Difference between revisions of "Majority"
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A function <math>f:\{-1,1\}^n \to \{-1,1\}</math> is called a '''majority function''' if <math>f(x)</math> returns the most common bit in the input: | A function <math>f:\{-1,1\}^n \to \{-1,1\}</math> is called a '''majority function''' if <math>f(x)</math> returns the most common bit in the input: | ||
| − | <math> f(x) = \begin{cases} 1, & if ~ \sum_i x_i \geq 0 \\ | + | :::{| class="wikitable" |
| + | |- | ||
| + | |<math> f(x) = \begin{cases} 1, & if ~ \sum_i x_i \geq 0 \\ | ||
-1 & otherwise \end{cases}</math> | -1 & otherwise \end{cases}</math> | ||
| + | |} | ||
For even <math>n</math>, the above definition breaks ties in favor of 1, although any arbitrary rule may be used instead. | For even <math>n</math>, the above definition breaks ties in favor of 1, although any arbitrary rule may be used instead. | ||
Revision as of 13:39, 20 November 2019
Definition
A function [math]f:\{-1,1\}^n \to \{-1,1\}[/math] is called a majority function if [math]f(x)[/math] returns the most common bit in the input:
[math] f(x) = \begin{cases} 1, & if ~ \sum_i x_i \geq 0 \\ -1 & otherwise \end{cases}[/math]
For even [math]n[/math], the above definition breaks ties in favor of 1, although any arbitrary rule may be used instead.
Majority is a special case of the perceptron function.
Properties
- Majority only depends on the number of ones and is therefore a symmetric Boolean function.
- Majority is a monotone function.
- TODO: a description of Majority's Fourier Transform. See http://www.contrib.andrew.cmu.edu/~ryanod/?p=877 for details.
- Majority is the stablest Boolean function. [1]
- Majority is the unique function that is symmetric, monotone and odd function. TODO May's theorem, credit.
- Majority is not in AC0, even if we allow using mod q functions as gates for prime [math]q[/math]. [2]
- For every [math]\varepsilon \gt 0 [/math], Majority can be [math]\varepsilon[/math]-approximated by a DNF of size [math]2^{O(\sqrt{n})}[/math]. [3]
References
- ↑ Ryan O'Donnell, Analysis of Boolean functions, [1]
- ↑ A. Razborov, Lower bounds on the size of bounded-depth networks over a complete basis with logical addition (Russian), in Matematicheskie Zametki, Vol. 41, No 4, 1987, pages 598-607. English translation in Mathematical Notes of the Academy of Sci. of the USSR, 41(4):333-338, 1987.
- ↑ O’Donnell R., Wimmer K. (2007) | Approximation by DNF: Examples and Counterexamples. In: Arge L., Cachin C., Jurdziński T., Tarlecki A. (eds) Automata, Languages and Programming. ICALP 2007. Lecture Notes in Computer Science, vol 4596. Springer, Berlin, Heidelberg
