 Reflection principle (Wiener process)

Reflecting a Brownian path after its first hit of a level $b>0$ preserves Wiener measure and maps an endpoint $x\le b$ to $2b-x\ge b$.

The Reflection Principle is a fundamental concept in the study of stochastic processes, particularly in the context of the Wiener process (also known as Brownian motion). The principle provides a method for analyzing the behavior of Brownian paths, especially concerning their maximum or minimum values.