Patrik Floréen

Balanced Data Gathering in Energy-Constrained Sensor Networks

We consider the problem of gathering data from a wireless multi-hop network of energy-constrained sensor nodes to a common base station. Specifically, we aim to balance the total amount of data received from the sensor network during its lifetime against a requirement of sufficient coverage for all the sensor locations surveyed. Our main contribution lies in formulating this balanced data gathering task and in studying the effects of balancing. We give an LP network flow formulation and present experimental results on optimal data routing designs also with impenetrable obstacles between the nodes. We then proceed to consider the effect of augmenting the basic sensor network with a small number of auxiliary relay nodes with less stringent energy constraints. We present an algorithm for finding approximately optimal placements for the relay nodes, given a system of basic sensor locations, and compare it with a straightforward grid arrangement of the relays.

Almost Stable Matchings by Truncating the Gale–Shapley Algorithm

We show that the ratio of matched individuals to blocking pairs grows linearly with the number of propose–accept rounds executed by the Gale–Shapley algorithm for the stable marriage problem. Consequently, the participants can arrive at an almost stable matching even without full information about the problem instance; for each participant, knowing only its local neighbourhood is enough. In distributed-systems parlance, this means that if each person has only a constant number of acceptable partners, an almost stable matching emerges after a constant number of synchronous communication rounds.We apply our results to give a distributed (2+ε)-approximation algorithm for maximum-weight matching in bicoloured graphs and a centralised randomised constant-time approximation scheme for estimating the size of a stable matching.

A local 2-approximation algorithm for the vertex cover problem

We present a distributed 2-approximation algorithm for the minimum vertex cover problem. The algorithm is deterministic, and it runs in (Δ + 1)2 synchronous communication rounds, where Δ is the maximum degree of the graph. For Δ = 3, we give a 2-approximation algorithm also for the weighted version of the problem.

Evaluating the role of a shared document-based annotation tool in learner-centered collaborative learning

This study presents a shared document-based annotation tool, EDUCOSM. Usefulness of the system is empirically evaluated in a real-life collaborative learning context. Relationships between learners self-rated use of learning strategies, cognitive outcomes, and completion of various tasks in the system are investigated. An empirical study (n=31) was conducted in order to investigate various dependencies between variables from precourse self-rated questionnaire, system log file data collected during the course and postcourse e-mail survey.

The convergence of Hamming memory networks

The convergence properties of Hamming memory networks are studied. It is shown how to construct the network so that it probably converges to an appropriate result, and a tight bound is given on the convergence time. The bound on the convergence time is largest when several stored vectors are at the minimum distance from the input vector. For random binary vectors, the probability for such situations to occur is not small. With a specific choice of parameter values, the worst-case convergence time is on the order of p ln (pn), where p is the memory capacity and n is the vector length. By allowing the connection weights to change during the computation, the convergence time can be decreased considerably.

Attraction radii in binary hopfield nets are hard to compute

We prove that it is an NP-hard problem to determine the attraction radius of a stable vector in a binary Hopfield memory network, and even that the attraction radius is hard to approximate. Under synchronous updating, the problems are already NP-hard for two-step attraction radii; direct (one-step) attraction radii can be computed in polynomial time.

Exact and approximate balanced data gathering in energy-constrained sensor networks

We consider the problem of gathering data from a wireless multi-hop network of energy-constrained sensor nodes to a common base station. Specifically, we aim to balance the total amount of data received from the sensor network during its lifetime against a requirement of sufficient coverage for all the sensor locations surveyed. Our main contribution lies in formulating this balanced data gathering task, studying the effects of balancing, and proposing an approximation algorithm for the problem. Based on an LP network flow formulation, we present experimental results on both optimal and approximate data routing designs, in open transmission ranges and with impenetrable obstacles between the nodes.

Product retrieval for grocery stores

We introduce a grocery retrieval system that maps shopping lists written in natural language into actual products in a grocery store. We have developed the system using nine months of shopping basket data from a large Finnish supermarket. To evaluate the system, we used 70 real shopping lists gathered from customers of the supermarket. Our system achieves over 80% precision for products at rank one, and the precision is around 70% for products at rank 5.

Capricorn: an intelligent interface for mobile widgets

Widgets are embeddable objects that provide easy and ubiquitous access to dynamic information sources, for example weather, news or TV program information. Widgets are typically rather static - they provide the information regardless of whether the information is relevant to the users current information needs. In this paper we introduce Capricorn, which is an intelligent interface for mobile widgets. The interface uses various adaptive web techniques for facilitating navigation. For example, we use collaborative filtering to recommend suitable widgets and we dim infrequently used widgets. The demonstration presents the Capricorn interface focusing on the adaptive parts of the interface. The user interface is web-based, and as such platform independent. However, our target environment is mobile phones, and thus the interface has been optimized for mobile phones.

EDUCOSM - personalized writable Web for learning communities

Many of the possibilities of Web-based education are still unexplored. It seems that novel ways of thinking about both learning and technology are needed to get beyond the limitations of the traditional classroom setting. In this paper we introduce EDUCOSM, which focusses on the possibilities of collaboration and the open-ended nature of the Web. Its main features include sharing and annotation of arbitrary Web-pages, and an adaptive desktop for accessing the evolving contents of the system. EDUCOSM has been used in a real Web-based course, and the experiences are discussed along with a description of the tool. Although the approach requires both the teacher and the students to rethink their roles, the feedback received so far has been encouraging.