Negation-Limited complexity of parity and inverters

  • Authors:
  • Kazuo Iwama;Hiroki Morizumi;Jun Tarui

  • Affiliations:
  • Graduate School of Informatics, Kyoto University, Kyoto, Japan;Graduate School of Informatics, Kyoto University, Kyoto, Japan;Department of Information and Communication Engineering, University of Electro-Communications, Chofu, Tokyo, Japan

  • Venue:
  • ISAAC'06 Proceedings of the 17th international conference on Algorithms and Computation
  • Year:
  • 2006

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Abstract

We give improved lower bounds for the size of negation-limited circuits computing Parity and for the size of negation-limited inverters. An inverter is a circuit with inputs x1,...,xn and outputs ¬x1,...,¬xn. We show that (1) For n=2r–1, circuits computing Parity with r–1 NOT gates have size at least 6n–log2(n+1)–O(1) and (2) For n=2r–1, inverters with r NOT gates have size at least 8n–log2(n+1)–O(1). We derive our bounds above by considering the minimum size of a circuit with at most r NOT gates that computes Parity for sorted inputs x1≥⋯≥xn. For an arbitraryr, we completely determine the minimum size. For odd n, it is 2n–r–2 for ⌈log2(n+1)⌉–1≤r ≤n/2, and it is $\lfloor 3/2 \: n\rfloor-1$ for r ≥n/2. We also determine the minimum size of an inverter for sorted inputs with at most r NOT gates. It is 4n–3r for ⌈log2(n+1) ⌉≤r ≤n. In particular, the negation-limited inverter for sorted inputs due to Fischer, which is a core component in all the known constructions of negation-limited inverters, is shown to have the minimum possible size. Our fairly simple lower bound proofs use gate elimination arguments.