New Geometric Representations and Domination Problems on Tolerance and Multitolerance Graphs

Transcription

1 New Geometric Representations and Domination Problems on Tolerance and Multitolerance Graphs Archontia Giannopoulou George B. Mertzios School of Engineering and Computing Sciences, Durham University, UK Algorithmic Graph Theory on the Adriatic Coast June 16 19, 2015 Koper, Slovenia George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

2 Intersection graphs Definition An undirected graph G = (V, E ) is called an intersection graph, if each vertex v V can be assigned to a set S v, such that two vertices of G are adjacent if and only if the corresponding sets have a nonempty intersection, i.e. E = {uv S u S v = }. George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

3 Intersection graphs Definition An undirected graph G = (V, E ) is called an intersection graph, if each vertex v V can be assigned to a set S v, such that two vertices of G are adjacent if and only if the corresponding sets have a nonempty intersection, i.e. E = {uv S u S v = }. Definition A graph G is called an interval graph, if G is the intersection graph of a set of intervals on the real line. b c a d e b c a e d George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

4 Tolerance graphs Definition (Golumbic, Monma, 1982) A graph G = (V, E ) is called a tolerance graph, if there is a set I = {I v v V } of intervals and a set t = {t v v V } of positive numbers, such that uv E if and only if I u I v min{t u, t v }. George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

5 Tolerance graphs Definition (Golumbic, Monma, 1982) A graph G = (V, E ) is called a tolerance graph, if there is a set I = {I v v V } of intervals and a set t = {t v v V } of positive numbers, such that uv E if and only if I u I v min{t u, t v }. I a I c a b d c I b I d t a = t c = 1 t b = 8 t d = 7 George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

6 Tolerance graphs Definition (Golumbic, Monma, 1982) A graph G = (V, E ) is called a tolerance graph, if there is a set I = {I v v V } of intervals and a set t = {t v v V } of positive numbers, such that uv E if and only if I u I v min{t u, t v }. Definition A vertex v of a tolerance graph G = (V, E ) with a tolerance representation I, t is called a bounded vertex, if t v I v. George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

7 Tolerance graphs Definition (Golumbic, Monma, 1982) A graph G = (V, E ) is called a tolerance graph, if there is a set I = {I v v V } of intervals and a set t = {t v v V } of positive numbers, such that uv E if and only if I u I v min{t u, t v }. Definition A vertex v of a tolerance graph G = (V, E ) with a tolerance representation I, t is called a bounded vertex, if t v I v. Otherwise, if t v > I v, v is called an unbounded vertex. George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

8 Multitolerance graphs Motivation and definition Tolerance graphs have important applications [Golumbic, Trenk, Tolerance graphs, 2004]: biology and bioinformatics (comparison of DNA sequences between organisms, e.g. in BLAST software) George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

9 Multitolerance graphs Motivation and definition Tolerance graphs have important applications [Golumbic, Trenk, Tolerance graphs, 2004]: biology and bioinformatics (comparison of DNA sequences between organisms, e.g. in BLAST software) interval DNA sub-sequence tolerance permissible number of errors George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

10 Multitolerance graphs Motivation and definition Tolerance graphs have important applications [Golumbic, Trenk, Tolerance graphs, 2004]: biology and bioinformatics (comparison of DNA sequences between organisms, e.g. in BLAST software) interval DNA sub-sequence tolerance permissible number of errors temporal reasoning, resource allocation, scheduling... George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

11 Multitolerance graphs Motivation and definition Tolerance graphs have important applications [Golumbic, Trenk, Tolerance graphs, 2004]: biology and bioinformatics (comparison of DNA sequences between organisms, e.g. in BLAST software) interval DNA sub-sequence tolerance permissible number of errors temporal reasoning, resource allocation, scheduling... In applications of BLAST, some genomic regions may be: biologically less significant, or George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

12 Multitolerance graphs Motivation and definition Tolerance graphs have important applications [Golumbic, Trenk, Tolerance graphs, 2004]: biology and bioinformatics (comparison of DNA sequences between organisms, e.g. in BLAST software) interval DNA sub-sequence tolerance permissible number of errors temporal reasoning, resource allocation, scheduling... In applications of BLAST, some genomic regions may be: biologically less significant, or more error prone than others = we want to treat several genomic parts non-uniformly. George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

13 Multitolerance graphs Motivation and definition Multitolerance graphs: t 1 t 2 I = [l, r] : l l t r t r from left and right: different tolerances. George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

14 Multitolerance graphs Motivation and definition Multitolerance graphs: t 1 t 2 I = [l, r] : l l t r t r from left and right: different tolerances. in the interior part: tolerate a convex combination of t 1 and t 2. George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

15 Multitolerance graphs Motivation and definition Multitolerance graphs: t 1 t 2 I = [l, r] : l l t r t r Formally: I(I, l t, r t ) = {λ [l, l t ] + (1 λ) [r t, r] : λ [0, 1]} (convex hull of [l, l t ] and [r t, r]) George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

16 Multitolerance graphs Motivation and definition Multitolerance graphs: t 1 t 2 I = [l, r] : l l t r t r Formally: I(I, l t, r t ) = {λ [l, l t ] + (1 λ) [r t, r] : λ [0, 1]} (convex hull of [l, l t ] and [r t, r]) Set τ of tolerance intervals of I : either τ = I(I, l t, r t ) for two values l t, r t I (bounded case), George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

17 Multitolerance graphs Motivation and definition Multitolerance graphs: t 1 t 2 I = [l, r] : l l t r t r Formally: I(I, l t, r t ) = {λ [l, l t ] + (1 λ) [r t, r] : λ [0, 1]} (convex hull of [l, l t ] and [r t, r]) Set τ of tolerance intervals of I : either τ = I(I, l t, r t ) for two values l t, r t I (bounded case), or τ = R (unbounded case). George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

18 Multitolerance graphs Motivation and definition Multitolerance graphs: t 1 t 2 I = [l, r] : l l t r t r Formally: I(I, l t, r t ) = {λ [l, l t ] + (1 λ) [r t, r] : λ [0, 1]} (convex hull of [l, l t ] and [r t, r]) Set τ of tolerance intervals of I : either τ = I(I, l t, r t ) for two values l t, r t I (bounded case), or τ = R (unbounded case). In a multitolerance graph G = (V, E ), uv E whenever: there exists a tolerance-interval Q u τ u such that Q u I v, or there exists a tolerance-interval Q v τ v such that Q v I u. George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

19 Complete classification in the hierarchy of perfect graphs perfect alternately orientable weakly chordal co-perfectly orderable multitolerance cocomparability tolerance bounded multitolerance trapezoid bounded tolerance parallelogram [Golumbic, Trenk, Tolerance Graphs, 2004] [Mertzios, SODA, 2011; Algorithmica, 2014] George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

20 Tolerance and multitolerance graphs Several NP-complete problems are known to be polynomially solvable on tolerance / multitolerance graphs George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

21 Tolerance and multitolerance graphs Several NP-complete problems are known to be polynomially solvable on tolerance / multitolerance graphs Some (few) algorithms used the (multi)tolerance representation: [Parra, Discr. Appl. Math., 1998] [Golumbic, Siani, AISC, 2002] [Golumbic, Trenk, Tolerance Graphs, 2004] Most followed by the containment in weakly chordal / perfect graphs George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

22 Tolerance and multitolerance graphs Several NP-complete problems are known to be polynomially solvable on tolerance / multitolerance graphs Some (few) algorithms used the (multi)tolerance representation: [Parra, Discr. Appl. Math., 1998] [Golumbic, Siani, AISC, 2002] [Golumbic, Trenk, Tolerance Graphs, 2004] Most followed by the containment in weakly chordal / perfect graphs It seems to be essential to assume (some) given representation: Tolerance graphs are NP-complete to recognize [Mertzios, Sau, Zaks, STACS, 2010; SIAM J. Comp., 2011] Recognition of multitolerance graphs: Open! George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

23 Tolerance and multitolerance graphs Previously known models Succinct intersection models are known for: bounded tolerance graphs (parallelogram representation) [Langley, PhD, 1993; Bogart et al., Discr. Appl. Math., 1995] George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

24 Tolerance and multitolerance graphs Previously known models Succinct intersection models are known for: bounded tolerance graphs (parallelogram representation) [Langley, PhD, 1993; Bogart et al., Discr. Appl. Math., 1995] bounded multitolerance graphs (trapezoid representation) [Parra, Discr. Appl. Math., 1998] George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

25 Tolerance and multitolerance graphs Previously known models Succinct intersection models are known for: bounded tolerance graphs (parallelogram representation) [Langley, PhD, 1993; Bogart et al., Discr. Appl. Math., 1995] bounded multitolerance graphs (trapezoid representation) [Parra, Discr. Appl. Math., 1998] general tolerance graphs (3D-parallelepiped representation) [Mertzios, Sau, Zaks, SIAM J. Discr. Math., 2009] George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

26 Tolerance and multitolerance graphs Previously known models Succinct intersection models are known for: bounded tolerance graphs (parallelogram representation) [Langley, PhD, 1993; Bogart et al., Discr. Appl. Math., 1995] bounded multitolerance graphs (trapezoid representation) [Parra, Discr. Appl. Math., 1998] general tolerance graphs (3D-parallelepiped representation) [Mertzios, Sau, Zaks, SIAM J. Discr. Math., 2009] general multitolerance graphs (3D-trapezoepiped representation) [Mertzios, SODA, 2011; Algorithmica, 2014] George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

27 Tolerance and multitolerance graphs Previously known models Succinct intersection models are known for: bounded tolerance graphs (parallelogram representation) [Langley, PhD, 1993; Bogart et al., Discr. Appl. Math., 1995] bounded multitolerance graphs (trapezoid representation) [Parra, Discr. Appl. Math., 1998] general tolerance graphs (3D-parallelepiped representation) [Mertzios, Sau, Zaks, SIAM J. Discr. Math., 2009] general multitolerance graphs (3D-trapezoepiped representation) [Mertzios, SODA, 2011; Algorithmica, 2014] These representations enabled the design of algorithms: for clique, coloring, independent set,... in most cases with (optimal) O(n log n) running time George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

28 Tolerance and multitolerance graphs Previously known models In spite of research in the area since [Golumbic, Monma, 1982]: a few problems remained open for (multi)tolerance graphs Dominating Set, Hamiltonian Cycle [Spinrad, Efficient Graph Representations, 2003] George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

29 Tolerance and multitolerance graphs Previously known models In spite of research in the area since [Golumbic, Monma, 1982]: a few problems remained open for (multi)tolerance graphs Dominating Set, Hamiltonian Cycle [Spinrad, Efficient Graph Representations, 2003] both these problems are: NP-complete on weakly chordal graphs [Booth, Johnson, SIAM J. Computing, 1982] [Müller, Discr. Math, 1996] polynomial on bounded (multi)tolerance (and cocomparability) graphs [Kratsch, Stewart, SIAM J. Discr. Math, 1993] [Deogun, Steiner, SIAM J. Computing, 1994] the known models do not provide (enough) insight for these problems new models are needed! George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

30 Our results New geometric representations: shadow representation for multitolerance graphs special case: horizontal shadow representation for tolerance graphs George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

31 Our results New geometric representations: shadow representation for multitolerance graphs special case: horizontal shadow representation for tolerance graphs Applications of these new models: Dominating Set is APX-hard on multitolerance graphs (i.e. no PTAS unless P = NP) Dominating Set is polynomially solvable on tolerance graphs Independent Dominating Set is polynomially solvable on multitolerance graphs (by a sweep-line algorithm) George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

32 Our results New geometric representations: shadow representation for multitolerance graphs special case: horizontal shadow representation for tolerance graphs Implications of the new representations: we can reduce optimization problems on these graphs to problems in computational geometry Dominating Set is the first problem with different complexity in tolerance & multitolerance graphs surprising dichotomy result useful for sweep-line algorithms George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

33 Bounded multitolerance graphs Lemma (Parra, 1998) Bounded multitolerance graphs coincide with trapezoid graphs. tv1 tv2 av cv dv bv cv T v bv L1 av tv1 dv L2 George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

34 Bounded multitolerance graphs Lemma (Parra, 1998) Bounded multitolerance graphs coincide with trapezoid graphs. tv1 tv2 av cv dv bv cv T v bv L1 av dv L2 tv1 tv2 George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

35 Bounded multitolerance graphs Lemma (Parra, 1998) Bounded multitolerance graphs coincide with trapezoid graphs. tv1 tv2 av cv dv bv cu T u cv bu T v bv L1 tu1 tu2 au du av dv L2 au cu du bu tv1 tv2 tu2 George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

36 Bounded multitolerance graphs Lemma (Parra, 1998) Bounded multitolerance graphs coincide with trapezoid graphs. tv1 tv2 av cv dv bv cu T u cv bu T v bv L1 tu1 tu2 au du av dv L2 au cu du bu tv1 tv2 tu2 Theorem (Langley 1993; Bogart et al. 1995) Bounded tolerance graphs coincide with parallelogram graphs. George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

37 Bounded multitolerance graphs Lemma (Parra, 1998) Bounded multitolerance graphs coincide with trapezoid graphs. tv1 tv2 av cv dv bv cu T u cv bu T v bv L1 tu1 tu2 au du av dv L2 au cu du bu tv1 tv2 tu2 Theorem (Langley 1993; Bogart et al. 1995) Bounded tolerance graphs coincide with parallelogram graphs. tv tv tu av tu cv dv bv cu P u cv bu P v bv L1 au cu du bu au du av tv dv L2 tu George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

38 The trapezoepiped representation A 3D-intersection model for multitolerance graphs bounded vertices 3D-trapezoepipeds T v5 T v6 T v2 T v3 T v4 T v1 T v7 L 1 z y L 2 x George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

39 The trapezoepiped representation A 3D-intersection model for multitolerance graphs bounded vertices 3D-trapezoepipeds unbounded vertices lifted line segments T v5 T v6 T v2 T v3 T v4 T v1 T v7 L 1 z y L 2 x George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

40 The trapezoepiped representation A 3D-intersection model for multitolerance graphs bounded vertices 3D-trapezoepipeds unbounded vertices lifted line segments an intersection model for multitolerance graphs: [Mertzios, SODA, 2011; Algorithmica, 2014] T v5 T v6 T v2 T v3 T v4 T v1 T v7 L 1 z y L 2 x George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

41 The trapezoepiped representation A 3D-intersection model for multitolerance graphs Special case: parallelepiped representation for tolerance graphs: [Mertzios, Sau, Zaks, SIAM J. Discr. Math., 2009] aaa aaa P v P w φ v φ u φ w P u L 1 z y L 2 x George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

42 The shadow representation All information is captured by the intersection of every 3D-object with the plane y = 0 z x z y x George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

43 The shadow representation All information is captured by the intersection of every 3D-object with the plane y = 0 Associate to every bounded vertex u: a line segment L u on the plane z x z y x George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

44 The shadow representation All information is captured by the intersection of every 3D-object with the plane y = 0 Associate to every bounded vertex u: a line segment L u on the plane Associate to unbounded vertex v: a point p v on the plane z x z y x George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

45 The shadow representation All information is captured by the intersection of every 3D-object with the plane y = 0 Associate to every bounded vertex u: a line segment L u on the plane Associate to unbounded vertex v: a point p v on the plane z x z y x George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

46 The shadow representation Definition The shadow representation of a multitolerance graph G is a tuple (P, L): P is the set of all points p v and L is the set of all line segments L u z z y x x George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

47 The shadow representation Definition The shadow representation of a multitolerance graph G is a tuple (P, L): P is the set of all points p v and L is the set of all line segments L u Special case: tolerance graphs parallelepipeds horizontal line segments horizontal shadow representation z z y x x George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

48 The shadow representation Definition The shadow representation of a multitolerance graph G is a tuple (P, L): P is the set of all points p v and L is the set of all line segments L u Question: How do we interpret adjacencies in such a representation? George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

49 The shadow representation Definition The shadow representation of a multitolerance graph G is a tuple (P, L): P is the set of all points p v and L is the set of all line segments L u Question: How do we interpret adjacencies in such a representation? Answer: We exploit the shadows of the line segments and the points. George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

50 The shadow representation Definition (shadow) For a point t = (t x, t y ) R 2 the shadow of t is the region S t = {(x, y) R 2 : x t x, y x t y t x }. p u,1 t L u S t S Lu p u,2 George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

51 The shadow representation Definition (shadow) For a point t = (t x, t y ) R 2 the shadow of t is the region S t = {(x, y) R 2 : x t x, y x t y t x }. For every line segment L u the shadow of L u is the region S Lu = t L u S t. p u,1 t L u S t S Lu p u,2 George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

52 The shadow representation The shadows capture all adjacencies: Lemma Let G = (V, E ) be a multitolerance graph and u, v be bounded vertices. Then uv E if and only if L v S Lu = or L u S Lv =. uv E : uv / E : L v L u L v L u S Lv S Lu S Lv S Lu George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

53 The shadow representation The shadows capture all adjacencies: Lemma Let G = (V, E ) be a multitolerance graph and u, v be bounded vertices. Then uv E if and only if L v S Lu = or L u S Lv =. uv E : uv / E : L v L u L v L u S Lv S Lu S Lv S Lu George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

54 The shadow representation The shadows capture all adjacencies: Lemma Let G = (V, E ) be a multitolerance graph, u be a bounded vertex and v be an unbounded vertex. Then uv E if and only if p v S Lu. uv E : uv / E : p v p v S Lu L u S Lu L u George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

55 The shadow representation The shadows capture all adjacencies: Lemma Let G = (V, E ) be a multitolerance graph, u be a bounded vertex and v be an unbounded vertex. Then uv E if and only if p v S Lu. uv E : uv / E : p v p v S Lu L u S Lu L u George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

56 The shadow representation Main idea for the adjacencies: uv / E : z L u L v x z y x George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

57 The shadow representation Main idea for the adjacencies: uv / E : z L u L v x z y x George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

58 The shadow representation Main idea for the adjacencies: uv E : z L u L v x z y x George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

59 The shadow representation Main idea for the adjacencies: uv E : z L u L v x z y x Observation The shadow representation is not an intersection model. George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

60 Inevitable vertices Definition Let v be an unbounded vertex of a multitolerance graph G (in a certain trapezoepiped representation). If making v a bounded vertex creates a new edge in G, then v is called inevitable. T v5 T v6 T v2 T v3 T v4 T v1 T v7 L 1 L 2 George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

61 Inevitable vertices Definition Let v be an unbounded vertex of a multitolerance graph G (in a certain trapezoepiped representation). If making v a bounded vertex creates a new edge in G, then v is called inevitable. Otherwise, v is called evitable. T v5 T v6 T v2 T v3 T v4 T v1 T v7 L 1 L 2 George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

62 Inevitable vertices Definition Let v be an inevitable unbounded vertex of a multitolerance graph G (in a certain trapezoepiped representation). A vertex u is called a hovering vertex of v if T v lies above T u. a T v5 T v6 T v2 T v3 T v4 T v1 T v7 L 1 L 2 George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

63 Inevitable vertices In a shadow representation: Lemma Let v be an inevitable unbounded vertex. Then a vertex u is a hovering vertex of v if and only if: L u S v = (when u is bounded) u is a hovering vertex of v: p v p v L u p u S Lu George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

64 Inevitable vertices In a shadow representation: Lemma Let v be an inevitable unbounded vertex. Then a vertex u is a hovering vertex of v if and only if: L u S v = (when u is bounded) p u S v (when u is unbounded) u is a hovering vertex of v: p v p v L u p u S Lu George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

65 Canonical trapezoepiped representations Definition A trapezoepiped representation of a multitolerance graph G is called canonical if every unbounded vertex is inevitable. George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

66 Canonical trapezoepiped representations Definition A trapezoepiped representation of a multitolerance graph G is called canonical if every unbounded vertex is inevitable. Theorem (Mertzios, SODA, 2011; Algorithmica, 2014) Given a trapezoepiped representation of a multitolerance graph G, a canonical representation of G can be computed in O(n log n) time. George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

67 Canonical trapezoepiped representations Definition A trapezoepiped representation of a multitolerance graph G is called canonical if every unbounded vertex is inevitable. Theorem (Mertzios, SODA, 2011; Algorithmica, 2014) Given a trapezoepiped representation of a multitolerance graph G, a canonical representation of G can be computed in O(n log n) time. Definition A shadow representation of a multitolerance graph G is called canonical if it can be obtained by a canonical trapezoepiped representation. In the algorithms: it is useful to assume canonical representations George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

68 Dominating set on tolerance graphs W.l.o.g. we assume: a connected tolerance graph a canonical horizontal shadow representation George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

69 Dominating set on tolerance graphs W.l.o.g. we assume: a connected tolerance graph a canonical horizontal shadow representation Lemma If an unbounded vertex v is in a minimum dominating set S, then w.l.o.g.: S does not contain any neighbor of v, S does not contain any hovering vertex of v. George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

70 Dominating set on tolerance graphs W.l.o.g. we assume: a connected tolerance graph a canonical horizontal shadow representation Lemma If an unbounded vertex v is in a minimum dominating set S, then w.l.o.g.: S does not contain any neighbor of v, S does not contain any hovering vertex of v. Therefore: an unbounded vertex v in the solution cuts the representation into left and right dynamic programming, using the position of the unbounded vertices George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

71 Dominating set on tolerance graphs Dynamic programming: L z L z r z r z p q p 1 p 2 p q l w l w L w L w L i L i r i r i p j remaining solution only unbounded only bounded last unbounded (so far) George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

72 Dominating set on tolerance graphs Dynamic programming: L z L z r z r z p q p 1 p 2 p q l w l w L w L w L i L i r i r i p j remaining solution only unbounded only bounded last unbounded (so far) George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

73 Dominating set on tolerance graphs Dynamic programming: L z L z r z r z p q p 1 p 2 p q l w l w L w L w L i L i r i r i p j remaining solution only unbounded only bounded last unbounded (so far) George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

74 Dominating set on tolerance graphs Dynamic programming: L z L z r z r z p q p 1 p 2 p q l w l w L w L w L i L i r i r i p j remaining solution only unbounded only bounded last unbounded (so far) George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

75 Dominating set on tolerance graphs Dynamic programming: L z L z r z r z p q p 1 p 2 p q l w l w L w L w L i L i r i r i p j Separate dynamic programming: bounded dominating set use only bounded remaining only vertices unbounded to dominate the (sub)graph last unbounded solution only bounded specifying the leftmost and rightmost bounded vertices (so far) not always possible to find a feasible solution! George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

76 Dominating set on multitolerance graphs On a general (non-horizontal) shadow representation: domination set is APX-hard reduction from Special 3-Set Cover (special case of the set cover problem) heavily use the different slopes of the line segments the spirit of the reduction is inspired from: [Chan, Grant, Comp. Geometry, 2014] p zt p wt p xt p yt p ai p aj L 5m+1 L5m+2 George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

77 Dominating set on multitolerance graphs On a general (non-horizontal) shadow representation: domination set is APX-hard reduction from Special 3-Set Cover (special case of the set cover problem) heavily use the different slopes of the line segments the spirit of the reduction is inspired from: [Chan, Grant, Comp. Geometry, 2014] In contrast to dominating set: independent dominating set is polynomial on multitolerance graphs Sweep line algorithm from right to left George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

78 Open problems Can we significantly improve the time complexity of dominating set on tolerance graphs? George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

79 Open problems Can we significantly improve the time complexity of dominating set on tolerance graphs? Can we solve in polynomial time the Hamiltonian Path / Cycle problems: on tolerance graphs? on multitolerance graphs? George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

80 Open problems Can we significantly improve the time complexity of dominating set on tolerance graphs? Can we solve in polynomial time the Hamiltonian Path / Cycle problems: on tolerance graphs? on multitolerance graphs? Recognition of multitolerance graphs? George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

81 Open problems Can we significantly improve the time complexity of dominating set on tolerance graphs? Can we solve in polynomial time the Hamiltonian Path / Cycle problems: on tolerance graphs? on multitolerance graphs? Recognition of multitolerance graphs? recognition of trapezoid graphs polynomial recognition of tolerance and bounded tolerance (parallelogram) graphs NP-complete [Mertzios, Sau, Zaks, STACS, 2010; SIAM J. Comp., 2011] George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

82 Open problems Can we significantly improve the time complexity of dominating set on tolerance graphs? Can we solve in polynomial time the Hamiltonian Path / Cycle problems: on tolerance graphs? on multitolerance graphs? Recognition of multitolerance graphs? recognition of trapezoid graphs polynomial recognition of tolerance and bounded tolerance (parallelogram) graphs NP-complete [Mertzios, Sau, Zaks, STACS, 2010; SIAM J. Comp., 2011] Recognition of unit / proper (multi)tolerance graphs? George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23

83 Thank you for your attention! George Mertzios (Durham) Dominating Set on (multi)tolerance graphs AGTAC / 23