In 1989, Erdős conjectured that for a sufficiently large n it is impossible to place n points in general position in a plane such that for every 1 ≤ i ≤ n − 1 there is a distance that occurs exactly i times. For small n this is possible and in his paper he provided constructions for n ≤ 8. The one for n = 5 was due to Pomerance while Palásti came up with the constructions for n = 7, 8. Constructions for n = 9 and above remain undiscovered, and little headway has been made toward a proof that for sufficiently large n no configuration exists. In this paper we consider a natural generalization to higher dimensions and provide a construction which shows that for any given n there exists a sufficiently large dimension d such that there is a configuration in d-dimensional space meeting Erdős' criteria.
Several recent papers have considered the Ramsey-theoretic problem of how large a subset of integers can be without containing any 3-term geometric progressions. This problem has also recently been generalized to number fields and F q [x]. We study the analogous problem in two noncommutative settings, quaternions and free groups, to see how lack of commutivity affected the problem. In the quaternion case, we show bounds for the supremum of upper densities of 3-term geometric progression avoiding sets. In the free groups case, we calculate the decay rate for the greedy set in x, y : x 2 = y 2 = 1 avoiding 3-term geometric progressions.
Let 1 ≤ k ≤ d and consider a subset E ⊂ R d . In this paper, we study the problem of how large the Hausdorff dimension of E must be in order for the set of distinct noncongruent k-simplices in E (that is, noncongruent point configurations of k + 1 points from E) to have positive Lebesgue measure. This generalizes the k = 1 case, the wellknown Falconer distance problem and a major open problem in geometric measure theory. Many results on Falconer type theorems have been established through incidence theorems, which generally establish sufficient but not necessary conditions for the point configuration theorems. We establish a dimensional lower threshold of d+1 2 on incidence theorems for k-simplices where k ≤ d ≤ 2k + 1 by generalizing an example of Mattila. We also prove a dimensional lower threshold of d+1 2 on incidence theorems for triangles in a convex setting in every dimension greater than 3. This last result generalizes work by Iosevich and Senger on distances that was built on a construction by Valtr. The final result utilizes number-theoretic machinery to estimate the number of solutions to a Diophantine equation.
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