TAILIEUCHUNG - Báo cáo toán học: "Equilateral Triangles in Finite Metric Spaces"

Tuyển tập các báo cáo nghiên cứu khoa học trên tạp chí toán học quốc tế đề tài:Equilateral Triangles in Finite Metric Spaces. | Equilateral Triangles in Finite Metric Spaces Vania Mascioni Department of Mathematical Sciences Ball State University vdm@ Submitted Aug 2 2003 Accepted Feb 7 2004 Published Feb 27 2004 MR Subject Classifications 05C55 05C12 Abstract In the context of finite metric spaces with integer distances we investigate the new Ramsey-type question of how many points can a space contain and yet be free of equilateral triangles. In particular for finite metric spaces with distances in the set . n the number Dn is defined as the least number of points the space must contain in order to be sure that there will be an equilateral triangle in it. Several issues related to these numbers are studied mostly focusing on low values of n. Apart from the trivial D1 3 D2 6 we prove that D3 12 D4 33 and 81 D5 95. In classical combinatorial theory the following is a well-known widely open problem determine the minimal order of a complete graph such that when coloring the edges with n colors with n E N fixed we can find at least one monochromatic triangle. Such a smallest integer has been among others proved to exist by Ramsey 10 and is typically denoted by Rn 3 3 . 3J n times since we won t consider any of the many variations of the problem we will be using the simpler notation Rn for short . People not acquainted with the theory are invariably surprised when learning that very little is known about Rn see 9 for Radziszowski s continuously updated survey of results beyond the trivial cases R1 3 R2 6 the only known value is R3 17 while for R4 just the range 51 R4 62 has been established the quality of the latter result should not be underestimated since it took almost fifty years to improve the upper bound from 66 to 62 a computer-free proof due to . Kramer that R4 62 is more than 100 pages long 3 7 . In this paper we will study a very much related but technically different problem to the best of our knowledge this problem appears to be new which explains the lack of direct .

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