Explicit Constructions of Depth-2 Majority Circuits for Comparison and Addition

  • Authors:
  • Noga Alon;Jehoshua Bruck

  • Affiliations:
  • -;-

  • Venue:
  • SIAM Journal on Discrete Mathematics
  • Year:
  • 1994

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Abstract

All Boolean variables here range over the two-element set {-1, 1}. Given n Boolean variables $x_{i},\ldots , x_{n}, a nonmonotone MAJORITY gate (in the variables $x_{i}$) is a Boolean function whose value is the sign of $\sum^{n}_{i=1} \varepsilon_{i} x_{i}$, where each $\varepsilon_{i}$ is either 1 or -1. The COMPARISON function is the Boolean function of two $n$-bits integers $X$ and $Y$ whose value is -1 if and only if $X \geq Y$. An explicit sparse polynomial whose sign computes this function is constructed. Similar polynomials are constructed for computing all the bits of the summation of the two numbers $X$ and $Y$. This supplies explicit constructions of depth-2 polynomial-size circuits computing these functions, which use only nonmonotone MAJORITY gates. These constructions are optimal in terms of the depth and can be used to obtain the best-known explicit constructions of MAJORITY circuits for other functions like the product of two $n$-bits wamlmys and the maximum of $n$ $n$-bit numbers. A crucial ingredient is the construction of a discrete version of a sparse "delta polynomial"---one that has a large absolute value for a single assignment and extremely small absolute values for all other assignments.