275 research outputs found
Thom series of contact singularities
Thom polynomials measure how global topology forces singularities. The power
of Thom polynomials predestine them to be a useful tool not only in
differential topology, but also in algebraic geometry (enumerative geometry,
moduli spaces) and algebraic combinatorics. The main obstacle of their
widespread application is that only a few, sporadic Thom polynomials have been
known explicitly. In this paper we develop a general method for calculating
Thom polynomials of contact singularities. Along the way, relations with the
equivariant geometry of (punctual, local) Hilbert schemes, and with iterated
residue identities are revealed
Indeterminancy and Sunspots with Constant Returns
We show that indeterminacy can easily arise in multi-sector models that have constant variable returns to scale and very small market imperfections. This is in sharp contrast to models that require increasing returns to generate indeterminacy, and which have been criticized on the basis of recent empirical estimates indicating that returns to scale are roughly constant, and that market imperfections are small. We also show that we can calibrate our constant returns model with sunspots, using standard parametrizations to produce a close match to the moments of aggregate consumption, investment, output and employment in U.S. data.Indeterminacy, multiple equilibria, sunspots
Riemannian Optimization via Frank-Wolfe Methods
We study projection-free methods for constrained Riemannian optimization. In
particular, we propose the Riemannian Frank-Wolfe (RFW) method. We analyze
non-asymptotic convergence rates of RFW to an optimum for (geodesically) convex
problems, and to a critical point for nonconvex objectives. We also present a
practical setting under which RFW can attain a linear convergence rate. As a
concrete example, we specialize Rfw to the manifold of positive definite
matrices and apply it to two tasks: (i) computing the matrix geometric mean
(Riemannian centroid); and (ii) computing the Bures-Wasserstein barycenter.
Both tasks involve geodesically convex interval constraints, for which we show
that the Riemannian "linear oracle" required by RFW admits a closed-form
solution; this result may be of independent interest. We further specialize RFW
to the special orthogonal group and show that here too, the Riemannian "linear
oracle" can be solved in closed form. Here, we describe an application to the
synchronization of data matrices (Procrustes problem). We complement our
theoretical results with an empirical comparison of Rfw against
state-of-the-art Riemannian optimization methods and observe that RFW performs
competitively on the task of computing Riemannian centroids.Comment: Under Review. Largely revised version, including an extended
experimental section and an application to the special orthogonal group and
the Procrustes proble
Clustering by compression
We present a new method for clustering based on compression. The method
doesn't use subject-specific features or background knowledge, and works as
follows: First, we determine a universal similarity distance, the normalized
compression distance or NCD, computed from the lengths of compressed data files
(singly and in pairwise concatenation). Second, we apply a hierarchical
clustering method. The NCD is universal in that it is not restricted to a
specific application area, and works across application area boundaries. A
theoretical precursor, the normalized information distance, co-developed by one
of the authors, is provably optimal but uses the non-computable notion of
Kolmogorov complexity. We propose precise notions of similarity metric, normal
compressor, and show that the NCD based on a normal compressor is a similarity
metric that approximates universality. To extract a hierarchy of clusters from
the distance matrix, we determine a dendrogram (binary tree) by a new quartet
method and a fast heuristic to implement it. The method is implemented and
available as public software, and is robust under choice of different
compressors. To substantiate our claims of universality and robustness, we
report evidence of successful application in areas as diverse as genomics,
virology, languages, literature, music, handwritten digits, astronomy, and
combinations of objects from completely different domains, using statistical,
dictionary, and block sorting compressors. In genomics we presented new
evidence for major questions in Mammalian evolution, based on
whole-mitochondrial genomic analysis: the Eutherian orders and the Marsupionta
hypothesis against the Theria hypothesis.Comment: LaTeX, 27 pages, 20 figure
SMARANDACHE NON-ASSOCIATIVE RINGS
An associative ring is just realized or built using reals or complex; finite or infinite
by defining two binary operations on it. But on the contrary when we want to define or study or even introduce a non-associative ring we need two separate algebraic structures say a commutative ring with 1 (or a field) together with a loop or a groupoid or a vector space or a linear algebra. The two non-associative well-known algebras viz. Lie algebras and Jordan algebras are mainly built using a vector space over a field satisfying special identities called the Jacobi identity and Jordan identity respectively. Study of these algebras started as early as 1940s. Hence the study of non-associative algebras or even non-associative rings boils down to the study of properties of vector spaces or linear algebras over fields
Smarandache Non-Associative Rings
Generally, in any human field, a Smarandache Structure on a set A means a
weak structure W on A such that there exists a proper subset B contained in A
which is embedded with a stronger structure S. These types of structures occur
in our everyday's life, that's why we study them in this book.
Thus, as a particular case:
A non-associative ring is a non-empty set R together with two binary
operations '+' and '.' such that (R, +) is an additive abelian group and (R, .)
is a groupoid. For all a, b, c belonging to R we have (a + b) . c = a . c + b .
c and c . (a + b) = c . a + c . b. A Smarandache non-associative ring is a
non-associative ring (R, +, .) which has a proper subset P contained in R, that
is an associative ring (with respect to the same binary operations on R).Comment: 150 pages, several new definitions, 44 tables and 150 problem
A parameter estimation subroutine package
Linear least squares estimation and regression analyses continue to play a major role in orbit determination and related areas. FORTRAN subroutines have been developed to facilitate analyses of a variety of parameter estimation problems. Easy to use multipurpose sets of algorithms are reported that are reasonably efficient and which use a minimal amount of computer storage. Subroutine inputs, outputs, usage and listings are given, along with examples of how these routines can be used
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