# Kernelization Basics

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33 %
Technology

Published on March 5, 2014

Author: ASPAK2014

Source: slideshare.net

Kernelization Evil is whatever distracts. ― Franz Kafka

Kernelization Evil is whatever distracts. ― Franz Kafka

Preprocess

Preprocess

Preprocess

(x, k) (x , k ) Preprocess

(x, k) (x , k ) FAST SMALL SANE Preprocess

(x, k) (x , k ) |x|O(1) time SMALL SANE Preprocess

(x, k) (x , k ) |x|O(1) time |x | = f(k) and k SANE Preprocess k

(x, k) (x , k ) |x|O(1) time and |x | = f(k) (x, k) k (x , k ) Preprocess k

Examples

Max Sat

Input A CNF Formula over n variables with m clauses. Max Sat

Input A CNF Formula over n variables with m clauses. Is there an assignment satisfying at least k clauses? Max Sat Question

Input A CNF Formula over n variables with m clauses. Is there an assignment satisfying at least k clauses? What if k is at most m/2? Max Sat Question

Input A CNF Formula over n variables with m clauses. Is there an assignment satisfying at least k clauses? What if k is at most m/2? Max Sat Question

Input A CNF Formula over n variables with m clauses. Is there an assignment satisfying at least k clauses? What if k is at most m/2? Max Sat Question

Input A CNF Formula over n variables with m clauses. Is there an assignment satisfying at least k clauses? What if k is at most m/2? Say YES. Max Sat Question

Input A CNF Formula over n variables with m clauses. Is there an assignment satisfying at least k clauses? What if k is at most m/2? Say YES. k > m/2? Max Sat Question

Input A CNF Formula over n variables with m clauses. Is there an assignment satisfying at least k clauses? What if k is at most m/2? Say YES. k > m/2? The number of clauses is bounded by 2k. Max Sat Question

Input A CNF Formula over n variables with m clauses. Is there an assignment satisfying at least k clauses? Max Sat Question

Input A CNF Formula over n variables with m clauses. Is there an assignment satisfying at least k clauses? Max Sat Question

Input A CNF Formula over n variables with m clauses. Is there an assignment satisfying at least k clauses? variables Max Sat Question

Input A CNF Formula over n variables with m clauses. Is there an assignment satisfying at least k clauses? variables Max Sat Question

Input A CNF Formula over n variables with m clauses. Is there an assignment satisfying at least k clauses? variables Configuration of x Max Sat Question

Input A CNF Formula over n variables with m clauses. Is there an assignment satisfying at least k clauses? Question variables Configuration of x If two variables x and y have the same “configuration” in the formula, then delete one of them. Max Sat

Input A CNF Formula over n variables with m clauses. Is there an assignment satisfying at least k clauses? variables Configuration of x Max Sat Question

Input A CNF Formula over n variables with m clauses. Is there an assignment satisfying at least k clauses? variables Configuration of x We have at most 3(2k) variables left. Max Sat Question

Input A CNF Formula over n variables with m clauses. Is there an assignment satisfying at least k clauses? variables Configuration of x We have at most 3(2k) variables left. Max Sat Question

Input A CNF Formula over n variables with m clauses. Is there an assignment satisfying at least k clauses? GOAL We have at most k variables left. Max Sat Question

Input A CNF Formula over n variables with m clauses. Is there an assignment satisfying at least k clauses? Variables Clauses Max Sat Question

Input A CNF Formula over n variables with m clauses. Is there an assignment satisfying at least k clauses? Variables Clauses Max Sat Question

Input A CNF Formula over n variables with m clauses. Is there an assignment satisfying at least k clauses? Question If we have at most k variables - nothing to do. If we have at least k variables and we have a matching from Variables — Clauses then we can say YES. Max Sat

Input A CNF Formula over n variables with m clauses. Is there an assignment satisfying at least k clauses? Else, we have at least k variables, but no matching from Variables — Clauses. Max Sat Question

Hall’s Theorem

If, in a bipartite graph with parts A and B, there is no matching from A to B, then there is a subset X of A such that |N(X)| < |X| Hall’s Theorem

Hall’s Theorem

Such an “obstructing set” can be computed in polynomial time. Hall’s Theorem

[inclusion-minimal] Such an “obstructing set” can be computed in polynomial time. Hall’s Theorem

Input A CNF Formula over n variables with m clauses. Is there an assignment satisfying at least k clauses? Question [inclusion-minimal] Such an “obstructing set” can be computed in polynomial time. V C Max Sat

Input A CNF Formula over n variables with m clauses. Is there an assignment satisfying at least k clauses? Question [inclusion-minimal] Such an “obstructing set” can be computed in polynomial time. V C Max Sat

Input A CNF Formula over n variables with m clauses. Is there an assignment satisfying at least k clauses? removed |X| variables V removed |N(X)| clauses C Max Sat Question

Input A CNF Formula over n variables with m clauses. Is there an assignment satisfying at least k clauses? Question Ask now if k - |N(X)| clauses can be satisfied. removed |X| variables V removed |N(X)| clauses C Max Sat

Input A CNF Formula over n variables with m clauses. Is there an assignment satisfying at least k clauses? V C Max Sat Question

Input A CNF Formula over n variables with m clauses. Is there an assignment satisfying at least k clauses? Question We removed an inclusion-minimal violating set. V C Max Sat

Input A CNF Formula over n variables with m clauses. Is there an assignment satisfying at least k clauses? Get rid of one vertex… V C MAX SAT Question

Input A CNF Formula over n variables with m clauses. Is there an assignment satisfying at least k clauses? Get rid of one vertex… V C MAX SAT Question

Input A CNF Formula over n variables with m clauses. Is there an assignment satisfying at least k clauses? Get rid of one vertex… The rest of it can be matched! V C MAX SAT Question

Input A CNF Formula over n variables with m clauses. Is there an assignment satisfying at least k clauses? Question This implies a kernel with at most k variables and 2k clauses. MAX SAT

Input A CNF Formula over n variables with m clauses. Is there an assignment satisfying at least k clauses? Question This implies a kernel with at most k variables and 2k clauses. MAX SAT

Vertex Cover

Input A graph G = (V,E) with n vertices, m edges, and k. Vertex Cover

Input A graph G = (V,E) with n vertices, m edges, and k. Is there a subset of vertices S of size at most k that intersects all the edges? Vertex Cover Question

Input A graph G = (V,E) with n vertices, m edges, and k. Is there a subset of vertices S of size at most k that intersects all the edges? What if a vertex has more than k neighbors? Vertex Cover Question

Input A graph G = (V,E) with n vertices, m edges, and k. Is there a subset of vertices S of size at most k that intersects all the edges? What if a vertex has more than k neighbors? Vertex Cover Question

Input A graph G = (V,E) with n vertices, m edges, and k. Is there a subset of vertices S of size at most k that intersects all the edges? What if a vertex has more than k neighbors? Vertex Cover Question

Input A graph G = (V,E) with n vertices, m edges, and k. Is there a subset of vertices S of size at most k that intersects all the edges? What if a vertex has more than k neighbors? Vertex Cover Question

Input A graph G = (V,E) with n vertices, m edges, and k. Is there a subset of vertices S of size at most k that intersects all the edges? What if a vertex has more than k neighbors? We cannot afford to leave v out of any vertex cover of size at most k. Vertex Cover Question

Input A graph G = (V,E) with n vertices, m edges, and k. Is there a subset of vertices S of size at most k that intersects all the edges? If a vertex has more than k neighbors, Vertex Cover Question

Input A graph G = (V,E) with n vertices, m edges, and k. Is there a subset of vertices S of size at most k that intersects all the edges? Question If a vertex has more than k neighbors, delete it from the graph and reduce the budget by one. Vertex Cover

Input A graph G = (V,E) with n vertices, m edges, and k. Is there a subset of vertices S of size at most k that intersects all the edges? When we have nothing more to do… Vertex Cover Question

Input A graph G = (V,E) with n vertices, m edges, and k. Is there a subset of vertices S of size at most k that intersects all the edges? When we have nothing more to do… every vertex has degree at most k. Vertex Cover Question

Input A graph G = (V,E) with n vertices, m edges, and k. Is there a subset of vertices S of size at most k that intersects all the edges? When we have nothing more to do… every vertex has degree at most k. Vertex Cover Question

Input A graph G = (V,E) with n vertices, m edges, and k. Is there a subset of vertices S of size at most k that intersects all the edges? If the graph has more than k2 edges, Vertex Cover Question

Input A graph G = (V,E) with n vertices, m edges, and k. Is there a subset of vertices S of size at most k that intersects all the edges? If the graph has more than k2 edges, reject the instance. Vertex Cover Question

Input A graph G = (V,E) with n vertices, m edges, and k. Is there a subset of vertices S of size at most k that intersects all the edges? Otherwise: Vertex Cover Question

Input A graph G = (V,E) with n vertices, m edges, and k. Is there a subset of vertices S of size at most k that intersects all the edges? Otherwise: the number of edges is at most k2. Vertex Cover Question

Input A graph G = (V,E) with n vertices, m edges, and k. Is there a subset of vertices S of size at most k that intersects all the edges? Otherwise: the number of edges is at most k2. Vertices? Vertex Cover Question

Input A graph G = (V,E) with n vertices, m edges, and k. Is there a subset of vertices S of size at most k that intersects all the edges? Question Otherwise: the number of edges is at most k2. Vertices? k2 edges can be involved in at most 2k2 vertices. Throw away isolated vertices. Vertex Cover

Input A graph G = (V,E) with n vertices, m edges, and k. Is there a subset of vertices S of size at most k that intersects all the edges? Question This implies a kernel with at most 2k2 vertices and k2 edges. Vertex Cover

Input A graph G = (V,E) with n vertices, m edges, and k. Is there a subset of vertices S of size at most k that intersects all the edges? Question This implies a kernel with at most 2k2 vertices and k2 edges. Vertex Cover

Input A graph G = (V,E) with n vertices, m edges, and k. Is there a subset of vertices S of size at most k that intersects all the edges? Question This implies a kernel with at most 2k2 vertices and k2 edges. [BUSS KERNELIZATION] Vertex Cover

Feedback Arc Set on Tournaments

Input A tournament on n vertices and an integer k. Feedback Arc Set on Tournaments

Input A tournament on n vertices and an integer k. Is there a subset of k arcs that can be reversed to make the tournament acyclic? Question Feedback Arc Set on Tournaments

Input A tournament on n vertices and an integer k. Is there a subset of k arcs that can be reversed to make the tournament acyclic? Question A tournament has a cycle if, and only if, it has a triangle. Feedback Arc Set on Tournaments

Input A tournament on n vertices and an integer k. Is there a subset of k arcs that can be reversed to make the tournament acyclic? Question A tournament has a cycle if, and only if, it has a triangle. Delete a vertex if it does not belong to any triangle. Feedback Arc Set on Tournaments

Input A tournament on n vertices and an integer k. Is there a subset of k arcs that can be reversed to make the tournament acyclic? Question A tournament has a cycle if, and only if, it has a triangle. Delete a vertex if it does not belong to any triangle. Feedback Arc Set on Tournaments

Input A tournament on n vertices and an integer k. Is there a subset of k arcs that can be reversed to make the tournament acyclic? Question Delete a vertex if it does not belong to any triangle. Feedback Arc Set on Tournaments

Input A tournament on n vertices and an integer k. Is there a subset of k arcs that can be reversed to make the tournament acyclic? Question Delete a vertex if it does not belong to any triangle. Feedback Arc Set on Tournaments

Input A tournament on n vertices and an integer k. Is there a subset of k arcs that can be reversed to make the tournament acyclic? Question Delete a vertex if it does not belong to any triangle. Feedback Arc Set on Tournaments

Input A tournament on n vertices and an integer k. Is there a subset of k arcs that can be reversed to make the tournament acyclic? Question What about an edge that participates in more than k triangles? Feedback Arc Set on Tournaments

Input A tournament on n vertices and an integer k. Is there a subset of k arcs that can be reversed to make the tournament acyclic? Question What about an edge that participates in more than k triangles? Feedback Arc Set on Tournaments

Input A tournament on n vertices and an integer k. Is there a subset of k arcs that can be reversed to make the tournament acyclic? Question What about an edge that participates in more than k triangles? Reverse it and decrease the budget by one. Feedback Arc Set on Tournaments

Input A tournament T on n vertices and an integer k. Is there a subset of k arcs that can be reversed to make the tournament acyclic? Question Feedback Arc Set on Tournaments

Input A tournament T on n vertices and an integer k. Is there a subset of k arcs that can be reversed to make the tournament acyclic? Question At most k arcs in any solution. Feedback Arc Set on Tournaments

Input A tournament T on n vertices and an integer k. Is there a subset of k arcs that can be reversed to make the tournament acyclic? Question At most k arcs in any solution. Feedback Arc Set on Tournaments

Input A tournament T on n vertices and an integer k. Is there a subset of k arcs that can be reversed to make the tournament acyclic? Question At most k arcs in any solution. Feedback Arc Set on Tournaments

Input A tournament T on n vertices and an integer k. Is there a subset of k arcs that can be reversed to make the tournament acyclic? Question At most k arcs in any solution. Every vertex is in a triangle, and every arc “sees” at most k triangles. Feedback Arc Set on Tournaments

Input A tournament T on n vertices and an integer k. Is there a subset of k arcs that can be reversed to make the tournament acyclic? Question Feedback Arc Set on Tournaments

Input A tournament T on n vertices and an integer k. Is there a subset of k arcs that can be reversed to make the tournament acyclic? Question After reducing the graph, if there are more than k2 + 2k vertices, reject the instance. Feedback Arc Set on Tournaments

Input A tournament T on n vertices and an integer k. Is there a subset of k arcs that can be reversed to make the tournament acyclic? Question This implies a kernel with at most k2 + 2k vertices. Feedback Arc Set on Tournaments

Input A tournament T on n vertices and an integer k. Is there a subset of k arcs that can be reversed to make the tournament acyclic? Question This implies a kernel with at most k2 + 2k vertices. Feedback Arc Set on Tournaments

d-Hitting Set

Input A family F of d-sized sets over an universe U, and k. d-Hitting Set

Input A family F of d-sized sets over an universe U, and k. Does U have a subset S of size at most k that intersects every set in F? d-Hitting Set Question

Input A family F of d-sized sets over an universe U, and k. Does U have a subset S of size at most k that intersects every set in F? d-Hitting Set Question

Input A family F of d-sized sets over an universe U, and k. Does U have a subset S of size at most k that intersects every set in F? Question What if there are more than k sets with one element in common? d-Hitting Set

Input A family F of d-sized sets over an universe U, and k. Does U have a subset S of size at most k that intersects every set in F? Question What if there are more than k sets with one element in common? ✂✍✎☕♢ ☕♤♧★☼ ♕♖☕♘♤ ⚖☕✜✍♖ d-Hitting Set

Input A family F of d-sized sets over an universe U, and k. Does U have a subset S of size at most k that intersects every set in F? Question What if there are more than k sets with one element in common? ✂✍✎☕♢ ☕♤♧★☼ ♕♖☕♘♤ ⚖☕✜✍♖ d-Hitting Set

Input A family F of d-sized sets over an universe U, and k. Does U have a subset S of size at most k that intersects every set in F? Question What if there are more than k sets with one element in common, and every set is mutually disjoint otherwise? d-Hitting Set

Input A family F of d-sized sets over an universe U, and k. Does U have a subset S of size at most k that intersects every set in F? Question What if there are more than k sets with one element in common, and every set is mutually disjoint otherwise? ✂☞✎☕♢ ☕♤♧★☼ ♕♖☕♘❀ ⚖☕✜✍♖ d-Hitting Set

Input A family F of d-sized sets over an universe U, and k. Does U have a subset S of size at most k that intersects every set in F? ❀ Sunflower Lemma d-Hitting Set Question

Input A family F of d-sized sets over an universe U, and k. Does U have a subset S of size at most k that intersects every set in F? {1,2,53,54,55} {1,2,68,69,67} {1,2,15,12,11} {1,2,23,24,29} {1,2,72,71,70} Sunflower Lemma d-Hitting Set Question

Input A family F of d-sized sets over an universe U, and k. Does U have a subset S of size at most k that intersects every set in F? Question If a d-uniform family F has at least d!(k-1)d sets, then it has a subcollection of at least k sets that: ! have a common mutual intersection (a core) are pairwise disjoint otherwise (petals). Sunflower Lemma d-Hitting Set

Input A family F of d-sized sets over an universe U, and k. Does U have a subset S of size at most k that intersects every set in F? Question Sunflowers can also be computed in polynomial time. Sunflower Lemma [By the way…] d-Hitting Set

Input A family F of d-sized sets over an universe U, and k. Does U have a subset S of size at most k that intersects every set in F? Question As long as there is a sunflower involving at least k+1 petals, remove it and replace it with its core. Intuition d-Hitting Set

Input A family F of d-sized sets over an universe U, and k. Does U have a subset S of size at most k that intersects every set in F? {1,2,53,54,55} {1,2,68,69,67} {1,2,15,12,11} {1,2,23,24,29} {1,2,72,71,70} Intuition d-Hitting Set Question

Input A family F of d-sized sets over an universe U, and k. Does U have a subset S of size at most k that intersects every set in F? {1,2} Intuition d-Hitting Set Question

Input A family F of d-sized sets over an universe U, and k. Does U have a subset S of size at most k that intersects every set in F? Question The rule destroys d-uniformity. Fix this by iterating over d. What if the core is empty? This means we have more than k disjoint sets, so we can reject the instance. Sunflower Lemma d-Hitting Set

Input A family F of d-sized sets over an universe U, and k. Does U have a subset S of size at most k that intersects every set in F? Question The rule destroys d-uniformity. Fix this by iterating over d. What if the core is empty? This means we have more than k disjoint sets, so we can reject the instance. Sunflower Lemma d-Hitting Set

Input A family F of d-sized sets over an universe U, and k. Does U have a subset S of size at most k that intersects every set in F? Question The rule destroys d-uniformity. Fix this by iterating over d. What if the core is empty? This means we have more than k disjoint sets, so we can reject the instance. Sunflower Lemma d-Hitting Set

Input A family F of d-sized sets over an universe U, and k. Does U have a subset S of size at most k that intersects every set in F? Question This implies a kernel with at most d!kd sets, and d(d!)kd elements. d-Hitting Set

Input A family F of d-sized sets over an universe U, and k. Does U have a subset S of size at most k that intersects every set in F? Question This implies a kernel with at most d!kd sets, and d(d!)kd elements. d-Hitting Set

Take Away…

Eliminate “high” degree vertices The problem is simple on “bounded degree” graphs

Eliminate “high” degree vertices The problem is simple on “bounded degree” graphs May have one, but not the other, eg., Dominating Set, Connected Vertex Cover.

Eliminate “high” degree vertices The problem is simple on “bounded degree” graphs May have one, but not the other, eg., Dominating Set, Connected Vertex Cover. High Degree Rules may be involved. Eg: Feedback Vertex Set

Combinatorial facts (such as the Sunflower Lemma) and structure theorems (like Hall’s theorem) can be often (surprisingly) useful.

The Big Picture

A fundamental connection with FPT-land… A parameterized problem is fixed-parameter tractable ! if and only if ! it has a kernelization algorithm.

If you have a f(k)-sized kernel…

If you have a f(k)-sized kernel… Obtain kernel and solve the problem using some algorithm with running time g(). poly(n) + g(f(k))

If you have a f(k)p(n) FPT Algorithm…

If you have a f(k)p(n) FPT Algorithm… We are done if n is at most f(k).

If you have a f(k)p(n) FPT Algorithm… We are done if n is at most f(k). If not, f(k)p(n) is at most n(p(n)), so the “FPT” algorithm can be run in polynomial time to obtain a trivial kernel.

Do FPT problems always have polynomial-sized kernels?

Do FPT problems always have polynomial-sized kernels? Remember to ask the question with an “above guarantee” parameterization after you find a ‘cheat’ kernel.

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