Collinearity

Summary

In geometry, collinearity of a set of points is the property of their lying on a single line. A set of points with this property is said to be collinear (sometimes spelled as colinear). In greater generality, the term has been used for aligned objects, that is, things being "in a line" or "in a row". Points on a line In any geometry, the set of points on a line are said to be collinear. In Euclidean geometry this relation is intuitively visualized by points lying in a row on a "straight line". However, in most geometries (including Euclidean) a line is typically a primitive (undefined) object type, so such visualizations will not necessarily be appropriate. A model for the geometry offers an interpretation of how the points, lines and other object types relate to one another and a notion such as collinearity must be interpreted within the context of that model. For instance, in spherical geometry, where lines are represented in the standard model by great circles of a sphe

Official source

https://en.wikipedia.org/wiki/Collinearity

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A note on coloring line arrangements

János Pach, Rom Pinchasi

We show that the lines of every arrangement of n lines in the plane can be colored with O(root n/log n) colors such that no face of the arrangement is monochromatic. This improves a bound of Bose et al. by a circle minus(root/log n) factor. Any further improvement on this bound would also improve the best known lower bound on the following problem of Erdos: estimate the maximum number of points in general position within a set of n points containing no four collinear points.

Electronic Journal Of Combinatorics2014

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Motivated by the numerous examples of 1/3 magnetization plateaux in the triangular-lattice Heisenberg antiferromagnet with spins ranging from 1/2 to 5/2, we revisit the semiclassical calculation of the magnetization curve of that model, with the aim of coming up with a simple method that allows one to calculate the full magnetization curve and not just the critical fields of the 1/3 plateau. We show that it is actually possible to calculate the magnetization curve including the first quantum corrections and the appearance of the 1/3 plateau entirely within linear spin-wave theory, with predictions for the critical fields that agree to order 1/S with those derived a long time ago on the basis of arguments that required going beyond linear spin-wave theory. This calculation relies on the central observation that there is a kink in the semiclassical energy at the field where the classical ground state is the collinear up-up-down structure and that this kink gives rise to a locally linear behavior of the energy with the field when all semiclassical ground states are compared to each other for all fields. The magnetization curves calculated in this way for spin 1/2, 1, and 5/2 are shown to be in good agreement with available experimental data.

2016

The effect of “objecthood” in the Ebbinghaus illusion

Aline Françoise Cretenoud, Michael Herzog, Einat Rashal

In the Ebbinghaus illusion, observers overestimate the size of a target when it is surrounded by large flakers and compared with a control target surrounded by small flankers. We examined whether “objecthood” – the degree to which an object is a cohesive entity – plays a role in the illusion. We predicted that the magnitude of the illusion will decrease with decreased objecthood. To test this hypothesis, we presented observers with squares as target and flankers, and manipulated the degree of their objecthood in two ways; a) the squares were composed of different parts (i.e., corners or edges) in different trials. Corners usually produce more cohesive squares than edges because the former combine closure, symmetry and collinearity, whereas the latter lacks collinearity; b) the gap size between the object parts was varied, so that larger gaps produced less cohesive objects than smaller gaps. The participants adjusted the test target to match a control target in size. Our results show that increasing the gap size between object parts reduced the magnitude of the illusion. In addition, the magnitude of the illusion was greater when the target was composed of its edges compared with when it was composed of its corners. These results suggest that objecthood plays a role in the Ebbinghaus illusion.

2017