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"Some commenters mentioned that the authors already use Adam and batchnorm, and Adam and batchnorm already are approximating second order behavior". [this really nice blog]

I can see how (correct me if I am wrong) Nesterov accelerated optimization (in some sense) solves an online convex optimization, where we choose which previously observed gradients to pick using Follow The Regularized Leader (FTRL, FoReL). But I can not see how adaptiveness emulates using second order statistics. And the statement about batchnorm is even more mysterious to me.

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It's kind of an imprecise statement, so It's hard to give a firm answer. Momentum and normalisation methods such as Adam, (diagonal-)AdaGrad and batch-normalization are (effectively) using diagonal approximations to the Hessian. Obviously, that's a very crude approximation, but it is approximating second-order (hessian) information.

I would associate second order methods with estimation of curvature, which is not something that can be done with diagonal approximations. IMHO it's too strong a statement to say they are approximating second order information.

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  • $\begingroup$ right; my point was that articles about BFGS method, for example, explicitly discuss this "diagonal approximation of Hessian", but I was not able to find an easy explanation of how does momentum and normalization archive that $\endgroup$ – MInner Apr 6 '17 at 17:00

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