Cluster Derivation of the Parisi Scheme for Disordered Systems

We propose a general quantitative scheme in which systems are given the freedom to sacrifice energy equi-partitioning on the relevant time-scales of observation, and have phase transitions by separating autonomously into ergodic sub-systems (clusters) with different characteristic time-scales and temperatures. The details of the break-up follow uniquely from the requirement of zero entropy for the slower cluster. Complex systems, such as the Sherrington-Kirkpatrick model, are found to minimise their free energy by spontaneously decomposing into a hierarchy of ergodically equilibrating degrees of freedom at different (effective) temperatures. This leads exactly and uniquely to Parisi's replica symmetry breaking scheme. Our approach, which is somewhat akin to an earlier one by Sompolinsky, gives new insight into the physical interpretation of the Parisi scheme and its relations with other approaches, numerical experiments, and short range models. Furthermore, our approach shows that the Parisi scheme can be derived quantitatively and uniquely from plausible physical principles.Comment: 6 pages, 3 figures, proceedings of international conference on "Disordered And Complex Systems", 10-14 July 2000 King's College Londo

Solvable Lattice Gas Models of Random Heteropolymers at Finite Density: II. Dynamics and Transitions to Compact States

In this paper we analyse both the dynamics and the high density physics of the infinite dimensional lattice gas model for random heteropolymers recently introduced in \cite{jort}. Restricting ourselves to site-disordered heteropolymers, we derive exact closed deterministic evolution equations for a suitable set of dynamic order parameters (in the thermodynamic limit), and use these to study the dynamics of the system for different choices of the monomer polarity parameters. We also study the equilibrium properties of the system in the high density limit, which leads to a phase diagram exhibiting transitions between swollen states, compact states, and regions with partial compactification. Our results find excellent verification in numerical simulations, and have a natural and appealing interpretation in terms of real heteropolymers.Comment: 12 pages, 8 eps figures, revised version (to be published in EPJ

A Solvable Model of Secondary Structure Formation in Random Hetero-Polymers

We propose and solve a simple model describing secondary structure formation in random hetero-polymers. It describes monomers with a combination of one-dimensional short-range interactions (representing steric forces and hydrogen bonds) and infinite range interactions (representing polarity forces). We solve our model using a combination of mean field and random field techniques, leading to phase diagrams exhibiting second-order transitions between folded, partially folded and unfolded states, including regions where folding depends on initial conditions. Our theoretical results, which are in excellent agreement with numerical simulations, lead to an appealing physical picture of the folding process: the polarity forces drive the transition to a collapsed state, the steric forces introduce monomer specificity, and the hydrogen bonds stabilise the conformation by damping the frustration-induced multiplicity of states.Comment: 24 pages, 14 figure

Restoration of former agricultural fields on acid sandy soils:Conversion to heathland, rangeland or forest?

Three restoration strategies on agricultural fields with acid sandy soil were evaluated after 18 and 25 years: conversion to heathland, rangeland and forest. Changes in soil microstructure, chemical characteristics, availability of N and P, and vegetation composition were analyzed in agricultural soils, three undisturbed reference sites and five types of restored former agricultural fields. Agricultural soils were characterized by organic slurry without much soil life. Soil nutrients were especially high for P, mostly in mineral form, and P-desorption rates were high. Partial and complete topsoil removal, aiming at heathland restoration, led to (much) lower soil organic matter and nutrients, but not to recovery of soil life, nor to P-limited soils. Heather was accompanied by many grassland species, even with complete topsoil removal. Conversion to rangeland did not decrease nutrient stocks, but led to improved soil life, although different from reference grasslands due to the higher pH. P-availability remained high, but net N-mineralization and plant N-content were clearly lower after 25 than after 18 years. Plant diversity was relatively high, and cover of eutrophic grasses decreased to 8–39% in intermediate and productive rangelands. Nutrient-poor species remained absent, but the slightly higher pH improved conditions for many grassland herbs. Afforestation did also not lead to nutrient-poor conditions, but soil life clearly increased and nutrients were used for rapid tree growth. Undergrowth species however remained eutrophic. It was impossible to retrieve the P-limited reference ecosystems within 25 years of restoration, not even with complete topsoil removal. Differences in plant diversity between expensive topsoil removal and much cheaper conversion to rangeland were also relatively small. For restoration on a landscape scale, it may thus be better to focus on conversion to semi-natural grasslands and afforestation. The half-open, nutrient-, mineral-, and species-rich landscape offers opportunities for large grazers, but also for many insects and birds

Cluster derivation of Parisi's RSB solution for disordered systems

We propose a general scheme in which disordered systems are allowed to sacrifice energy equi-partitioning and separate into a hierarchy of ergodic sub-systems (clusters) with different characteristic time-scales and temperatures. The details of the break-up follow from the requirement of stationarity of the entropy of the slower cluster, at every level in the hierarchy. We apply our ideas to the Sherrington-Kirkpatrick model, and show how the Parisi solution can be {\it derived} quantitatively from plausible physical principles. Our approach gives new insight into the physics behind Parisi's solution and its relations with other theories, numerical experiments, and short range models.Comment: 7 pages 5 figure

Bronchoscopic treatment of intraluminal typical carcinoid: A pilot study

AbstractObjective: The curative potential of various bronchoscopic treatments such as Nd:YAG laser, photodynamic therapy, and brachytherapy for the treatment of intraluminal tumor has been reported previously. Bronchoscopic treatment can be used to treat small intraluminal tumor with curative intent, such as in patients with roentgenologically occult squamous cell cancer. In a retrospective study, we showed that bronchoscopic treatment provided excellent local control with surgical proof of cure in 6 of 11 patients with intraluminal typical bronchial carcinoid. Methods: In a prospective study, 19 patients (8 women and 11 men) with resectable intraluminal typical bronchial carcinoid have undergone bronchoscopic treatment under general anesthesia. Median age was 44 years (range, 20-74 years). If tumor persisted after 2 bronchoscopic treatment sessions, surgery was performed within 4 months after the treatment. Results: Bronchoscopic treatment was able to completely eradicate tumor in 14 of the 19 patients (complete response rate 73%, 95% CI: 49%-91%). Median follow-up of these patients is 29 months (range, 8-62 months). One patient had severe cicatricial stenosis after bronchoscopic treatment, and sleeve lobectomy was necessary. No residual carcinoid was found in the resected specimen. In the remaining 5 patients, bronchoscopic treatment did not result in a complete response and radical surgical resection was performed afterward with confirmation of residual carcinoid in the resected specimen. Median follow-up of the surgical group is 34 months (range, 12-62 months). Conclusions: Current data suggest that bronchoscopic treatment may be an effective alternative to surgical resection in a subgroup of patients with resectable intraluminal typical bronchial carcinoid. It alleviated the necessity of surgical resection in 68% (95% CI: 43%-87%) of the patients. (J Thorac Cardiovasc Surg 1998;116:402-6

Verbetering trosvorming bij trosrozen door teelthandelingen

Het jaarrond telen van trosrozen met voldoende bloemen op het bloemgestel is voor veel telers een probleem. In dit onderzoek is onderzocht of door gewasopbouw (snoeimethode) en de keuze van cultivar en onderstam een verbetering kan worden bereikt van het percentage trossen bij trosrozen. In totaal zijn in dit onderzoek drie knipmethoden en twee cultivars vergeleken. Bij één cultivar zijn twee onderstammen, in combinatie met eigen wortel, met elkaar vergeleken. Met behulp van de gewasopbouw het percentage trossen beïnvloeden is mogelijk gebleken. Een opbouw waarbij de basis van de plant dichter bij de grond blijft en waarbij er minder ogen zijn waar de plant mee uit kan lopen beïnvloedt het percentage trossen en het takgewicht positief. Het aantal geoogste takken wordt echter negatief beïnvloed, zodat per saldo er geen verschil in het totaal geoogste aantal kilogrammen per snoeimethode is. Bij de snoeimethode 'knotroos', waarbij contant nieuwe grondscheuten gevormd worden, is de groeiduur van knippen tot de volgende oogst langer dan bij de andere methoden

Colouring random graphs and maximising local diversity

We study a variation of the graph colouring problem on random graphs of finite average connectivity. Given the number of colours, we aim to maximise the number of different colours at neighbouring vertices (i.e. one edge distance) of any vertex. Two efficient algorithms, belief propagation and Walksat are adapted to carry out this task. We present experimental results based on two types of random graphs for different system sizes and identify the critical value of the connectivity for the algorithms to find a perfect solution. The problem and the suggested algorithms have practical relevance since various applications, such as distributed storage, can be mapped onto this problem.Comment: 10 pages, 10 figure

Video-Assisted Thoracoscopic Surgery in Patients With Clinically Resectable Lung Tumors

To investigate the feasibility of thoracoscopic resection, a pilot study was performed in patients with clinically resectable lung tumors. In 40 patients, Video-assisted thoracic surgery (VATS) was performed because of suspicion of malignancy. There were 29 men and 11 women with a median age of 54.8 years (range 18 to 78). Preoperative indications were suspected lung cancer and tumor in 27 patients, assessment of tumor resectability in 7 patients, and probability of metastatic tumors in 6 patients. The final diagnoses in the 27 patients with suspected lung cancer were 12 primary lung cancers, 6 lung metastases, and 9 benign lesions. The success rates for VATS (no conversion to thoracotomy) were 1 of 12 (8.3%) for resectable stage I lung cancer, 8 of 12 (66.7%) for metastatic tumors, and 9 of 9 (100%) for benign tumors. With VATS, 6 of 7 patients (85.7%), possible stage III non-small cell lung cancer, an explorative thoracotomy with was avoided, significantly reducing morbidity. The reasons for conversion to thoracotomy were 1) oncological (N2 lymph node dissection and prevention of tumor spillage) and 2) technical (inability to locate the nodule, central localization, no anatomical fissure, or poor lung function requiring full lung ventilation). The ultimate diagnoses were 19 lung cancers, 12 metastatic lung tumors, and 9 benign lung tumors. Our data show the limitations of VATS for malignant tumors in general use. These findings, together with the fact that experience in performing thoracoscopic procedures demonstrates a learning curve, may limit the use of thoracoscopic resection as a routine surgical procedure, especially when strict oncological rules are respected