منابع مشابه
Adjacency, Inseparability, and Base Orderability in Matroids
Two elements in an oriented matroid are inseparable if they have either the same sign in every signed circuit containing them both or opposite signs in every signed circuit containing them both. Two elements of a matroid are adjacent if there is no M(K4)-minor using them both, and in which they correspond to a matching of K4. We prove that two elements e, { of an oriented matroid are inseparabl...
متن کاملSelections
It was not until the sixteenth century that dead bodies were submitted to medico-legal investigation. The horror which human dissections inspired in the ancients, and the religious awe with which they beheld the mortal wreck of their fellows, not only retarded the progress of anatomy and physiology, but excluded the light of medicine from the researches of the forum. The Jewish law, indeed, con...
متن کاملDiagram Groups, Braid Groups, and Orderability
We prove that all diagram groups (in the sense of Guba and Sapir) are left-orderable. The proof is in two steps: firstly, it is proved that all diagram groups inject in a certain braid group on infinitely many strings, and secondly, this group is then shown to be left-orderable.
متن کاملSelections Reprinted
MR1631259 (2000d:11019) 11B25; 11N13 Gowers, W. T. A new proof of Szemerédi’s theorem for arithmetic progressions of length four. Geometric and Functional Analysis 8 (1998), no. 3, 529–551. This remarkable paper gives a new proof that every subset of the integers with positive density must contain arithmetic progressions of length four. This was conjectured by P. Erdős and P. Turán [J. London M...
متن کاملBichromatic P4-composition schemes for perfect orderability
A P4 is an induced path with four vertices. A bichromatic P4 composition scheme is as follows: (1) start with two graphs with vertex sets of di1erent colour, say black • and white ◦, (2) select a set of allowable four-vertex bichromatic sequences, for example {• • ••; ◦ ◦ ◦◦; • ◦ ◦•; ◦ • •◦}, (3) add edges between the graphs so that in the composed graph each P4 is coloured with an allowable se...
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ژورنال
عنوان ژورنال: Proceedings of the American Mathematical Society
سال: 1981
ISSN: 0002-9939
DOI: 10.2307/2044129