26 research outputs found
Dynamic Limits on Planar Libration-Orbit Coupling Around an Oblate Primary
This paper explores the dynamic properties of the planar system of an
ellipsoidal satellite in an equatorial orbit about an oblate primary. In
particular, we investigate the conditions for which the satellite is bound in
librational motion or when the satellite will circulate with respect to the
primary. We find the existence of stable equilibrium points about which the
satellite can librate, and explore both the linearized and non-linear dynamics
around these points. Absolute bounds are placed on the phase space of the
libration-orbit coupling through the use of zero-velocity curves that exist in
the system. These zero-velocity curves are used to derive a sufficient
condition for when the satellite's libration is bound to less than 90 degrees.
When this condition is not satisfied so that circulation of the satellite is
possible, the initial conditions at zero libration angle are determined which
lead to circulation of the satellite. Exact analytical conditions for
circulation and the maximum libration angle are derived for the case of a small
satellite in orbits of any eccentricity.Comment: Submitted to Celestial Mechanics and Dynamical Astronom
Demographic, clinical and antibody characteristics of patients with digital ulcers in systemic sclerosis: data from the DUO Registry
OBJECTIVES: The Digital Ulcers Outcome (DUO) Registry was designed to describe the clinical and antibody characteristics, disease course and outcomes of patients with digital ulcers associated with systemic sclerosis (SSc).
METHODS: The DUO Registry is a European, prospective, multicentre, observational, registry of SSc patients with ongoing digital ulcer disease, irrespective of treatment regimen. Data collected included demographics, SSc duration, SSc subset, internal organ manifestations, autoantibodies, previous and ongoing interventions and complications related to digital ulcers.
RESULTS: Up to 19 November 2010 a total of 2439 patients had enrolled into the registry. Most were classified as either limited cutaneous SSc (lcSSc; 52.2%) or diffuse cutaneous SSc (dcSSc; 36.9%). Digital ulcers developed earlier in patients with dcSSc compared with lcSSc. Almost all patients (95.7%) tested positive for antinuclear antibodies, 45.2% for anti-scleroderma-70 and 43.6% for anticentromere antibodies (ACA). The first digital ulcer in the anti-scleroderma-70-positive patient cohort occurred approximately 5 years earlier than the ACA-positive patient group.
CONCLUSIONS: This study provides data from a large cohort of SSc patients with a history of digital ulcers. The early occurrence and high frequency of digital ulcer complications are especially seen in patients with dcSSc and/or anti-scleroderma-70 antibodies
Approximating the th root by composite rational functions
A landmark result from rational approximation theory states that x
1/p on [0, 1] can be approximated
by a type-(n, n) rational function with root-exponential accuracy. Motivated by the recursive optimality
property of Zolotarev functions (for the square root and sign functions), we investigate approximating
x
1/p by composite rational functions of the form rk
(x,rk−1(x,rk−2(· · · (x,r1(x, 1))))). While this class
of rational functions ceases to contain the minimax (best) approximant for p ≥ 3, we show that it
achieves approximately pth-root exponential convergence with respect to the degree. Moreover, crucially,
the convergence is doubly exponential with respect to the number of degrees of freedom, suggesting that
composite rational functions are able to approximate x
1/p
and related functions (such as |x| and the
sector function) with exceptional efficienc
A backward stable algorithm for computing the CS decomposition via the Polar decomposition
We introduce a backward stable algorithm for computing the CS decomposition of a partitioned matrix with orthonormal columns, or a rank-deficient partial isometry. The algorithm computes two polar decompositions (which can be carried out in parallel) followed by an eigendecomposition of a judiciously crafted Hermitian matrix. We prove that the algorithm is backward stable whenever the aforementioned decompositions are computed in a backward stable way. Our algorithm can also be adapted to compute the complete CS decomposition of a square orthogonal or unitary matrix. Since the polar decomposition and the symmetric eigendecomposition are highly amenable to parallelization, the algorithm inherits this feature. We illustrate this fact by invoking recently developed algorithms for the polar decomposition and symmetric eigendecomposition that leverage Zolotarev's best rational approximations of the sign function. Numerical examples demonstrate that the resulting algorithm for computing the CS decomposition enjoys excellent numerical stability
A backward stable algorithm for computing the CS decomposition via the Polar decomposition
We introduce a backward stable algorithm for computing the CS decomposition of a partitioned matrix with orthonormal columns, or a rank-deficient partial isometry. The algorithm computes two polar decompositions (which can be carried out in parallel) followed by an eigendecomposition of a judiciously crafted Hermitian matrix. We prove that the algorithm is backward stable whenever the aforementioned decompositions are computed in a backward stable way. Our algorithm can also be adapted to compute the complete CS decomposition of a square orthogonal or unitary matrix. Since the polar decomposition and the symmetric eigendecomposition are highly amenable to parallelization, the algorithm inherits this feature. We illustrate this fact by invoking recently developed algorithms for the polar decomposition and symmetric eigendecomposition that leverage Zolotarev's best rational approximations of the sign function. Numerical examples demonstrate that the resulting algorithm for computing the CS decomposition enjoys excellent numerical stability