7,501 research outputs found
Congruences and Canonical Forms for a Positive Matrix: Application to the Schweinler-Wigner Extremum Principle
It is shown that a real symmetric [complex hermitian] positive
definite matrix is congruent to a diagonal matrix modulo a
pseudo-orthogonal [pseudo-unitary] matrix in [ ], for any
choice of partition . It is further shown that the method of proof in
this context can easily be adapted to obtain a rather simple proof of
Williamson's theorem which states that if is even then is congruent
also to a diagonal matrix modulo a symplectic matrix in
[]. Applications of these results considered include a
generalization of the Schweinler-Wigner method of `orthogonalization based on
an extremum principle' to construct pseudo-orthogonal and symplectic bases from
a given set of linearly independent vectors.Comment: 7 pages, latex, no figure
Arithmetic Circuits and the Hadamard Product of Polynomials
Motivated by the Hadamard product of matrices we define the Hadamard product
of multivariate polynomials and study its arithmetic circuit and branching
program complexity. We also give applications and connections to polynomial
identity testing. Our main results are the following. 1. We show that
noncommutative polynomial identity testing for algebraic branching programs
over rationals is complete for the logspace counting class \ceql, and over
fields of characteristic the problem is in \ModpL/\Poly. 2.We show an
exponential lower bound for expressing the Raz-Yehudayoff polynomial as the
Hadamard product of two monotone multilinear polynomials. In contrast the
Permanent can be expressed as the Hadamard product of two monotone multilinear
formulas of quadratic size.Comment: 20 page
Hilbert von Neumann modules
We introduce a way of regarding Hilbert von Neumann modules as spaces of
operators between Hilbert space, not unlike [Skei], but in an apparently much
simpler manner and involving far less machinery. We verify that our definition
is equivalent to that of [Skei], by verifying the `Riesz lemma' or what is
called `self-duality' in [Skei]. An advantage with our approach is that we can
totally side-step the need to go through -modules and avoid the two stages
of completion - first in norm, then in the strong operator topology - involved
in the former approach.
We establish the analogue of the Stinespring dilation theorem for Hilbert von
Neumann bimodules, and we develop our version of `internal tensor products'
which we refer to as Connes fusion for obvious reasons.
In our discussion of examples, we examine the bimodules arising from
automorphisms of von Neumann algebras, verify that fusion of bimodules
corresponds to composition of automorphisms in this case, and that the
isomorphism class of such a bimodule depends only on the inner conjugacy class
of the automorphism. We also relate Jones' basic construction to the
Stinespring dilation associated to the conditional expectation onto a
finite-index inclusion (by invoking the uniqueness assertion regarding the
latter).Comment: 20 page
Turbojet engine blade damping
The potentials of various sources of nonaerodynamic damping in engine blading are evaluated through a combination of advanced analysis and testing. The sources studied include material hysteresis, dry friction at shroud and root disk interfaces as well as at platform type external dampers. A limited seris of tests was conducted to evaluate damping capacities of composite materials (B/AL, B/AL/Ti) and thermal barrier coatings. Further, basic experiments were performed on titanium specimens to establish the characteristics of sliding friction and to determine material damping constants J and n. All the tests were conducted on single blades. Mathematical models were develthe several mechanisms of damping. Procedures to apply this data to predict damping levels in an assembly of blades are developed and discussed
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