Abstract
We consider the black-box polynomial identity testing (\({\small \mathrm {PIT}}\)) problem for a sub-class of depth-4 \(\varSigma \varPi \varSigma \varPi (k,r)\) circuits. Such circuits compute polynomials of the following type: \( C({\small \mathrm X}) = \sum _{i=1}^k \prod _{j=1}^{d_i} Q_{i,j}, \) where k is the fan-in of the top \(\varSigma \) gate and r is the maximum degree of the polynomials \(\{Q_{i,j}\}_{i\in [k], j\in [d_i]}\), and \(k,r=O(1)\). We consider a sub-class of such circuits satisfying a generic algebraic-geometric restriction, and we give a deterministic polynomial-time black-box \({\small \mathrm {PIT}}\) algorithm for such circuits.
Our study is motivated by two recent results of Mulmuley (FOCS 2012, [Mul12]), and Gupta (ECCC 2014, [Gup14]). In particular, we obtain the derandomization by solving a particular instance of derandomization problem of Noether’s Normalization Lemma (\(\mathrm{NNL}\)). Our result can also be considered as a unified way of viewing the depth-4 \({\small \mathrm {PIT}}\) problems closely related to the work of Gupta [Gup14], and the approach suggested by Mulmuley [Mul12].
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- 1.
We work in the projective spaces \(\mathbb {P}^n\), since by standard reason the polynomials \(Q_{i,j}\)s can be assumed to be homogeneous polynomials over \(n+1\) variables.
- 2.
Notice that a zero dimensional variety contains a finite number of points.
- 3.
We do not explicitly use the fact that \(Q_{i,j}\)s are irreducible in the analysis. This fact is useful if we would like to formulate a conjecture in the similar spirit of Conjecture 1 in [Gup14].
- 4.
Notice that \({{\mathrm{{\small \mathrm codim}}}}(\varLambda _S) = n-(2k-1)\le n-k-1\) for \(k\ge 2\).
- 5.
In the next section, we show how to construct such a subspace deterministically.
- 6.
Recall that \(\deg (Q_S) \le r^{k+1}\) for any S.
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I thank K.V. Subrahmanyam for many helpful discussions.
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Mukhopadhyay, P. (2016). Depth-4 Identity Testing and Noether’s Normalization Lemma. In: Kulikov, A., Woeginger, G. (eds) Computer Science – Theory and Applications. CSR 2016. Lecture Notes in Computer Science(), vol 9691. Springer, Cham. https://doi.org/10.1007/978-3-319-34171-2_22
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