Abstract
We investigate the power of Non-commutative Arithmetic Circuits, which compute polynomials over the free non-commutative polynomial ring \({\mathbb{F}\langle{x_1,\ldots,x_N\rangle}}\), where variables do not commute. We consider circuits that are restricted in the ways in which they can compute monomials: this can be seen as restricting the families of parse trees that appear in the circuit. Such restrictions capture essentially all non-commutative circuit models for which lower bounds are known. We prove several results about such circuits.
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1.
We show exponential lower bounds for circuits with up to an exponential number of parse trees, strengthening the work of Lagarde et al. [Electronic Colloquium on Comput Complexity (ECCC) vol 23, no 94, 2016], who prove such a result for Unique Parse Tree (UPT) circuits which have a single parse tree. The polynomial we prove a lower bound for is in fact computable by a polynomial-sized non-commutative circuit.
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2.
We show exponential lower bounds for circuits whose parse trees are rotations of a single tree. This simultaneously generalizes recent lower bounds of Limaye et al. (Theory Comput 12(1):1–38, 2016) and the above lower bounds of Lagarde et al. (2016), which are known to be incomparable. Here too, the hard polynomial is computable by a polynomial-sized non-commutative circuit.
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3.
We make progress on a question of Nisan (STOC, pp 410–418, 1991) regarding separating the power of Algebraic Branching Programs (ABPs) and Formulas in the non-commutative setting by showing a tight lower bound of \({n^{\Omega(\log d)}}\) for any UPT formula computing the product of d\({n \times n}\) matrices.
When \({d \leq \log n}\), we can also prove superpolynomial lower bounds for formulas with up to \({2^{o(d)}}\) many parse trees (for computing the same polynomial). Improving this bound to allow for \({2^{o(d)}}\) trees would give an unconditional separation between ABPs and Formulas.
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4.
We give deterministic whitebox PIT algorithms for UPT circuits over any field, strengthening a result of Lagarde et al. (2016), and also for sums of a constant number of UPT circuits with different parse trees.
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Acknowledgments
We thank the anonymous reviewers of Computational Complexity for their many suggestions that helped improve the quality of the exposition. Many open questions in Section 7 were suggested by one of the reviewers. An extended abstract of this paper appeared in Lagarde et al. (2017).
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Lagarde, G., Limaye, N. & Srinivasan, S. Lower Bounds and PIT for Non-commutative Arithmetic Circuits with Restricted Parse Trees. comput. complex. 28, 471–542 (2019). https://doi.org/10.1007/s00037-018-0171-9
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DOI: https://doi.org/10.1007/s00037-018-0171-9
Keywords
- Non-commutative arithmetic circuits
- Algebraic branching programs
- Formulas
- Lower bounds
- Polynomial identity testing
- Parse trees of circuits