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Computability and Complexity

Computability and Complexity. 32-1. Boolean Circuits. Computability and Complexity Andrei Bulatov. Computability and Complexity. 32-2. A Non-Parallelizable Problem. Let us consider the TSP(D) problem. Suppose there is a parallel algorithm solving this problem.

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Computability and Complexity

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  1. Computability and Complexity 32-1 Boolean Circuits Computability and Complexity Andrei Bulatov

  2. Computability and Complexity 32-2 A Non-Parallelizable Problem Let us consider the TSP(D) problem Suppose there is a parallel algorithm solving this problem Then there is a sequential algorithm that simulates the parallel one By Brent’s Principle, we have parallel time  no. of processors = total amount of work where the total amount of work is actually not larger than the time complexity of the sequential simulation Either parallel time or the number of processors is exponential!! Bad News Unless P=NP, no NP-complete problem can be parallelized

  3. Computability and Complexity    32-3 Gates To model parallel computation we use extremely simple processors The three types of them can only compute the three logic connectives inputs outputs AND OR NOT

  4. Computability and Complexity 32-4 Circuits Definition A Boolean circuit is a collection of gates and inputs connected by wires such that: • every gate input is connected to exactly one circuit input or one gate output • every gate output except for one, called the circuit output, is connected to at least one gate input • cycles are not permitted

  5. Computability and Complexity We use the fact that      32-5 Example XxorY X Y 1 0 0 1 1 1 1

  6. Computability and Complexity Suppose we have two 2-bit numbers: xor xor     xor   32-6 Addition As the sum is 3-bit long, we need 3 circuits

  7. Computability and Complexity Definition A circuit familyC is an infinite list of circuits, where has n inputs A circuit family is said to be uniform if there is a log-space Turing machine that on the input of n1s produces the circuit 32-7 Circuit Families For example, for computing the sum of two integers (of unlimited length), we need a circuit family: the even members of the family computes the sum for the least valuable segment of the numbers the odd members are not needed, so they can be defined to be empty It is not hard to show that this family is uniform

  8. Computability and Complexity The size complexity of a circuit family C is the function f on positive integers such that f(n) is the size of 32-8 Parameters of Circuits The size of a circuit is the number of gates it contains Two circuits are equivalent if they have the same inputs and output the same value on every input assignment A circuit is minimal if no smaller circuit is equivalent to it A circuit family is minimal if every its member is minimal The depth of a circuit is the length of a longest path from an input to the output gate Depth minimal circuits and circuit families and the depth complexity of a circuit family are defined in the same way as for size

  9. Computability and Complexity Definition A circuit family Cdecides a language L over {0,1} if, for every string w L if and only if with input outputs 1 32-9 Languages and Circuits Definition The size complexity of a language is the size complexity of a minimal circuit family that decides this language. The depth complexity of a language is the depth complexity of a minimal circuit family that decides this language.

  10. Computability and Complexity We built a circuit family that decides L …     32-10 Example Let L= {1,11,111,1111,…} Size complexity = n – 1 … Depth complexity = n This circuit is not minimal Size complexity of L= n – 1 Depth complexity of L= log n

  11. Computability and Complexity Theorem Let p be a function on positive integers. Then if L TIME(p(n)) then L has circuit complexity 32-11 Circuit Complexity Corollary If L P, then the circuit complexity of L is polynomial

  12. Computability and Complexity Definition For i 1 the class is the class of languages that can be decided by a uniform circuit family with polynomial size complexity and depth complexity in Then 32-12 The Class NC

  13. Computability and Complexity Theorem Let . That is there is a log-space transducer that generates a circuit family C of logarithmic depth that decides L • using the log-space transducer for C generate 32-13 NC and Other Classes Proof Idea We have to present a log-space algorithm that decides L On an input w of length n • using depth-first search from the output gate check if on w the • circuit outputs 1

  14. Computability and Complexity Theorem Theorem 32-14 More Theorems

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