Taylor expansion to show that for Stratonovich stochastic calculus the chain rule takes the form of the classical one
How can I show with a heuristic argument based on a Taylor expansion that for Stratonovich stochastic calculus the chain rule takes the form of the classical (Newtonian) one?
Concerning Ito calculus the fact that $dX^2 = dt$ results via a Taylor expansion in Ito's lemma -- this fact should stay the same with Stratonovich, but it should somehow cancel out in there -- I just don't know how....
In the following, $W_t$ is a Wiener process with increments having mean zero and standard deviation 1.
Basically, the Stratonovich formula does the following in terms of the Itô formula:
$$f(W_t) \circ dW_t = \frac{f(W_t+dW_t)+f(W_t)}{2}dW_t \; ,$$
which can be rewritten as
$$f(W_t) \circ dW_t = \frac{f(W_t+dW_t)-f(W_t)}{2}dW_t + f(W_t)dW_t \; .$$
Using Taylor expansion on the first term and Itô calculus rules, this can be simplified to
$$f(W_t) \circ dW_t = \frac{f'(W_t)}{2}dt + f(W_t)dW_t \; .$$
Now, we can replace $f$ with $f'$ in the formula to give
$$f'(W_t) \circ dW_t = \frac{f''(W_t)}{2}dt + f'(W_t)dW_t \; .$$
But you can easily check that the right hand side is nothing but $df(W_t)$ according to Itô calculus rules, therefore
$$df(W_t) = f'(W_t) \circ dW_t \; .$$
Which is just a special case of the chain rule. I guess from here on out you can generalize the argument.