1,082 research outputs found
The VC-Dimension of Limited Visibility Terrains
No full textVisibility problems are fundamental to computational geometry, and many versions of geometric set cover where coverage is based on visibility have been considered. In most settings, points can see "infinitely far" so long as visibility is not "blocked" by some obstacle. In many applications, this may be an unreasonable assumption. In this paper, we consider a new model of visibility where no point can see any other point beyond a sight radius ρ. In particular, we consider this visibility model in the context of terrains. We show that the VC-dimension of limited visibility terrains is exactly 7. We give lower bound construction that shatters a set of 7 points, and we prove that shattering 8 points is not possible
Benefits of the chelate effect: Preparation of an unsymmetrical ansa-bis(imido)molybdenum complex containing a seven-membered chelate ring
No full textSiemeling U, Turk T, Schoeller W, Redshaw C, Gibson VC. Benefits of the chelate effect: Preparation of an unsymmetrical ansa-bis(imido)molybdenum complex containing a seven-membered chelate ring. INORGANIC CHEMISTRY. 1998;37(18):4738-4739
Chromium(III) and chromium(IV) bis(trimethylsilyl) amido complexes as ethylene polymerisation catalysts
No full textOxidation of Cr[N(SiMe3)(2)](2)(THF)(2) with iodine and dicumyl peroxide results in tetrahedral Cr(IV)Cr[N(SiMe3)(2)](2)I-2 and trigonal planar Cr(III) Cr[N(SiMe3)(2)](OCMe2Ph)(2), respectively; both complexes have been characterised by single-crystal X-ray diffraction, and both are active for ethylene polymerisation with alkylaluminium co-catalysts.PT: J; CR: AJJOU JAN, 1997, ORGANOMETALLICS, V16, P86 AJJOU JAN, 2000, J AM CHEM SOC, V122, P8968 ALONSO PJ, 2002, CHEM-EUR J, V8, P4056 BASI JS, 1971, J CHEM SOC A, P1433 BEAUDOIN MC, 2002, J MOL CATAL A-CHEM, V190, P159 BERNO P, 1994, ORGANOMETALLICS, V13, P1052 BLANCHARD H, 1988, J ORGANOMET CHEM, V341, P367 BOCHMANN M, 1980, J CHEM SOC DA, P1863 BRADLEY DC, 1972, J CHEM SOC CHEM COMM, P567 BRADLEY DC, 1976, ACCOUNTS CHEM RES, V9, P273 BRADLEY DC, 1978, INORG SYNTH, V18, P112 BRITOVSEK GJP, 1999, ANGEW CHEM INT EDIT, V38, P428 BURGER H, 1964, MONATSH, V95, P1099 CUMMINS CC, 1992, ANGEW CHEM INT EDIT, V31, P1501 CUMMINS CC, 1998, PROG INORG CHEM, V47, P685 DRYKACZ G, 1973, J AM CHEM SOC, V95, P4756 ELLER PG, 1977, COORDIN CHEM REV, V24, P1 FILIPPOU AC, 2002, EUR J INORG CHEM NOV, P2928 FILIPPOU AC, 2003, ANGEW CHEM INT EDIT, V42, P4486 FILIPPOU AC, 2003, ORGANOMETALLICS, V22, P3010 FIRMAN TK, 2001, J ORGANOMET CHEM, V635, P153 FRYZUK MD, 1995, ORGANOMETALLICS, V14, P5193 GIBSON VC, 1998, CHEM COMMUN 0821, P1651 GIBSON VC, 1999, J CHEM SOC DALT 0321, P827 GIBSON VC, 2000, J CHEM SOC DALTON, P1969 GIBSON VC, 2001, EUR J INORG CHEM JUL, P1895 GIBSON VC, 2002, J CHEM SOC DALTON, P4017 GIBSON VC, 2003, CHEM REV, V103, P283 HAGIHARA N, 1959, J AM CHEM SOC, V81, P3160 IKEDA H, 2001, MACROMOL CHEM PHYSIC, V202, P1806 IKEDA H, 2002, J ORGANOMET CHEM, V642, P156 KAYAL A, 2002, INORG CHEM, V41, P321 LAPLAZA CE, 1996, J AM CHEM SOC, V118, P8623 MESSERE R, 2000, EUR J INORG CHEM JUN, P1151 MINDIOLA DJ, 1998, ANGEW CHEM INT EDIT, V37, P945 MOWAT W, 1972, J CHEM SOC DA, P533 MOWAT W, 1973, J CHEM SOC DA, P770 ROBERTSON NJ, 2003, INORG CHEM, V42, P6876 SCHMID R, 2000, CAN J CHEM, V78, P265 SCHMID R, 2000, ORGANOMETALLICS, V19, P2756 SCHNEIDER S, 2001, INORG CHEM, V40, P4674 SCHULZKE C, 2002, ORGANOMETALLICS, V21, P3810 SCOTT SL, 2001, CHEM ENG SCI, V56, P4155 SEIDEL W, 1976, Z ANORG ALLG CHEM, V426, P150 SEIDEL W, 1976, Z ANORG ALLG CHEM, V426, P155 SMALL BL, 2004, MACROMOLECULES, V37, P4375 THEOPOLD KH, 1998, EUR J INORG CHEM JAN, P15 VANRENSBURG WJ, 2004, ORGANOMETALLICS, V23, P1207; NR: 48; TC: 3; J9: DALTON TRANS; PG: 3; GA: 865WTSource type: Electronic(1
Shear transfer mechanism in reinforced engineered cementitious composite (ECC) beams: Quantification of Vs and Vc
No full textTo enhance the structural and seismic resistance, as well as durability of concrete structures, an ultra ductile fiber reinforced cementitious composites called Engineered Cementitious Composite (ECC), also known as Strain Hardening Cementitious Composite (SHCC), was developed. ECC has a similar compressive and tensile strength to conventional concrete, but it exhibits a pseudo-strain-hardening behaviour under uniaxial tension with excellent crack control ability. The ultimate tensile strain of ECC can reach 3–12%, which is 300–1200 times higher than that of concrete. It is reported that ECC can also exhibit at least twice as high shear carrying capacity compared to traditional concrete, signifying a potential to use ECC material in shear-resistance elements. However, the shear resisting mechanism of reinforced ECC (R/ECC) members is still not clear. In most existing codes and models, the shear strength of reinforced structural members (Vu) is divided into two parts, i.e., shear resistance coming from the matrix (Vc) and from the transverse reinforcement (Vs). To quantify accurately Vc and Vs and also their development throughout the loading, a well-designed testing method consisting of continuous strain quantification along the stirrups, was used in this research. Six steel reinforced beams incorporating different matrix (ECC, concrete and mortar) were tested under four-point bending. The test results indicated that Vc changed continuously with the propagation of shear crack, whereas the stirrups that crossed the critical shear crack, did not always yield at the ultimate shear resistance.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Concrete Structure
Market Potential and Operational Scenarios for Virtual Coupling
No full textThis document evaluates the attractiveness of Virtual Coupling (VC) for different market segments (high-speed, main line, regional, urban/suburban, freight) and defines operational scenarios for each of them. A SWOT analysis identifies main strengths and weaknesses of the Virtual Coupling concept and corresponding opportunities and threats to each specific railway market segment. The research relies on a Delphi method with an extensive survey of expert opinions and stated travel preferences assuming VC has been implemented. The survey involved subject matter experts of the wide European railway industry including infrastructure managers, railway undertakings, system suppliers, transport authorities, railway institutions, private consultants and academics. In addition, travel preferences have been collected by interviewing European representatives belonging to other socio professional categories. Results show that the implementation of Virtual Coupling can be attractive to customers of high-speed, main line, regional and especially freight segments. Virtual Coupling has the potential of completely changing the way in which such segments operate and attract a modal shift from other transport modes to railways. Customers are even willing to pay higher fares for more frequent and flexible train services, especially on the regional and freight segments which are currently perceived as not satisfactory. Several operational scenarios have been defined based on the outcomes of the survey, setting market-attractive VC service headways for each market segment as well as specifying characteristics of rolling stock, power supply, traffic, and platform crowd management. Principles to couple/decouple convoys of virtually coupled trains are also provided based on the specific network characteristics of the different market segments.A SWOT analysis is presented which builds on the outcomes of the survey, the operational scenarios and brainstorming sessions with experts of the European railway industry. The main strengths identified for VC are a substantial increase in capacity and reduced operational costs with respect to Moving Block while mitigating delay propagation and improving reliability of ground/train communication. On the other hand, weaknesses of this concept refer to the fact that capacity gains at diverging junctions equipped with current switch technologies might be marginal, since here trains still need to be separated by a full braking distance. Also, the implementation of VC operations would require an investment to upgrade the overhead line system, platform lengths (to allow platoons of trains to stop) and possibly the switch technology. An upgrade of the switch technology towards faster and more reliable ones (e.g. Railtaxi and REPOINT) will unleash the full potential of VC operations. Significant opportunities will be brought about Virtual Coupling such as potential increase in the profit of infrastructure managers and operators as well as a deregulation of the current railway market which could be opened also to smaller transport operators due to the increase of available train paths and the decrease of operational costs by full train automation. In addition, the train-to-train communication could lead to the institution of cooperative consortia of railway operators which can be more economically beneficial than the current competitive market model. This would also provide the chance to migrate obsolescent command and control systems towards future-proof digital railway architectures. Possible threats to the introduction of this concept mainly relate to potential increase of train control complexity increasing risks of approval from the railway industry. The need for an initial investment might be not well received by infrastructure managers and local governments. As well as the necessity of partially changing policies, operational procedures and engineering rules currently in place. When overcoming such challenges, Virtual Coupling has potentials to fully revolutionise and improve current train operations so to induce a sustainable shift to railways.Transport and Plannin
Computer-Aided Proofs for the VC-dimension of Art Gallery Variants
No full textVisibility problems are fundamental to computational geometry, and many versions of geometric set cover where coverage is based on visibility have been considered. In most settings, points can see "infinitely far" so long as visibility is not "blocked" by some obstacle. In many applications, this may be an unreasonable assumption. We consider a new model of visibility where no point can see any other point beyond a sight radius ρ. We consider this visibility model in the context of art gallery variants. We show that the VC-dimension of limited visibility terrains is exactly 7. We give lower bound construction that shatters a set of 7 points, and we prove that shattering 8 points for limited visibility terrains is not possible. We also prove a collection of lemmas about the structure of visibility and that help finding the VC-dimension of limited visibility on the boundary of simple polygons.
We also consider the VC-dimension of visibility on the boundary of simple polygons under this limited visibility model. That is, we restrict all guards/viewpoints to be on the boundary of the simple polygon. We show that the VC-dimension under the limited visibility model is exactly 8. That is, we give an example of a simple polygon with 8 "guards" on the boundary such that every one of the 2<sup>8</sup> subsets of the 8 guards has a "viewpoint" that sees exactly the guards in the subset. We then show that this is not possible with any set of 9 guards.
Finally, we consider the problem of lowering the upper bound on the VC-dimension of the classic art gallery problem (no limited visibility constraints), and the guards and viewpoints can lie anywhere in the interior of the polygon. This problem has not seen an improvement in a decade. We extend our code to address this problem, relying on counterclockwise systems defined by Knuth to discretize the set of possible polygons. We give a dynamic programming approach for finding all of the ways that certain guard and viewpoint sets can be placed while satisfying their visibility requirements.Computer Scienc
Identification vs. Self-Verification in Virtual Communities (VC)
No full textIdentity-related processes have been identified as important in explaining virtual community (VC) member behavior as well as informing system design of VCs. In particular, the two distinct identity processes of self-verification and identification have been identified and investigated separately, portrayed as two distinctive or contradictory identity processes with different practical implications. This chapter compares and reconciles these two theoretical perspectives in explaining VC participation. Based on a critical and comprehensive review of prior literature, the author identifies three major theoretical gaps that suggest how VC research and management can be advanced through an identity perspective. Finally, the chapter is concluded by discussing key implications of applying identity perspectives in VC research and future research agenda.</jats:p
VC valuation and multiples: an exploration of comparable analysis of software start-ups
No full textThis master thesis seeks to better understand the investment valuation procedure followed by software venture capitalists (VC) in the European context. I explain how VCs perform fair value estimations of software start-ups with the emerging comparable analysis technique. Furthermore, this study examines the relative importance of start-up characteristics in determining the multiple and how these factors influence the VC’s valuation behaviour. Additionally, I explore whether this behaviour and the multiples paid can be explained by differences in VC firm experience at a time of historically low interest rates and record-breaking fund inflows. Based on 36 interviews with European VCs, primarily from the Benelux region, I find that all start-up characteristics matter in the determination of the multiple, but the management team a little more. As a result, software VCs are willing to pay higher multiples for stellar management teams than for exceptional business characteristics. In contrast with the other characteristics, poor traction does not necessarily kill the deal, but VCs might rather use it to enforce a lower valuation. Overall, VC firm experience is not a strong predictor of the valuation behaviour and ARR multiples paid for deals. However, I do find that more experienced VCs are willing to pay higher premiums for benchmark-exceeding traction than their less experienced counterparts.Management of Technology (MoT
Almost Linear VC Dimension Bounds for Piecewise Polynomial Networks
No full textWe compute upper and lower bounds on the VC dimension and pseudo dimension of feedforward neural networks composed of piecewise polynomial activation functions. We show that if the number of layers is fixed, then the VC dimension and pseudo-dimension grow as W log W , where W is the number of parameters in the network. This result stands in opposition to the case where the the number of layers is unbounded, in which case the VC dimension and pseudo-dimension grow as W 2 . We combine our results with recently established approximation error rates, and determine error bounds for the problem of regression estimation by piecewise polynomial networks with unbounded weights. 1 Motivation It is a well known result of much recent work, that in order to derive useful performance bounds for classification, regression, and time series prediction, use must be made of both approximation as well as estimation error bounds. Approximation errors result Part of this work was done while the author..
The impact of the quality of VC financing and monitoring on the survival of IPO firms
No full text
Purpose
The purpose of this paper is to analyze the impact of venture capital (VC) involvement on the survival rate of French initial public offerings (IPOs) during the period 1996-2006. The paper examines the link between the survival rates of IPO companies, and several proxies for the quality of venture capitalist financing and monitoring.
Design/methodology/approach
To analyze the impact of the involvement of VC on both long and short run post-IPO survival, two methods are used: survival analysis (the Cox proportional hazard), and a logit model.
Findings
This paper shows that the quality of venture capitalist monitoring, measured by the duration of their investment before the IPO, is positively correlated with company survival rates. However, the author does not find the expected result when the author considers the experience of venture capitalists measured by their age.
Research limitations/implications
The findings are limited to a sample of VC-backed companies that went public.
Practical implications
The findings have implications for entrepreneurs. When analyzing the advantages and disadvantages linked to the presence of VC firms in the capital of their companies, entrepreneurs should consider that certain types of venture capitalists might be more or less able to be involved in the monitoring and value adding process.
Originality/value
To date, there is no comprehensive study on the French IPO market analyzing both long and short run post-IPO survival of VC-backed companies. This paper fills this gap.
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