5,557 research outputs found
On equiangular lines in 17 dimensions and the characteristic polynomial of a Seidel matrix
For a positive integer, we find restrictions modulo on the
coefficients of the characteristic polynomial of a Seidel matrix
. We show that, for a Seidel matrix of order even (resp. odd), there are
at most (resp. ) possibilities for
the congruence class of modulo . As an application
of these results, we obtain an improvement to the upper bound for the number of
equiangular lines in , that is, we reduce the known upper bound
from to .Comment: 21 pages, fixed typo in Lemma 2.
A new non-arithmetic lattice in PU(3,1)
We study the arithmeticity of the Couwenberg-Heckman-Looijenga lattices in
PU(n,1), and show that they contain a non-arithmetic lattice in PU(3,1) which
is not commensurable to the non-arithmetic Deligne-Mostow lattice in PU(3,1)
Perfect forms over totally real number fields
A rational positive-definite quadratic form is perfect if it can be
reconstructed from the knowledge of its minimal nonzero value m and the finite
set of integral vectors v such that f(v) = m. This concept was introduced by
Voronoi and later generalized by Koecher to arbitrary number fields. One knows
that up to a natural "change of variables'' equivalence, there are only
finitely many perfect forms, and given an initial perfect form one knows how to
explicitly compute all perfect forms up to equivalence. In this paper we
investigate perfect forms over totally real number fields. Our main result
explains how to find an initial perfect form for any such field. We also
compute the inequivalent binary perfect forms over real quadratic fields
Q(\sqrt{d}) with d \leq 66.Comment: 11 pages, 2 figures, 1 tabl
Abelian Spiders
If G is a finite graph, then the largest eigenvalue L of the adjacency matrix
of G is a totally real algebraic integer (L is the Perron-Frobenius eigenvalue
of G). We say that G is abelian if the field generated by L^2 is abelian. Given
a fixed graph G and a fixed set of vertices of G, we define a spider graph to
be a graph obtained by attaching to each of the chosen vertices of G some
2-valent trees of finite length. The main result is that only finitely many of
the corresponding spider graphs are both abelian and not Dynkin diagrams, and
that all such spiders can be effectively enumerated; this generalizes a
previous result of Calegari, Morrison, and Snyder. The main theorem has
applications to the classification of finite index subfactors. We also prove
that the set of Salem numbers of "abelian type" is discrete.Comment: This work represents, in part, the PhD thesis of the second autho
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