Let X be a finite set and F a family of subsets of X such that...
Let X be a finite set and F a family of subsets of X such that
every element of X appears in at least one subset in F. We say that
a subset C of F is a set cover for X if X =U SEC S (that is, the
union of the sets in C is X). The cardinality of a set cover C is
the number of elements in C. (Note that an element of C is a subset
of X.) The Set Cover problem is: Given an instance (X,F), and a
nonnegative integer k, decide if X has a set cover C in F of
cardinality k. For example, if X = {1; 2; 3; 4; 5}, F= {{1}, {2},
{3}, {2, 5}, {1,3, 5}, {1, 2, 5}, {2, 4, 5}, {1, 4, 5}}, and k = 2,
then the answer is Yes because X has a set cover C in F of
cardinality 2, namely C = {{1, 3, 5}, {2, 4, 5}}.
Show that the Set Cover problem is NP-complete. (Hint. Reduce from
Vertex Cover.)
Homework Answers
5. The Hitting Set Problem (HS) is the following decision problem. Input. A finite set S,...
5. The Hitting Set Problem (HS) is the following decision problem. Input. A finite set S, a collection (s1, s2,... , sn) of subsets of S, and a positive integer K. Question. Does there exist a subset t of S such that (a) t has exactly K members and (b) for i 1,..., n, sint6For example, suppose S # {1, 2, 3, 4, 5, 6, 7. the collection of subsets is (2.45), (34).(1,35) and K - 2. Then the answer...
3. (20 pts) Let ụ be a finite set, and let S = {Si, S , S,n} be a collection of subsets of U. Given an integer k, w...
3. (20 pts) Let ụ be a finite set, and let S = {Si, S , S,n} be a collection of subsets of U. Given an integer k, we want to know if there is a sub-collection of k sets S' C S whose union covers all the elements of U. That is, S k, and Us es SU. Prove that this problem is NP-complete. 992 m SES, si
3. (20 pts) Let ụ be a finite set, and let...
8. The problem 'SET COVER gives two numbers n, k, and a family of n subsets of (,.. n It asks whe...
Algorithms
8. The problem 'SET COVER gives two numbers n, k, and a family of n subsets of (,.. n It asks whether it is possible to select k of these subsets such that each number in (1,... ,n) occurs in at least one of the selected subsets. (8.1) Show that the problem 'SET COVER' is in the class NP (8.2) The simplest algorithm to solve set cover just tests all the possible choices of k subsets. How long will...
4. Ranking/Unranking Subsets. Let A be a set of n elements and set Sk(A) be the collection of all k-element subsets of A. Recall that |Sk(A)I - (a.) (8 points) Describe a ranking algorithm to rank a...
4. Ranking/Unranking Subsets. Let A be a set of n elements and set Sk(A) be the collection of all k-element subsets of A. Recall that |Sk(A)I - (a.) (8 points) Describe a ranking algorithm to rank a k-element subset of an n-element set. (b.) (8 points) Describe an unranking algorithm to unrank an integer 0 < s< [into a ithm to unrank an integer 0 S s <C) k-element subset of an n-element set. (c.) (10 points) As examples, let...
. Let C be a collection of open subsets of R. Thus, C is a set...
. Let C be a collection of open subsets of R. Thus, C is a set whose elements are open subsets of R. Note that C need not be finite, or even countable. (a) Prove that the union U S is also an open subset of R. SEC (b) Assuming C is finite, prove that the intersection n S is an open subset of R. SEC (c) Give an example where C is infinite and n S is not open....
5- Recall that a set KCR is said to be compact if every open cover for...
5- Recall that a set KCR is said to be compact if every open cover for K has a finite subcover 5-1) Use the above definition to prove that if A and B are two compact subsets of R then AUB is compact induction to show that a finite union of compact subsets of R is compact. 5-2) Now use 5-3) Let A be a nonempty finite subset of R. Prove that A is compact 5-4) Give an example of...
Problem 7. (20 pts) Let n EN be a natural nmber and X a finite set...
Problem 7. (20 pts) Let n EN be a natural nmber and X a finite set with n elements. Show that the number of permutations of X such that no element stays in the same position is n! k! k o For instance, there are 6 = 3! permutations of 3 elements, but only 2 of them are permutations which fix no element. Similarly, there are 24-4! permutations of 4 elements, but only 9 which fix no element. Hint: Use...
where Problem 36. Assume f : X → [0, oo]. Prove that if Σ f(x) < 00, then {x E X (z) > 0} is a countable set. (HINT: Show that for every k E N the set {x E X | f(x) > k-1} is finite.)...
where
Problem 36. Assume f : X → [0, oo]. Prove that if Σ f(x) < 00, then {x E X (z) > 0} is a countable set. (HINT: Show that for every k E N the set {x E X | f(x) > k-1} is finite.) f(x)-sup f(x) | F is any finite subset of X TEF
Problem 36. Assume f : X → [0, oo]. Prove that if Σ f(x) 0} is a countable set. (HINT: Show that...
Problem 7. (20 pts) Let n N be a natural number and X a finite set...
Problem 7. (20 pts) Let n N be a natural number and X a finite set with n elements. Show that the number of permutations of X such that no element stays in the same position is n. n! k! For instance, there are 6-3! permutations of 3 elements, but only 2 of them are permutations which fix no element. Similarly, there are 24 41 permutations of 4 elements, but only 9 which fix no element Hint: Use the Inclusion-Erclusion...
7. Let A, , An be non-empty subsets of a finite set Ω. If 1 k n and Ek is the set of elements in ...
7. Let A, , An be non-empty subsets of a finite set Ω. If 1 k n and Ek is the set of elements in Ω which belong to at least k of the Ai's show that Pal i-1
7. Let A, , An be non-empty subsets of a finite set Ω. If 1 k n and Ek is the set of elements in Ω which belong to at least k of the Ai's show that Pal i-1
5. The Hitting Set Problem (HS) is the following decision problem. Input. A finite set S, a collection (s1, s2,... , sn) of subsets of S, and a positive integer K. Question. Does there exist a subset t of S such that (a) t has exactly K members and (b) for i 1,..., n, sint6For example, suppose S # {1, 2, 3, 4, 5, 6, 7. the collection of subsets is (2.45), (34).(1,35) and K - 2. Then the answer...
3. (20 pts) Let ụ be a finite set, and let S = {Si, S , S,n} be a collection of subsets of U. Given an integer k, we want to know if there is a sub-collection of k sets S' C S whose union covers all the elements of U. That is, S k, and Us es SU. Prove that this problem is NP-complete. 992 m SES, si
3. (20 pts) Let ụ be a finite set, and let...
Algorithms
8. The problem 'SET COVER gives two numbers n, k, and a family of n subsets of (,.. n It asks whether it is possible to select k of these subsets such that each number in (1,... ,n) occurs in at least one of the selected subsets. (8.1) Show that the problem 'SET COVER' is in the class NP (8.2) The simplest algorithm to solve set cover just tests all the possible choices of k subsets. How long will...
4. Ranking/Unranking Subsets. Let A be a set of n elements and set Sk(A) be the collection of all k-element subsets of A. Recall that |Sk(A)I - (a.) (8 points) Describe a ranking algorithm to rank a k-element subset of an n-element set. (b.) (8 points) Describe an unranking algorithm to unrank an integer 0 < s< [into a ithm to unrank an integer 0 S s <C) k-element subset of an n-element set. (c.) (10 points) As examples, let...
. Let C be a collection of open subsets of R. Thus, C is a set whose elements are open subsets of R. Note that C need not be finite, or even countable. (a) Prove that the union U S is also an open subset of R. SEC (b) Assuming C is finite, prove that the intersection n S is an open subset of R. SEC (c) Give an example where C is infinite and n S is not open....
5- Recall that a set KCR is said to be compact if every open cover for K has a finite subcover 5-1) Use the above definition to prove that if A and B are two compact subsets of R then AUB is compact induction to show that a finite union of compact subsets of R is compact. 5-2) Now use 5-3) Let A be a nonempty finite subset of R. Prove that A is compact 5-4) Give an example of...
Problem 7. (20 pts) Let n EN be a natural nmber and X a finite set with n elements. Show that the number of permutations of X such that no element stays in the same position is n! k! k o For instance, there are 6 = 3! permutations of 3 elements, but only 2 of them are permutations which fix no element. Similarly, there are 24-4! permutations of 4 elements, but only 9 which fix no element. Hint: Use...
where
Problem 36. Assume f : X → [0, oo]. Prove that if Σ f(x) < 00, then {x E X (z) > 0} is a countable set. (HINT: Show that for every k E N the set {x E X | f(x) > k-1} is finite.) f(x)-sup f(x) | F is any finite subset of X TEF
Problem 36. Assume f : X → [0, oo]. Prove that if Σ f(x) 0} is a countable set. (HINT: Show that...
Problem 7. (20 pts) Let n N be a natural number and X a finite set with n elements. Show that the number of permutations of X such that no element stays in the same position is n. n! k! For instance, there are 6-3! permutations of 3 elements, but only 2 of them are permutations which fix no element. Similarly, there are 24 41 permutations of 4 elements, but only 9 which fix no element Hint: Use the Inclusion-Erclusion...
7. Let A, , An be non-empty subsets of a finite set Ω. If 1 k n and Ek is the set of elements in Ω which belong to at least k of the Ai's show that Pal i-1
7. Let A, , An be non-empty subsets of a finite set Ω. If 1 k n and Ek is the set of elements in Ω which belong to at least k of the Ai's show that Pal i-1
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