# Does the $\mathsf{TC^0}$ hierarchy collapse?

Do we know that the $\mathsf{TC^0}$ hierarchy does not collapse ($\mathsf{TC^0_d} \subsetneq \mathsf{TC^0_{d+1}}$ for all $d$)?

The Zoo entry for $\mathsf{TC^0}$ only mentions the separation between depth 2 and 3.

Also is there a standard reference for the fact that $\mathsf{AC^0_d}$ hierarchy does not collapse?

• A related question would be, how many distinct functions are there in $AC_d^0$ / $TC_d^0$ ? A reasonable lower bound on these quantities would answer your questions. Also a proof of tightness for Hastad's switching lemma would perhaps answer your second question. – MCH Jun 11 '13 at 18:34
• For the second question, I believe it was first proved in Sipser's STOC '83 paper "Borel sets and circuit complexity". This only gives super-polynomial lower bounds though. The first exponential lower bounds were given by Yao, later improved by Håstad. – Robin Kothari Jun 11 '13 at 19:03
• @MCH, did you mean to write $\mathsf{TC^0_d}/\mathsf{AC^0_d}$? Or do you mean the number of equivalence classes of problems in $\mathsf{TC^0_d}$ w.r.t. $\mathsf{AC^0_d}$ reductions? – Kaveh Jun 11 '13 at 21:34
• What I mean is very simple: How many distinct functions can the class of $AC_d^0$ circuits of size $s$ represent? (We can estimate the number of circuits very easily but we should be careful that some of them may compute the same function.) Once you show that this quantity grows with $d$, you are done. – MCH Jun 12 '13 at 17:53
• @Dilworth, nonuniform. Counting doesn't seem to work, otherwise as I noted below we could then separate $\mathsf{TC^0}$ from $\mathsf{NC^1}$ which is open. – Kaveh Jul 24 '13 at 13:36

We know of no good lower bounds (meaning, say, a superpolynomial lower bound for a language in $\mathsf{NEXP}$) for depth 2 threshold circuits (unbounded weights). Depth 3 circuits built from majority gates, i.e. $\mathsf{TC}^0_3$ contains this class, and thus we know no good lower bounds for this class either.

• This answers my question. Thank you Kristoffer. – Kaveh Jun 11 '13 at 21:46
• As I wrote in the comment above, even if a problem in NEXP is not known to be outside TC$^0_2$, isn't it still possible that the non-uniform TC$^0$ hierarchy is proper via a counting argument lower bound? – Dilworth Jul 24 '13 at 13:28
• Also, may I inquire, how is this consistent with the known exponential lower bounds on TC$_2^0$ and the separation of depth 3 from depth 2 threshold circuits, as reported in the complexity zoo? Am I missing something? – Dilworth Jul 24 '13 at 13:35
• @Dilworth, I think that is because it is defined using Majority not Threshold. – Kaveh Jul 24 '13 at 13:38
• Hmm.. what do you mean precisely? Is this related to the note made by Kristoffer about "unbounded weights"? – Dilworth Jul 24 '13 at 13:43

If I am not making a mistake, it seems that proving that the $\mathsf{TC^0_d}$ hierarchy does not collapse is at least as difficult as separating $\mathsf{NC^1}$ from $\mathsf{TC^0}$:

Let's denote the Boolean Formula Evaluation problem by $BFE$. $BFE$ is complete for $\mathsf{NC^1}$ under $\mathsf{AC^0}$ reductions.

By Manindra Agrawal, Eric Allender, and Steven Rudich, "Reductions in Circuit Complexity: An Isomorphism Theorem and a Gap Theorem", 1999, $BFE$ is complete for $\mathsf{NC^1}$ under $\mathsf{AC^0_2}$ reductions.

Assume $\mathsf{NC^1}=\mathsf{TC^0}$. Then $BFE \in \mathsf{TC^0_d}$ for some $d$. Therefore $\mathsf{NC^1} \subseteq \mathsf{TC^0_{d+2}}$. Which means that $\mathsf{TC^0} \subseteq \mathsf{TC^0_{d+2}}$.

So for all $d$ we have

$\mathsf{TC^0} \not\subseteq \mathsf{TC^0_d}$ implies $\mathsf{NC^1} \not\subseteq \mathsf{TC^0_{d+2}}$ and $BFE \notin \mathsf{TC^0_d}$.