# Boolean circuit

An -ary Boolean function is a function from with values in . The values , are called Boolean values, or bits, or truth values if they are identified with false and true (cf. also Boolean function). Elements of are Boolean strings of length (or bit strings). is the set of all -ary Boolean functions.

A basis of connectives is any non-empty set of Boolean functions, not necessarily of the same arity. An -formula for is a formula built from Boolean variables using connectives from . A basis is complete if all Boolean functions are definable by an -formula. Examples of complete bases are: , the set of all binary Boolean functions, or the DeMorgan basis .

A more general way to define (equivalently, compute) Boolean functions is by circuits (straight line programs) that are simplified mathematical models of integrated circuits. A Boolean circuit over a basis for is a sequence of functions from such that and such that any is either one of the projections or is equal to , for some and . Drawing directed edges from all such to , and labelling by , defines a labelled directed acyclic graph (cf. also Graph, oriented). The size of the circuit is . The maximum length of a path from a vertex corresponding to one of the inputs to is the depth of the circuit. Note that formulas are circuits whose graphs are trees.

There are three basic ways to measure the complexity of a Boolean function, given a basis :

the minimum size of an -formula defining ,

the minimum size of a circuit over defining , and

the minimum depth of a circuit over defining . For two complete bases , , the measures , and , are proportional to each other, and , are polynomially related. A relation exists between the formula size and the circuit depth (see [a13]): and are proportional to each other, provided is finite.

By estimating the number of circuits of a bounded size, C.E. Shannon [a11] proved (cf. [a14] for an optimal computation) that for almost-all , . On the other hand,

for all , see [a8].

For a formal language , let be the characteristic function of . The main open problem in Boolean complexity theory (cf. [a4]) is whether there is a language accepted by a non-deterministic polynomial-time Turing machine (cf. also Turing machine) such that is not bounded by any polynomial in . The importance of this question stems from the fact that the affirmative answer implies that the computational complexity classes and are different, as is bounded by a polynomial for any decidable in polynomial time (cf. [a10] and also ).

No super-linear lower bounds on are known for any . However, if additional restrictions are put on the circuits, then strong lower bounds are known. Most notable are two results: any circuit of depth over a DeMorgan-like basis , allowing of unbounded arity, computing the parity function must have size at least (see [a1], [a5], [a15], [a6]), and any circuit in the monotone basis computing whether or not a graph on vertices contains a clique of size at least must have size at least (see [a9], [a3]).

Interesting upper bounds are also known: a DeMorgan circuit of size for multiplication of two -bit long numbers, [a12] (it is still open (1998) if there are linear size circuits), and a monotone DeMorgan sorting network of simultaneous size and depth , see [a2].

Various parts of computational complexity (cf. also Complexity theory) are directly related to lower-bound problems for Boolean functions; for example, communication complexity, cf. [a7].

#### References

[a1] | M. Ajtai, " - formulae on finite structures" Ann. Pure Appl. Logic , 24 (1983) pp. 1–48 |

[a2] | M. Ajtai, J. Komlós, E. Szemerédi, "An sorting network" Combinatorica , 3 (1983) pp. 1–19 |

[a3] | N. Alon, R. Boppana, "The monotone circuit complexity of Boolean functions" Combinatorica , 7 : 1 (1987) pp. 1–22 |

[a4] | R. Boppana, M. Sipser, "Complexity of finite functions" J. van Leeuwen (ed.) , Handbook of Theoretical Computer Science , A (1990) pp. 758–804; Chap.14 |

[a5] | M. Furst, J.B. Saxe, M. Sipser, "Parity, circuits and the polynomial-time hierarchy" Math. Systems Theory , 17 (1984) pp. 13–27 |

[a6] | J. Hastad, "Almost optimal lower bounds for small depth circuits" S. Micali (ed.) , Randomness and Computation , Adv. Comput. Res. , 5 , JAI Press (1989) pp. 143–170 |

[a7] | E. Kushilevitz, N. Nisan, "Communication complexity" , Cambridge Univ. Press (1996) |

[a8] | O.B. Lupanov, "A method of circuit synthesis" Izv. V.U.Z. (Radiofizika) , 1 : 1 (1958) pp. 120–140 (In Russian) |

[a9] | A.A. Razborov, "Lower bounds on the monotone complexity of some Boolean functions" Soviet Math. Dokl. , 31 (1985) pp. 354–357 |

[a10] | J.E. Savage, "Computational work and time on finite machines" J. ACM , 19 : 4 (1972) pp. 660–674 |

[a11] | C.E. Shannon, "The synthesis of two-terminal switching circuits" Bell Systems Techn. J. , 28 : 1 (1949) pp. 59–98 |

[a12] | A. Schonhage, V. Strassen, "Schnelle Multiplikation grosser Zahlen" Computing , 7 (1971) pp. 281–292 |

[a13] | P.M. Spira, "On time-hardware complexity of tradeoffs for Boolean functions" , Proc. 4th Hawaii Symp. System Sciences , North Hollywood&Western Periodicals (1971) pp. 525–527 |

[a14] | I. Wegener, "The complexity of Boolean functions" , Wiley&Teubner (1987) |

[a15] | Y. Yao, "Separating the polynomial-time hierarchy by oracles" , Proc. 26th Ann. IEEE Symp. Found. Comput. Sci. (1985) pp. 1–10 |

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Boolean circuit.

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