12,690 research outputs found

    Diffraction-limited CCD imaging with faint reference stars

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    By selecting short exposure images taken using a CCD with negligible readout noise we obtained essentially diffraction-limited 810 nm images of faint objects using nearby reference stars brighter than I=16 at a 2.56 m telescope. The FWHM of the isoplanatic patch for the technique is found to be 50 arcseconds, providing ~20% sky coverage around suitable reference stars.Comment: 4 page letter accepted for publication in Astronomy and Astrophysic

    A Reciprocity Theorem for Monomer-Dimer Coverings

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    The problem of counting monomer-dimer coverings of a lattice is a longstanding problem in statistical mechanics. It has only been exactly solved for the special case of dimer coverings in two dimensions. In earlier work, Stanley proved a reciprocity principle governing the number N(m,n)N(m,n) of dimer coverings of an mm by nn rectangular grid (also known as perfect matchings), where mm is fixed and nn is allowed to vary. As reinterpreted by Propp, Stanley's result concerns the unique way of extending N(m,n)N(m,n) to n<0n < 0 so that the resulting bi-infinite sequence, N(m,n)N(m,n) for nZn \in {Z}, satisfies a linear recurrence relation with constant coefficients. In particular, Stanley shows that N(m,n)N(m,n) is always an integer satisfying the relation N(m,2n)=ϵm,nN(m,n)N(m,-2-n) = \epsilon_{m,n}N(m,n) where ϵm,n=1\epsilon_{m,n} = 1 unless mm\equiv 2(mod 4) and nn is odd, in which case ϵm,n=1\epsilon_{m,n} = -1. Furthermore, Propp's method is applicable to higher-dimensional cases. This paper discusses similar investigations of the numbers M(m,n)M(m,n), of monomer-dimer coverings, or equivalently (not necessarily perfect) matchings of an mm by nn rectangular grid. We show that for each fixed mm there is a unique way of extending M(m,n)M(m,n) to n<0n < 0 so that the resulting bi-infinite sequence, M(m,n)M(m,n) for nZn \in {Z}, satisfies a linear recurrence relation with constant coefficients. We show that M(m,n)M(m,n), a priori a rational number, is always an integer, using a generalization of the combinatorial model offered by Propp. Lastly, we give a new statement of reciprocity in terms of multivariate generating functions from which Stanley's result follows.Comment: 13 pages, 12 figures, to appear in the proceedings of the Discrete Models for Complex Systems (DMCS) 2003 conference. (v2 - some minor changes

    Presumed Nonhuman: Black Women Intellectuals and the Struggle for Humanity in the Academy

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    In this article I engage with the work of Sylvia Wynter, Christina Sharpe, and Kevin Quashie, weaving in my own personal narrative of being presumed nonhuman to detail the everyday struggles Black women academics face. Herein I also illustrate how these struggles become sites of resistance, building, and hope

    Decay of far flow field in trailing vortices

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    A finite difference machine code is used in the wake vortex problems in the quasi-cylindrical boundary layer approximation. A turbulent energy model containing new features is developed that accounts for the major effects disclosed by more advanced models in which the parameters are not yet established. Several puzzles that arose in previous theoretical investigations of wake vortices are resolved

    The United States Supreme Court: A Creative Check of Institutional Misdirection?

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    In the Comment which follows Professor Baldwin presents a brief for an extremely creative Supreme Court. In contrast to those who suggest limiting the function of the Court, either by subject matter or by judicial restraint, the author would have it protect the compact upon which the community is based, by taking an active role to insure that the compensation implied in the compact flows in fact not only to the community but to the individual
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