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Kashiwa-Nakamura-Convex Gemetries AAB 2012.PDF The rooted circuits and the rooted cocircuits of convex geometries, closure operators, and monotone extensive operators Kashiwabara Kenji and Nakamura Masataka Department of System Sciences, University of Tokyo, Komaba, Meguro, Tokyo 153‐8902, JAPAN. 1 Outline 1. Closure spaces, closure systems, matroids, and convex geometries 2. Supersolvable antimatroids 3. Lattice-embedding of convex geometries to convex geometries 4. Rooted circuits and implicational systems 5. Extensive and intensive operators 6. Relations of areas to convex geometries 2 1. Closure spaces, closure systems, matroids, and convex geometries Let E be a non‐empty finite set. Def. Closure operator :2E 2E A (A) (extensive) A B (A) (B)(monotone) ((A)) (A) (idempotent) Def. (, E) is called a closure space. 3 Def. Closure system Def. Closure operator E E K 2E :2 2 ・EK, ・A(A) ・X,Y K X Y K ・A B (A) (B) ・((A)) (A) A member of a closure system is called a closed set. (A) X K{AE: (A)A} XK,AX one-to-one correspondence 4 Ex. A closure system is a lattice under inclusion relation. {1,2,3,4,5} {1,2,4,5} {2,3,4,5} {1,2,3,5} {2,4,5} {1,2,5} {2,3,5} {1,5} {3,5} {2,5} {5} 5 Closure spaces (closure systems) Def. For each element e E,suppose X E e and X is minimal with respect to the property e (X). Then we say (X,e) is a rooted circuit of a closure operator , (or more generally a monotone extensive operator ). X is called the stem, and e is called the root. We denote the collection of the rooted circuits of by C(). Ex. Rooted circuits {x,y,z} ({y,z}, x) ({x,z}, {x} {y} {z} ({x,y},y) z) 6 Closure spaces (closure systems) Conversely, the rooted-circuit system C ( ) determines the closure operator . (A) A{eE A:(X,e)C(), X A} (A E). If (A)contains an element e E A here is a minimal A such that X A e (X) wirh (X,e) C. e X 7 Closure spaces (closure systems) Def. Rooted cocircuit For e E and Y E e , if Y is minimal with respect to the property e (E(Y e) ), (Y,e) is said to be a rooted cocircuit. We denote the collection of the rooted cocircuits by D(). Ex. {x,y,z} Rooted Cocircuits ({y}, x) ({z}, x) {x} {y} {z} ({z}, y) ({x}, y) ({x}, z) ({y}, z) a (E {y, x}) ({z}){z} 8 Ex. Affine-point configuration E {a,b,c,e} R2 (A) conv.hull(A) E Rooted Circuits a ({a,b,c},e) e b c Rooted Cocircuits ({a},e), ({b},e), ({c},e). 9 Closure spaces (closure systems) Ex. Affine-point configuration E {a,b,c,d,e} R2 Rooted Circuits (A) conv.hull(A) E ({a,b,c},e), ({c,d},e), a ({a,b},d). d e Rooted Cocircuits ({c},e), ({a,d},e), b c ({b,d},e). ({a},d), ({b},d) 10 Closure spaces (closure systems) Def. (A) {eA:e(Ae)} (A E) Is the extreme-point operator of a closure operator . Ex. Affine-point configuration E {a,b,c,d,e} R2. a ({a,b,c,d,e}){a,b,c} d ({a,c,d,e}){a,c,d} e b c ({a,b,e}){a,b,e} 11 Matroid Def. A closure space (,E) A is a matroid if b a(Ab) b(Aa) (Exchange Property) a Abstraction of linear dependency 12 Matroid a(Ab) b(Aa) (Exchange Property) Ex. A graphic matroid b b x x w y w a y a u z u z A{x, y,z} (A) {x, y,z,u,w} 13 Ex. A graphic matroid M(G) of a graph G E=E(G) b Rooted circuits with the root z x ({a, b, x, u}, z), y w a ({x, w, u}, z), ({w, y}, z), u z ({a, b, y}, z). Proposition In a matroid M, for any circuit C and any element eC, (Ce, e) is a rooted circuit, and vice versa. 14 Convex geometry Def. A closure space (, E) is a convex geometry if the corresponding closure operator satisfies the anti‐exchange property. a b, a,b(A), b(Aa) a(Ab) Ex. Affine‐point configuration E R2. (Anti‐exchange Property) (A) conv.hull (A) E (A) b a Abstraction of convexity (Jamison 1982) a(Ab) b(Aa) (Excange Property) 15 Convex geometry Theorem ( Edelman and Jamison 1985) Let (, E) be a closure space , K be the closure system associated with it, and be the extreme‐point operator. Then the following are equivalent. (1) is a closure operator with the anti‐exchange property. (2) For any closed set X K with X E, there exists e E X such that X e K. (3) ( (A)) (A) for A E. (4) ( (A)) (A) for A E. [ Krein‐Milman property] 16 Convex Geometries and Antimatroids Def. Convex geometry Def. Antimatroid K 2E F 2E ,EK, ・ٛ F・ ・X,Y K X Y K ・A,BF ABF ・X K, X E ・AF, A eE X : X eK eA: AeF Def. Antimatroid An element of K is called a F {E X : X K} convex set, and an element of F is a feasible set. 17 E X e A X Ae convex geometry antimatroid 18 {a,b.c,b} {a,b,c,d} {b,c,d} {a,c,d} {a,b,d} {c,d} {a,b} {c} {a} {b} A convex An antimatroid geometry 19 classes of convex geometries ・ Affine convex geometry ・ poset convex geometry ・ double shelling of a poset ・ simplicial shelling of a chordal graph ・ tree node‐shelling convex geometry ・ graph search convex geometry generalized affine convex geometries = all the convex geometries 20 Principle of shelling process of antimatroids Once an element is removable, it remains removable until it is deleted. (A) is the set of removable elements in A. A : E ; while( (A) is non-empty) do delete any element in (A) from A end 21 Ex. Poset double shelling antimatroid – Repeating the deletion of a minimal or maximal element. c {a,b,c,d} d {a,b,c,d} a b {a,c,d} {a,c} {a,b,d} {a,b,c} {a,c,d} {b,c,d} c {a} {a,b} {a,c} {b,c} {c,d} d a b {a} {b} {c} A poset double shelling antimatroid 22 Ex. poset convex geometries c {a,b,c,d} {a,b.c,b} d {a,b,d} {b,c,d} {a,c,d} a b {c,d} {a,b} Hasse diagram of a poset P {a} {b} {c} Deleting a minimal antimatroid convex geometry element of P of P antimatroid 23 Def. Monophonically convex sets of a chordal graph G : a graph, V(G): the vertex set of G X V(G) is monophonically convex if it holds that a,b X and c V is on a chordless path between a and b c X Theorem (Edelman and Jamison 1985) The collection of monophonically convex sets is a convex geometry if and only if G is a chordal graph. 24 Ex. Monophonically convex sets of a chordal graph 125 {12345} {2345} {1345} 3 4 {245} {345} {135} A chordal graph G {24}{25} {45} {34} {35} {15} {13} {2} {4} {5} {3} {1} Removable elements =Simplicial vertex A convex geometry of the chordal graph G 25 Antimatroids and rooted cocircuits Def. Let e E . In an antimatroid F , suppose that an element H F contains e and is minimal with respect to this property. Then we say that ( Y , e ) is a rooted cocircuit where Y H e . e is called the root and Y is the costem. [Remark] H is necessarily a join-irreducible element in the lattice F . 26 Ex. Poset double shelling antimatroid – Repeating the deletion of a minimal or maximal element. {a,b,c,d} c {a,b,d} {a,b,c} {a,c,d} {b,c,d} {a,b} {a,c} {b,c} {c,d} d a b {a} {b} {c} antimatroid Element: minimal feasible set Rooted cocircuits a {a} ( , a), b {b} ( , b), c {c} ( , c) d {a, b, d}, {c, d}, ({a, b}, d), ({c}, d) 27 clutter Def. A clutter is a collection of sets in which no member contains another properly. Def. For a clutter L on E, a subset of E is a transversal of L if it intersects every member of L . The collection of the minimal transversals called the blocker of L and denoted by b ( L ). [Note] If a set intersects every member of a clutter L , then it contains a member of the blocker b(L). 28 clutter Ex. clutters and their blockers L {{a, c},{b, c}}, b (L) {{c},{a, b}}, b(b(L)) L L {{a, b},{b, c},{c,a}}, b (L) {{a,b},{b,c},{c,a}}, b(b(L)) L Proposition For any clutter L, b (b (L)) L holds. 29 Rooted circuits: root stems costems ({a,c}, d), ({b,c}, d) a ー b ー Rooted cocircuits: c ー ( , a), ( , b), ( , c) d {a,c}, {b,c} {a,b}, {c} ({a,b}, d), ({c}, d) L={ {a, c}, {b, c} }, b(L)={ {c}, {a, b} } Proposition (Korte, Lovasz and Schrader) For a convex geometry (K,E) , let C be the collection of the rooted circuits, and D be the collection of the rooted cocircuits. Then for each eE, C(e) {X :(X,e)C}and D(e) {Y :(Y ,e)D}are the blocker of each other.
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