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{{Logical connectives sidebar}}
In [[mathematics]], a '''Boolean function''' is a [[function (mathematics)|function]] whose [[Argument of a function|arguments]] and result assume values from a two-element set (usually {true, false}, {0,1} or {−1,1}).<ref>{{Cite web|title=Boolean function - Encyclopedia of Mathematics|url=https://encyclopediaofmath.org/wiki/Boolean_function|access-date=2021-05-03|website=encyclopediaofmath.org}}</ref><ref>{{Cite web|last=Weisstein|first=Eric W.|title=Boolean Function|url=https://mathworld.wolfram.com/BooleanFunction.html|access-date=2021-05-03|website=mathworld.wolfram.com|language=en}}</ref> Alternative names are '''switching function''', used especially in older [[computer science]] literature,<ref>{{Cite web|title=switching function|url=https://encyclopedia2.thefreedictionary.com/switching+function|access-date=2021-05-03|website=TheFreeDictionary.com}}</ref><ref>{{Cite journal|last=Davies|first=D. W.|date=December 1957|title=Switching Functions of Three Variables|url=https://ieeexplore.ieee.org/document/5222038|journal=IRE Transactions on Electronic Computers|volume=EC-6|issue=4|pages=265–275|doi=10.1109/TEC.1957.5222038|issn=0367-9950|url-access=subscription}}</ref> and '''[[truth function]]''' (or '''logical function)''', used in [[logic]]. Boolean functions are the subject of [[Boolean algebra]] and [[switching theory]].<ref>{{Citation|last=McCluskey|first=Edward J.|title=Switching theory|date=2003-01-01|url=https://dl.acm.org/doi/10.5555/1074100.1074844|encyclopedia=Encyclopedia of Computer Science|pages=1727–1731|place=GBR|publisher=John Wiley and Sons Ltd.|doi=|isbn=978-0-470-86412-8|access-date=2021-05-03}}</ref>
A Boolean function takes the form <math>f:\{0,1\}^k \to \{0,1\}</math>, where <math>\{0,1\}</math> is known as the [[Boolean ___domain]] and <math>k</math> is a non-negative integer called the [[arity]] of the function. In the case where <math>k=0</math>, the function is a constant element of <math>\{0,1\}</math>. A Boolean function with multiple outputs, <math>f:\{0,1\}^k \to \{0,1\}^m</math> with <math>m>1</math> is a '''vectorial''' or ''vector-valued'' Boolean function (an [[S-box]] in symmetric [[cryptography]]).<ref name=":2" />
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The ''[[Boolean derivative]]'' of the function to one of the arguments is a (''k''−1)-ary function that is true when the output of the function is sensitive to the chosen input variable; it is the XOR of the two corresponding cofactors. A derivative and a cofactor are used in a [[Reed–Muller expansion]]. The concept can be generalized as a ''k''-ary derivative in the direction dx, obtained as the difference (XOR) of the function at x and x + dx.<ref name=":1" />
The ''[[Zhegalkin polynomial#Möbius transformation|Möbius transform]]'' (or ''Boole–Möbius transform'') of a Boolean function is the set of coefficients of its [[Zhegalkin polynomial|polynomial]] ([[algebraic normal form]]), as a function of the monomial exponent vectors. It is a [[Involution (mathematics)|self-inverse]] transform. It can be calculated efficiently using a [[Butterfly diagram|butterfly algorithm]] ("''Fast Möbius Transform''"), analogous to the [[fast Fourier transform]].<ref>{{Citation|last=Carlet|first=Claude|title=Boolean Functions for Cryptography and Error-Correcting Codes|date=2010|url=https://www.math.univ-paris13.fr/~carlet/chap-fcts-Bool-corr.pdf|work=Boolean Models and Methods in Mathematics, Computer Science, and Engineering|pages=257–397|editor-last=|editor-first=|series=Encyclopedia of Mathematics and its Applications|place=Cambridge|publisher=Cambridge University Press|isbn=978-0-521-84752-0|access-date=2021-05-17|editor2-last=|editor2-first=}}</ref> ''Coincident'' Boolean functions are equal to their Möbius transform, i.e. their truth table (minterm) values equal their algebraic (monomial) coefficients.<ref>{{Cite journal|last1=Pieprzyk|first1=Josef|last2=Wang|first2=Huaxiong|last3=Zhang|first3=Xian-Mo|date=2011-05-01|title=Mobius transforms, coincident Boolean functions and non-coincidence property of Boolean functions|url=https://doi.org/10.1080/00207160.2010.509428|journal=International Journal of Computer Mathematics|volume=88|issue=7|pages=1398–1416|doi=10.1080/00207160.2010.509428|s2cid=9580510 |issn=0020-7160|url-access=subscription}}</ref> There are 2^2^(''k''−1) coincident functions of ''k'' arguments.<ref>{{Cite journal|last1=Nitaj|first1=Abderrahmane|last2=Susilo|first2=Willy|last3=Tonien|first3=Joseph|date=2017-10-01|title=Dirichlet product for boolean functions|url=https://doi.org/10.1007/s12190-016-1037-4|journal=Journal of Applied Mathematics and Computing|language=en|volume=55|issue=1|pages=293–312|doi=10.1007/s12190-016-1037-4|s2cid=16760125 |issn=1865-2085}}</ref>
=== Cryptographic analysis ===
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