The Power of Depth 2 Circuits over Algebras

Authors Chandan Saha, Ramprasad Saptharishi, Nitin Saxena

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Chandan Saha
Ramprasad Saptharishi
Nitin Saxena

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Chandan Saha, Ramprasad Saptharishi, and Nitin Saxena. The Power of Depth 2 Circuits over Algebras. In IARCS Annual Conference on Foundations of Software Technology and Theoretical Computer Science. Leibniz International Proceedings in Informatics (LIPIcs), Volume 4, pp. 371-382, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2009)


We study the problem of polynomial identity testing (PIT) for depth $2$ arithmetic circuits over matrix algebra. We show that identity testing of depth $3$ ($\Sigma \Pi \Sigma$) arithmetic circuits over a field $\F$ is polynomial time equivalent to identity testing of depth $2$ ($\Pi \Sigma$) arithmetic circuits over $\mathsf{U}_2(\mathbb{F})$, the algebra of upper-triangular $2\times 2$ matrices with entries from $\F$. Such a connection is a bit surprising since we also show that, as computational models, $\Pi \Sigma$ circuits over $\mathsf{U}_2(\mathbb{F})$ are strictly `weaker' than $\Sigma \Pi \Sigma$ circuits over $\mathbb{F}$. The equivalence further implies that PIT of $\Sigma \Pi \Sigma$ circuits reduces to PIT of width-$2$ commutative \emph{Algebraic Branching Programs}(ABP). Further, we give a deterministic polynomial time identity testing algorithm for a $\Pi \Sigma$ circuit of size $s$ over commutative algebras of dimension $O(\log s/\log\log s)$ over $\F$. Over commutative algebras of dimension $\poly(s)$, we show that identity testing of $\Pi \Sigma$ circuits is at least as hard as that of $\Sigma \Pi \Sigma$ circuits over $\mathbb{F}$.
  • Polynomial identity testing
  • depth 3 circuits
  • matrix algebras
  • local rings


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