31 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.
Another construction of edge-regular graphs with regular cliques
We exhibit a new construction of edge-regular graphs with regular cliques
that are not strongly regular. The infinite family of graphs resulting from
this construction includes an edge-regular graph with parameters . We
also show that edge-regular graphs with -regular cliques that are not
strongly regular must have at least vertices.Comment: 7 page
Real equiangular lines in dimension 18 and the Jacobi identity for complementary subgraphs
We show that the maximum cardinality of an equiangular line system in
is at most . Our proof includes a novel application of the
Jacobi identity for complementary subgraphs. In particular, we show that there
does not exist a graph whose adjacency matrix has characteristic polynomial
.Comment: 26 pages. Updated to match the published, journal versio
Equiangular lines in low dimensional Euclidean spaces
We show that the maximum cardinality of an equiangular line system in 14 and
16 dimensions is 28 and 40, respectively, thereby solving a longstanding open
problem. We also improve the upper bounds on the cardinality of equiangular
line systems in 19 and 20 dimensions to 74 and 94, respectively.Comment: 23 page
Frames over finite fields: Equiangular lines in orthogonal geometry
We investigate equiangular lines in finite orthogonal geometries, focusing
specifically on equiangular tight frames (ETFs). In parallel with the known
correspondence between real ETFs and strongly regular graphs (SRGs) that
satisfy certain parameter constraints, we prove that ETFs in finite orthogonal
geometries are closely aligned with a modular generalization of SRGs. The
constraints in our finite field setting are weaker, and all but~18 known SRG
parameters on vertices satisfy at least one of them. Applying our
results to triangular graphs, we deduce that Gerzon's bound is attained in
finite orthogonal geometries of infinitely many dimensions. We also demonstrate
connections with real ETFs, and derive necessary conditions for ETFs in finite
orthogonal geometries. As an application, we show that Gerzon's bound cannot be
attained in a finite orthogonal geometry of dimension~5
Frames over finite fields: Equiangular lines in orthogonal geometry
We investigate equiangular lines in finite orthogonal geometries, focusing
specifically on equiangular tight frames (ETFs). In parallel with the known
correspondence between real ETFs and strongly regular graphs (SRGs) that
satisfy certain parameter constraints, we prove that ETFs in finite orthogonal
geometries are closely aligned with a modular generalization of SRGs. The
constraints in our finite field setting are weaker, and all but~18 known SRG
parameters on vertices satisfy at least one of them. Applying our
results to triangular graphs, we deduce that Gerzon's bound is attained in
finite orthogonal geometries of infinitely many dimensions. We also demonstrate
connections with real ETFs, and derive necessary conditions for ETFs in finite
orthogonal geometries. As an application, we show that Gerzon's bound cannot be
attained in a finite orthogonal geometry of dimension~5
The JCMT Gould Belt Survey: Evidence for radiative heating in Serpens MWC 297 and its influence on local star formation
We present SCUBA-2 450micron and 850micron observations of the Serpens MWC
297 region, part of the JCMT Gould Belt Survey of nearby star-forming regions.
Simulations suggest that radiative feedback influences the star-formation
process and we investigate observational evidence for this by constructing
temperature maps. Maps are derived from the ratio of SCUBA-2 fluxes and a two
component model of the JCMT beam for a fixed dust opacity spectral index of
beta = 1.8. Within 40 of the B1.5Ve Herbig star MWC 297, the submillimetre
fluxes are contaminated by free-free emission with a spectral index of
1.03+-0.02, consistent with an ultra-compact HII region and polar winds/jets.
Contamination accounts for 73+-5 per cent and 82+-4 per cent of peak flux at
450micron and 850micron respectively. The residual thermal disk of the star is
almost undetectable at these wavelengths. Young Stellar Objects are confirmed
where SCUBA-2 850micron clumps identified by the fellwalker algorithm coincide
with Spitzer Gould Belt Survey detections. We identify 23 objects and use Tbol
to classify nine YSOs with masses 0.09 to 5.1 Msun. We find two Class 0, one
Class 0/I, three Class I and three Class II sources. The mean temperature is
15+-2K for the nine YSOs and 32+-4K for the 14 starless clumps. We observe a
starless clump with an abnormally high mean temperature of 46+-2K and conclude
that it is radiatively heated by the star MWC 297. Jeans stability provides
evidence that radiative heating by the star MWC 297 may be suppressing clump
collapse.Comment: 24 pages, 13 figures, 7 table