Geometry

A conjecture on iterated circumcentres ★★

Author(s): Goddyn

Conjecture   Let $ p_1,p_2,p_3,\ldots $ be a sequence of points in $ {\mathbb R}^d $ with the property that for every $ i \ge d+2 $, the points $ p_{i-1}, p_{i-2}, \ldots p_{i-d-1} $ are distinct, lie on a unique sphere, and further, $ p_i $ is the center of this sphere. If this sequence is periodic, must its period be $ 2d+4 $?

Keywords: periodic; plane geometry; sequence

Posted by mdevos
updated June 8th, 2007
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Graph Theory » Algebraic G.T.

The sum of the two largest eigenvalues ★★

Author(s): Gernert

Problem   Let $ G $ be a graph on $ n $ vertices and let $ \lambda_1 \ge \lambda_2 \ge \ldots \ge \lambda_n $ be the eigenvalues of $ G $. Is $ \lambda_1 + \lambda_2 \le n $?

Keywords: eigenvalues; spectrum

Posted by mdevos
updated January 13th, 2009
4 comments
Combinatorics » Ramsey Theory

Diagonal Ramsey numbers ★★★★

Author(s): Erdos

Let $ R(k,k) $ denote the $ k^{th} $ diagonal Ramsey number.

Conjecture   $ \lim_{k \rightarrow \infty} R(k,k) ^{\frac{1}{k}} $ exists.
Problem   Determine the limit in the above conjecture (assuming it exists).

Keywords: Ramsey number

Posted by mdevos
updated September 13th, 2010
2 comments