209 research outputs found
Neighbors of knots in the Gordian graph
We show that every knot is one crossing change away from a knot of
arbitrarily high bridge number and arbitrarily high bridge distance.Comment: Accepted by American Mathematical Monthly. New version incorporates
referee comment
The relative -invariant of a compact -manifold
In this paper, we introduce the relative -invariant
of a smooth, orientable, compact 4-manifold with
boundary. This invariant is defined by measuring the lengths of certain paths
in the cut complex of a trisection surface for . This is motivated by the
definition of the -invariant for smooth, orientable, closed
4-manifolds by Kirby and Thompson. We show that if is a rational homology
ball, then if and only if .
In order to better understand relative trisections, we also produce an
algorithm to glue two relatively trisected 4-manifold by any Murasugi sum or
plumbing in the boundary, and also prove that any two relative trisections of a
given 4-manifold are related by interior stabilization, relative
stabilization, and the relative double twist, which we introduce in this paper
as a trisection version of one of Piergallini and Zuddas's moves on open book
decompositions. Previously, it was only known (by Gay and Kirby) that relative
trisections inducing equivalent open books on are related by interior
stabilizations.Comment: 39 pages, 14 figures. v2: Significantly improved discussion of
relative double twist (Section 2), changed some statements in Section 4,
restructured Sections 3 and
What Powers the 2006 Outburst of the Symbiotic Star BF Cygni?
BF Cygni is a classical symbiotic binary. Its optical light curve occasionally shows outbursts of the Z And-type, whose nature is not well understood. During the 2006 August, BF Cyg underwent the recent outburst, and continues its active phase to the present. The aim of this contribution is to determine the fundamental parameters of the hot component in the binary during the active phase. For this purpose we used a high- and low-resolution optical spectroscopy and the multicolour UBV RCIC photometry. Our photometric monitoring revealed that a high level of the star’s brightness lasts for unusually long time of > 7 years. A sharp violet-shifted absorption component and broad emission wings in the Hα profile developed during the whole active phase. From 2009, our spectra revealed a bipolar ejection from the white dwarf (WD). Modelling the spectral energy distribution (SED) of the low-resolution spectra showed simultaneous presence of a warm (< 10 000 K) disk-like pseudophotosphere and a strong nebular component of radiation (emission measure of ~1061 cm−3). The luminosity of the hot active object was estimated to > 5−8×103 Lʘ. Such high luminosity, sustained for the time of years, can be understood as a result of an enhanced transient accretion rate throughout a large disk, leading also to formation of collimated ejection from the WD
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