Note: Smallest formulas for the parity of 2 k variables are essentially unique

  • Authors:
  • Jun Tarui

  • Affiliations:
  • University of Electro-Comm, Chofu, Tokyo 182-8585, Japan

  • Venue:
  • Theoretical Computer Science
  • Year:
  • 2010

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Abstract

For n=2^k, we know that the size of a smallest AND/OR/NOT formula computing the Boolean function Parity(x"1,...,x"n)=Odd(x"1,...,x"n) is exactly n^2: For any n, it is at least n^2 by the classical Khrapchenko bound, and for n=2^k we easily obtain a formula of size n^2 by writing and recursively expanding Odd(x"1,...,x"n)=[Odd(x"1,...,x"n"/"2)@?Even(x"n"/"2"+"1,...,x"n)]@?[Even(x"1,...,x"n"/"2)@?Odd(x"n"/"2"+"1,...,x"n)]. We show that for n=2^k the formula obtained above is an essentially unique one that computes Parity(x"1,...,x"n) with size n^2. In the equivalent framework of the Karchmer-Wigderson communication game, our result means that an optimal protocol for the parity of 2^k variables is essentially unique.