16,545 research outputs found
Towards Unbiased BFS Sampling
Breadth First Search (BFS) is a widely used approach for sampling large
unknown Internet topologies. Its main advantage over random walks and other
exploration techniques is that a BFS sample is a plausible graph on its own,
and therefore we can study its topological characteristics. However, it has
been empirically observed that incomplete BFS is biased toward high-degree
nodes, which may strongly affect the measurements. In this paper, we first
analytically quantify the degree bias of BFS sampling. In particular, we
calculate the node degree distribution expected to be observed by BFS as a
function of the fraction f of covered nodes, in a random graph RG(pk) with an
arbitrary degree distribution pk. We also show that, for RG(pk), all commonly
used graph traversal techniques (BFS, DFS, Forest Fire, Snowball Sampling, RDS)
suffer from exactly the same bias. Next, based on our theoretical analysis, we
propose a practical BFS-bias correction procedure. It takes as input a
collected BFS sample together with its fraction f. Even though RG(pk) does not
capture many graph properties common in real-life graphs (such as
assortativity), our RG(pk)-based correction technique performs well on a broad
range of Internet topologies and on two large BFS samples of Facebook and Orkut
networks. Finally, we consider and evaluate a family of alternative correction
procedures, and demonstrate that, although they are unbiased for an arbitrary
topology, their large variance makes them far less effective than the
RG(pk)-based technique.Comment: BFS, RDS, graph traversal, sampling bias correctio
Exploring chemical compound space with a graph-based recommender system
With the availability of extensive databases of inorganic materials,
data-driven approaches leveraging machine learning have gained prominence in
materials science research. In this study, we propose an innovative adaptation
of data-driven concepts to the mapping and exploration of chemical compound
space. Recommender systems, widely utilized for suggesting items to users,
employ techniques such as collaborative filtering, which rely on bipartite
graphs composed of users, items, and their interactions. Building upon the Open
Quantum Materials Database (OQMD), we constructed a bipartite graph where
elements from the periodic table and sites within crystal structures are
treated as separate entities. The relationships between them, defined by the
presence of ions at specific sites and weighted according to the thermodynamic
stability of the respective compounds, allowed us to generate an embedding
space that contains vector representations for each ion and each site. Through
the correlation of ion-site occupancy with their respective distances within
the embedding space, we explored new ion-site occupancies, facilitating the
discovery of novel stable compounds. Moreover, the graph's embedding space
enabled a comprehensive examination of chemical similarities among elements,
and a detailed analysis of local geometries of sites. To demonstrate the
effectiveness and robustness of our method, we conducted a historical
evaluation using different versions of the OQMD and recommended new compounds
with Kagome lattices, showcasing the applicability of our approach to practical
materials design
Striking a Balance Between Physical and Digital Resources
In various configurations—be they academic, archival, county, juvenile, monastic, national, personal, public, reference, or research, the library has been a fixture in human affairs for a long time. Digital — meaning, content or communication that is delivered through the internet, is 20 years old (but younger in parts). Basically, both approaches to organizing serve to structure information for access. However, digital is multiplying very fast and libraries all-round contemplate an existential crisis; the more hopeful librarians fret about physical and digital space.
Yet, the crux of the matter is not about physical vs. digital: without doubt, the digital space of content or communication transmogrifies all walks of life and cannot be wished away; but, the physical space of libraries is time-tested, extremely valuable, and can surely offer more than currently meets the eye. Except for entirely virtual libraries, the symbiotic relationship between the physical and the digital is innately powerful: for superior outcomes, it must be recognized, nurtured, and leveraged; striking a balance between physical and digital resources can be accomplished. This paper examines the subject of delivering digital from macro, meso, and micro perspectives: it looks into complexity theory, digital strategy, and digitization
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