noticed in finance. Asked what happens if interest is compounded monthly, then daily, then every second, the answer kept converging to one fixed number — e.