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I was working on a proof to determine that a product cannot be done in AC0, how can a proof that can be done in AC1?

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Multiplication can be done even with stronger restrictions, like $AC^1$ with bounded fan-in.

Proof is little hard to typeset here, but I will outline the sketch and give a link.

  1. You shall prove, that addition of two m-bit numbers have $O(m)$ size and constant depth circuit and thus is in $AC^0$. This is pretty simple (start with looking for $O(m)$ depth and then simplify)
  2. Then you need to generalize this result to addition of many m-bit numbers and show that it have logarithmic depth and thus in $AC^1$
  3. And finally what you are looking for (product in $AC^1$) is simple corollary

Look into those lecture notes for example, where this proof is done in great details.

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  • $\begingroup$ AC$^1$ with bounded fan-in is known as NC$^1$. $\endgroup$ – Yuval Filmus Jun 29 at 15:53
  • $\begingroup$ @YuvalFilmus of course. If you read linked paper, it have this and many more definitions. I just tried not to overcomplicate things here. $\endgroup$ – Konstantin Vladimirov Jun 29 at 16:47

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