Papers by Alessio Orlandi
Let G=(V, E) be a directed graph. A vertex v��� V (respectively an edge e��� E) is a strong artic... more Let G=(V, E) be a directed graph. A vertex v��� V (respectively an edge e��� E) is a strong articulation point (respectively a strong bridge) if its removal increases the number of strongly connected components of G. We implement and engineer the linear-time algorithms in [9] for computing all the strong articulation points and all the strong bridges of a directed graph.
The creative approach when designing new algorithms is probably the major rewarding task for many... more The creative approach when designing new algorithms is probably the major rewarding task for many researchers. This love for solving algorithmic problems is cultivated by some very young and talented students at high school. The IOI (International Olympiad in Informatics) is a special programming contest for these students [1]. The main flavor is not just computer program- ming, but deep thinking in terms of algorithmics. The required skills are problem analysis, design of efficient algorithms and data structures, programming and testing [2].
Abstract We study the problem of estimating the number of occurrences of substrings in textual da... more Abstract We study the problem of estimating the number of occurrences of substrings in textual data: A text T on some alphabet�� of size �� is preprocessed and an index I is built. The index is used in lieu of the text to answer queries of the form CountH (P), returning an approximated number of the occurrences of an arbitrary pattern P as a substring of T. The problem has its main application in selectivity estimation related to the LIKE predicate in textual databases [15, 14, 5].
Abstract Collections are a fundamental tool for reproducible evaluation of information retrieval ... more Abstract Collections are a fundamental tool for reproducible evaluation of information retrieval techniques. We describe a new method for distributing the document lengths and term counts (aka within-document frequencies) of a web snapshot in a highly compressed and nonetheless quickly accessible form.
We give an interpretation of the temperature in de Sitter universe in terms of a dynamical Unruh ... more We give an interpretation of the temperature in de Sitter universe in terms of a dynamical Unruh effect associated with the Hubble sphere. As with the quantum noise perceived by a uniformly accelerated observer in static spacetimes, observers endowed with a proper motion can in principle detect the effect. In particular, we study a" Kodama observer" as a two-field Unruh detector for which we show the effect is approximately thermal.
Abstract: We study the formation of a black hole in the collision of two Gaussian packets. Rather... more Abstract: We study the formation of a black hole in the collision of two Gaussian packets. Rather than following their dynamical evolution in details, we assume a horizon forms when the mass function for the two packets becomes larger than half the flat areal radius, as it would occur in a spherically symmetric geometry. This simple approximation allows us to determine the existence of a minimum black hole mass solely related to the width of the packets.
Abstract: We consider the problem of representing, in a compressed format, a bit-vector $ S $ of ... more Abstract: We consider the problem of representing, in a compressed format, a bit-vector $ S $ of $ m $ bits with $ n $ 1s, supporting the following operations, where $ b\ in\{0, 1\} $: $ rank_b (S, i) $ returns the number of occurrences of bit $ b $ in the prefix $ S [1.. i] $; $ select_b (S, i) $ returns the position of the $ i $ th occurrence of bit $ b $ in $ S $. Such a data structure is called\ emph {fully indexable dictionary (FID)}[Raman et al., 2007], and is at least as powerful as predecessor data structures.
Abstract Three published papers are resumed in this thesis. Different aspects of the semiclassica... more Abstract Three published papers are resumed in this thesis. Different aspects of the semiclassical theory of gravity are discussed. In chapter 1 we find a new perturbative (yet analytical) solution to the unsolved problem of the metric junction between two Friedmann-Robertson-Walker using Israel's formalism. The case of an expanding radiation core inside an expanding or collapsing dust exterior is treated. This model can be useful in the" landscape" cosmology in string theory or for treating new gravastar configurations.
Abstract: We consider the problem of supporting Rank () and Select () operations on a bit vector ... more Abstract: We consider the problem of supporting Rank () and Select () operations on a bit vector of length m with n 1 bits. The problem is considered in the succinct index model, where the bit vector is stored in" read-only" memory and an additional data structure, called the index, is created during pre-processing to help answer the above queries.
Let s be a string whose symbols are solely available through access (i), a read-only operation th... more Let s be a string whose symbols are solely available through access (i), a read-only operation that probes s and returns the symbol at position i in s. Many compressed data structures for strings, trees, and graphs, require two kinds of queries on s: select (c, j), returning the position in s containing the jth occurrence of c, and rank (c, p), counting how many occurrences of c are found in the first p positions of s. We give matching upper and lower bounds for this problem.
Analysis and manipulation of large data sets is a driving force in today's development of compute... more Analysis and manipulation of large data sets is a driving force in today's development of computer science: information retrieval experts are pushing against the limits of computation capabilities and data processing every day, both in academic and industrial environment. The data to process can be expressed in different formats: unstructured text, semi-structured data like HTML or XML, fully structured data like MIDI files, JPEG images, etc..
La crescente mole di informazione sparsa in rete e la conseguente necessit�� di creare dei punti ... more La crescente mole di informazione sparsa in rete e la conseguente necessit�� di creare dei punti di accesso centrali alla stessa hanno spinto la comunit�� scientifica e le aziende ad interessarsi ai motori di ricerca, oggi diventati ormai strumento di uso comune per ogni utente della rete. Immediata conseguenza �� stato l'interesse per il problema di come collezionare efficientemente la base di dati (nel nostro caso, le pagine web disponibili su Internet) e come progettare dei software in grado di farlo.
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Papers by Alessio Orlandi