ItĂ´'s Lemma states that if $f(t, W(t))$ is a twice continuously differentiable function in $W$ and once in $t$, then
\[ df(t, W(t)) = \left(\frac{\partial f}{\partial t} + \frac{\partial f}{\partial W} dW(t) + \frac{1}{2} \frac{\partial^2 f}{\partial W^2}
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