What is the definition of the Itô integral?

The extra $dt$ term in the Itô expansion of $W^2$ arises from the randomness of the Brownian motion path. This term distinguishes Itô calculus from ordinary deterministic calculus, where the chain rule would only produce a $2W\,dW$ term.

Conditions: $W$ is a standard Brownian motion.; Itô's lemma is being applied to $W^2$.

The video connects Itô integrals to finance by explaining that they are used to track market swings and support option pricing. It visually represents this with a stylized random price path.

Conditions: The context is the application of stochastic calculus to financial markets.

Classical integration sums smooth and predictable quantities, like accumulating steady bond interest, and is visually represented by the area under a smooth curve. Stochastic integration sums highly irregular, random movements, such as wild stock price jumps, and is visually depicted as summing areas under a jagged, erratic path.

Conditions: Classical integration applies to deterministic, smooth functions.; Stochastic integration applies to processes with non-smooth, random paths.

In the formula $\frac{1}{2}W(T)^2 - \frac{1}{2}T$, the term $\frac{1}{2}W(T)^2$ depends on the terminal position of the path, while the term $-\frac{1}{2}T$ is a correction that comes from accumulated randomness over time.

Conditions: The formula is the result of evaluating the Itô integral $\int_0^T W\,dW$.; $W(T)$ is the value of the Brownian motion at time $T$.; $T$ is the elapsed time.

The closed-form value of the Itô integral of a standard Brownian motion with respect to itself is $\frac{1}{2}W(T)^2 - \frac{1}{2}T$. This result is derived by applying Itô's lemma to $W^2$, integrating the resulting differential identity over the interval $[0, T]$, and using the initial condition $W(0)=0$.

Conditions: $W(t)$ is a standard Brownian motion.; The integral is evaluated from $0$ to $T$.; The integral is interpreted in the Itô sense.