4,897 research outputs found
Better Actions
We explain why compact U(1) confines and how to fix it. We show that
plaquettes of negative trace carry most of the confinement signal in compact
SU(2). We show how to perform noncompact gauge-invariant simulations without
auxiliary fields. We suggest a way to simulate fermions without doublers.Comment: Talk presented at LATTICE96(algorithms), 4 pages, Latex, espcrc2,
picte
Connectivity and Irreducibility of Algebraic Varieties of Finite Unit Norm Tight Frames
In this paper, we settle a long-standing problem on the connectivity of
spaces of finite unit norm tight frames (FUNTFs), essentially affirming a
conjecture first appearing in [Dykema and Strawn, 2003]. Our central technique
involves continuous liftings of paths from the polytope of eigensteps to spaces
of FUNTFs. After demonstrating this connectivity result, we refine our analysis
to show that the set of nonsingular points on these spaces is also connected,
and we use this result to show that spaces of FUNTFs are irreducible in the
algebro-geometric sense, and also that generic FUNTFs are full spark.Comment: 33 pages, 4 figure
Saving phase: Injectivity and stability for phase retrieval
Recent advances in convex optimization have led to new strides in the phase
retrieval problem over finite-dimensional vector spaces. However, certain
fundamental questions remain: What sorts of measurement vectors uniquely
determine every signal up to a global phase factor, and how many are needed to
do so? Furthermore, which measurement ensembles lend stability? This paper
presents several results that address each of these questions. We begin by
characterizing injectivity, and we identify that the complement property is
indeed a necessary condition in the complex case. We then pose a conjecture
that 4M-4 generic measurement vectors are both necessary and sufficient for
injectivity in M dimensions, and we prove this conjecture in the special cases
where M=2,3. Next, we shift our attention to stability, both in the worst and
average cases. Here, we characterize worst-case stability in the real case by
introducing a numerical version of the complement property. This new property
bears some resemblance to the restricted isometry property of compressed
sensing and can be used to derive a sharp lower Lipschitz bound on the
intensity measurement mapping. Localized frames are shown to lack this property
(suggesting instability), whereas Gaussian random measurements are shown to
satisfy this property with high probability. We conclude by presenting results
that use a stochastic noise model in both the real and complex cases, and we
leverage Cramer-Rao lower bounds to identify stability with stronger versions
of the injectivity characterizations.Comment: 22 page
- …