Comprehensive Study on Discrete Geometry

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1.On the rigidity of frameworks : We presented a rigid unit-bar-framework in the 3-dimensional space that has no triangle, and a minimum rigid framework in the plane that cannot be constructed from the data of edge-lengths and graph structure. We proved that if a complete bipartite framework K (m, n) (m >= 3, n >= 5) in the plane admits a continuous deformation, then one of the partite-sets lies on a line L and the other partite-set lies on the line perpendicular to L.2.On embeddings of structures : We proved that for any planar graph G = (V,E), there is an emebedding f : V * R^2 such that x, y * V are adijacent if and only if the distance between f (x) and f (y) is an integer. We also proved that every n dimensional inner product space over the rational field can be isometrically embedded into 2n + 1 dimensional Euclidean space.3.On srrangements of spheres : A graph G is said to be representable by balls on the table, if we can place solid balls on a table, one ball for each vertex, so that two balls are tangent only when the corresponding vertices are adjacent. We proved that the family F of graphs representable by balls on a table is different from the family of planar graphs, and that F does not contain any member of the so-called Petersen family, and that the maximum value of the chromatic number of a graph in F is either 5 or 6.4.On random graphs, probability : We determined the probability distribution of the order of the maximum regular tournament in a dominance relation generated by a randam n points on a circle. We extended the classical ruin problem to 3 persons' game, and calculated the probability that a fixed gambler A ruined first, and the probability that A is the sole survivor.