Talk:Quadratic variation
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Why can we use quadratic variation to test differentiability? From Advanced Stochastic Processes by David Gamarnik: Even though Brownian motion is nowhere differentiable and has unbounded total variation, it turns out that it has bounded quadratic variation.
The theorem might be correct, but what is it useful for?
A differentiable function needn't have a continuous derivation:
x2sin(1 / x)
