Itô's Formula
Applying a smooth function to Brownian motion introduces a second-derivative correction absent from ordinary calculus. Work through the square-function example to connect that correction with quadratic variation.
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Where to read
Start here: Probability: Theory and Examples (Version 5)
Rick Durrett · Book · English · Free full text
§7.6 Itô's formula, Theorem 7.6.1 and Example 7.6.3
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Probability Theory in the library · Preparation and skip guidance