47,375 research outputs found
Statistics of weighted Poisson events and its applications
The statistics of the sum of random weights where the number of weights is
Poisson distributed has important applications in nuclear physics, particle
physics and astrophysics. Events are frequently weighted according to their
acceptance or relevance to a certain type of reaction. The sum is described by
the compound Poisson distribution (CPD) which is shortly reviewed. It is shown
that the CPD can be approximated by a scaled Poisson distribution (SPD). The
SPD is applied to parameter estimation in situations where the data are
distorted by resolution effects. It performs considerably better than the
normal approximation that is usually used. A special Poisson bootstrap
technique is presented which permits to derive confidence limits for
observations following the CPD.Comment: 14 pages, 2 figure
Identifiability and Unmixing of Latent Parse Trees
This paper explores unsupervised learning of parsing models along two
directions. First, which models are identifiable from infinite data? We use a
general technique for numerically checking identifiability based on the rank of
a Jacobian matrix, and apply it to several standard constituency and dependency
parsing models. Second, for identifiable models, how do we estimate the
parameters efficiently? EM suffers from local optima, while recent work using
spectral methods cannot be directly applied since the topology of the parse
tree varies across sentences. We develop a strategy, unmixing, which deals with
this additional complexity for restricted classes of parsing models
Relation between the weak itinerant magnetism in Ni compounds ( = Y, La) and their stacked crystal structures
The weak itinerant magnetic properties of Ni compounds with =
{Y, La} have been investigated using electronic band structure calculations in
the relation with their polymorphic crystal structures. These compounds
crystallizes in two structures resulting from the stacking of two and three
blocks of [Ni + 2 Ni] units for hexagonal -LaNi
(CeNi type) and rhombohedral -YNi (GdCo type)
respectively. Experimentally, -LaNi is a weak itinerant
antiferromagnet whereas -YNi is a weak itinerant ferromagnet. From
the present first principles calculation within non-spin polarized state, both
compounds present an electronic density of state with a sharp and narrow peak
centered at the Fermi level corresponding to flat bands from -Ni. This
induces a magnetic instability and both compounds are more stable in a
ferromagnetic (FM) order compared to a paramagnetic state (
-35 meV/f.u.). The magnetic moment of each of the five Ni sites varies with
their positions relative to the [Ni] and [Ni] units: they are
minimum in the [Ni] unit and maximum at the interface between two
[Ni] units. For -LaNi, an antiferromagnetic (AFM) structure
has been proposed and found with an energy comparable to that of the FM state.
This AFM structure is described by two FM unit blocks of opposite Ni spin sign
separated by a non-magnetic layer at z = 0 and . The Ni () atoms
belonging to this intermediate layer are located in the [LaNi] unit and
are at a center of symmetry of the hexagonal cell () where the
resultant molecular field is cancelled. Further non-collinear spin calculations
have been performed to determine the Ni moment orientations which are found
preferentially parallel to the axis for both FM and AFM structures.Comment: 19 pages, 7 figures, 2 table
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