Geometric Brownian Motion

dS = μS dt + σS dW — the Black-Scholes model for stock prices
E[S(T)] =
Median S(T) =
Mode S(T) =
GBM: S(t) = S₀·exp((μ−σ²/2)t + σW(t))

Log-normal S(T): log S(T) ~ N(log S₀ + (μ−σ²/2)T, σ²T)

Itô correction: μ−σ²/2 (not μ) is the growth rate of log S — Jensen's inequality gap.

Black-Scholes: risk-neutral pricing replaces μ with r; call price = S·N(d₁)−Ke^{−rT}·N(d₂).

Middle chart: distribution of S(T) at maturity. Bottom: log-returns are Gaussian.