# The Exponential Time Hypothesis

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Published on March 7, 2014

Author: ASPAK2014

Source: slideshare.net

The Exponential Time Hypothesis

“Easy” for small values of the parameter An O(nk) algorithm exists. Fixed-Parameter Tractable: f(k)poly(n) As hard as solving Clique

We are going to focus on problems that have O*(kk) algorithms, ! but are not expected to have O*(2o(k log k)) algorithms.

Input A graph over the vertex set [k] x [k]. Is there a clique that picks one vertex from each row, Question and one vertex from each column? Permutation Clique

Input A graph over the vertex set [k] x [k]. Is there a clique that picks one vertex from each row, Question and one vertex from each column? Permutation Clique

Unless ETH fails, there is no algorithm that solves Permutation Clique in 2o(k log k) time.

Input A family of subsets over the universe [k] x [k]. Is there a hitting set that picks one vertex from each row, and one vertex from each column? Permutation Hitting Set Question

Permutation Clique Permutation Hitting Set Permutation Clique

Permutation Clique Permutation Hitting Set Permutation Clique

Permutation Clique Permutation Hitting Set Permutation Hitting Set

Permutation Clique Permutation Hitting Set Every Clique is in fact a hitting set too.

Permutation Clique Permutation Hitting Set Every Clique is in fact a hitting set too.

Permutation Clique Permutation Hitting Set Every Clique is in fact a hitting set too.

Permutation Clique Permutation Hitting Set Every Clique is in fact a hitting set too.

Input A family of subsets over the universe [k] x [k], such that every set has at most one element from every row. Is there a hitting set that picks one vertex from each row, and one vertex from each column? Question Permutation Hitting Set With Thin Sets

Input n strings, x1, x2, …, xn of length L each over an alphabet A, and a budget d. Is there a string of length d over A whose hamming distance from each xi is at most d? x1 … x2 xn Closest String Question

Input n strings, x1, x2, …, xn of length L each over an alphabet A, and a budget d. Is there a string of length d over A whose hamming distance from each xi is at most d? x1 … x2 xn Closest String Question

A family of subsets over the universe [k] x [k], such that every set has at most one element from every row. Is there a hitting set that picks one vertex from each row, and one vertex from each column? Permutation Hitting Set With Thin Sets

A family of subsets over the universe [k] x [k], such that every set has at most one element from every row. Is there a hitting set that picks one vertex from each row, and one vertex from each column? 132♠♠ 1 Permutation Hitting Set With Thin Sets

A family of subsets over the universe [k] x [k], such that every set has at most one element from every row. Is there a hitting set that picks one vertex from each row, and one vertex from each column? 132♠♠ 1 4♠3555 Permutation Hitting Set With Thin Sets

A family of subsets over the universe [k] x [k], such that every set has at most one element from every row. Is there a hitting set that picks one vertex from each row, and one vertex from each column? 132♠♠ 1 4♠3555 ♠6543♠ Permutation Hitting Set With Thin Sets

A family of subsets over the universe [k] x [k], such that every set has at most one element from every row. Is there a hitting set that picks one vertex from each row, and one vertex from each column? 132♠♠ 1 4♠3555 ♠6543♠ ♠♠♠♠12 Permutation Hitting Set With Thin Sets

A family of subsets over the universe [k] x [k], such that every set has at most one element from every row. 111111 Is there a hitting set that picks one vertex from each row, and one vertex from each column? 333333 444444 555555 666666 222222 132♠♠ 1 4♠3555 ♠6543♠ ♠♠♠♠12 Permutation Hitting Set With Thin Sets

Permutation Hitting Set with Thin Sets is unlikely to admit a 2o(k log k) algorithm. Closest String is unlikely to admit a 2o(d log d) algorithm. Closest String is unlikely to admit a 2o(d log |A|) algorithm.

Input A graph over the vertex set [k] x [k]. Is there a clique that picks one vertex from each row? Question [k]x[k] Clique

Unless ETH fails, there is no algorithm that solves 3-Colorability in 2o(n) time.

Unless ETH fails, there is no algorithm that solves 3-Colorability in 2o(n) time. Unless ETH fails, there is no algorithm that solves [k]x[k] Clique in 2o(k log k) time.

No 2o(k) algorithm. A 2(k log k) algorithm.

A 2(k log k) algorithm.

A 2(k log k) algorithm.

Unless ETH fails, there is no algorithm that solves 3-Colorability in 2o(n) time. Unless ETH fails, there is no algorithm that solves [k]x[k] Clique in 2o(k log k) time.

3-Colorability [N] [k]x[k] Clique Reduce 3-COL to [k]x[k] Clique, and suppose n —> k* ! Run a 2o(k* log k*) algorithm. ! This should be a 2o(n) algorithm.

3-Sat [N] Edge Clique Cover [k] Reduce 3-SAT to Edge Clique Cover, and suppose n —> k* ! Run a 2 o(2k ) algorithm. ! This should be a 2o(n) algorithm.

3-Colorability for a graph with N vertices reduces to [k]x[k] Clique with k = O(N/log N).

2N k= log3 N V1 V2 … Vk

2N k= log3 N V1 V2 … All possible 3-colorings of the Vi’s. Vk

2N k= log3 N V1 V2 … Vk Add edges between compatible colorings…

Clique does not admit a f(k)no(k) algorithm unless ETH fails, for any computable function f. Specifically, if W[1] = FPT, then ETH fails.

Exponential Time Hypothesis [ETH] 3-SAT cannot be solved in 2o(n+m) time.

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