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THE QUANTUM ADVERSARY METHOD AND CLASSICAL FORMULA SIZE LOWER BOUNDS

Abstract.

We introduce two new complexity measures for Boolean functions, which we name sumPI and maxPI . The quantity sumPI has been emerging through a line of research on quantum query complexity lower bounds via the so-called quantum adversary method (Ambainis 2002, 2003; Barnum et al. 2003; Laplante & Magniez 2004; Zhang 2005), culminating in Špalek & Szegedy (2005) with the realization that these many different formulations are in fact equivalent. Given that sumPI turns out to be such a robust invariant of a function, we begin to investigate this quantity in its own right and see that it also has applications to classical complexity theory. As a surprising application we show that sumPI 2(f) is a lower bound on the formula size, and even, up to a constant multiplicative factor, the probabilistic formula size of f. We show that several formula size lower bounds in the literature, specifically Khrapchenko and its extensions (Khrapchenko 1971; Koutsoupias 1993), including a key lemma of Håstad (1998), are in fact special cases of our method. The second quantity we introduce, maxPI (f), is always at least as large as sumPI(f) , and is derived from sumPI in such a way that maxPI 2(f) remains a lower bound on formula size. Our main result is proven via a combinatorial lemma which relates the square of the spectral norm of a matrix to the squares of the spectral norms of its submatrices. The generality of this lemma implies that our methods can also be used to lower-bound the communication complexity of relations, and a related combinatorial quantity, the rectangle partition number. To exhibit the strengths and weaknesses of our methods, we look at the sumPI and maxPI complexity of a few examples, including the recursive majority of three function, a function defined by Ambainis (2003), and the collision problem.

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Correspondence to Sophie Laplante.

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Manuscript received 7 September 2005

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Laplante, S., Lee, T. & Szegedy, M. THE QUANTUM ADVERSARY METHOD AND CLASSICAL FORMULA SIZE LOWER BOUNDS. comput. complex. 15, 163–196 (2006). https://doi.org/10.1007/s00037-006-0212-7

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  • DOI: https://doi.org/10.1007/s00037-006-0212-7

Keywords.

  • Lower bounds
  • quantum computing
  • adversary method
  • formula size
  • communication complexity

Subject classification.

  • 68Q17
  • 68Q30