Julian Müller


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Müller

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Julian

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0000-0001-6440-8046

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Publications1 - 7 of 7
  • netroles: A Java library for role equivalence analysis
    Item type: Journal Article
    Müller, Julian (2024)
    Journal of Open Source Software
    An important problem in the analysis of networks is structural similarity. It has commonly been expressed in terms of role equivalences, which have often been considered to formalize the concepts of social role and position as discussed by Linton (1936), Merton (1957) and Nadel (1957) in their analyses of social structure (Borgatti & Everett, 1992a). Role equivalences are based on the idea that actors are equivalent or play the same role if they form ties to similar others in similar ways. For example, the role of “doctor” is characterized by a set of ties to others playing related roles like “nurses”, “patients” and “colleagues”. However, this equivalence idea has been interpreted by different authors in different ways, resulting in the proposition of diverse definitions of role equivalence such as structural (Lorrain & White, 1971) or regular equivalence (White & Reitz, 1983). The netroles library provides implementations of many established notions of role equivalence, but more importantly, it offers a unified approach to role equivalence analysis that generalizes beyond the classic role equivalences, allowing users to express more complex kinds of role notions suitable for networks with multiple relations and attributes.
  • Relation of diet-induced thermogenesis to brown adipose tissue activity in healthy men
    Item type: Journal Article
    Loeliger, Rahel C.; Maushart, Claudia I.; Gashi, Gani; et al. (2021)
    American Journal of Physiology. Endocrinology and Metabolism
    Human brown adipose tissue (BAT) is a thermogenic tissue activated by the sympathetic nervous system in response to cold exposure. It contributes to energy expenditure (EE) and takes up glucose and lipids from the circulation. Studies in rodents suggest that BAT contributes to the transient rise in EE after food intake, so-called diet-induced thermogenesis (DIT). We investigated the relationship between human BAT activity and DIT in response to glucose intake in 17 healthy volunteers. We assessed DIT, cold-induced thermogenesis (CIT), and maximum BAT activity at three separate study visits within 2 wk. DIT was measured by indirect calorimetry during an oral glucose tolerance test. CIT was assessed as the difference in EE after cold exposure of 2-h duration as compared with warm conditions. Maximal activity of BAT was assessed by 18-F-fluoro-deoxyglucose (18F-FDG) 18F-FDG-PET/MRI after cold exposure and concomitant pharmacological stimulation with mirabegron. Seventeen healthy men (mean age = 23.4 yr, mean body mass index = 23.2 kg/m2) participated in the study. EE increased from 1,908 (±181) kcal/24 h to 2,128 (±277) kcal/24 h (P < 0.0001, +11.5%) after mild cold exposure. An oral glucose load increased EE from 1,911 (±165) kcal/24 h to 2,096 (±167) kcal/24 h at 60 min (P < 0.0001, +9.7%). The increase in EE in response to cold was significantly associated with BAT activity (R2 = 0.43, P = 0.004). However, DIT was not associated with BAT activity (R2 = 0.015, P = 0.64). DIT after an oral glucose load was not associated with stimulated 18F-FDG uptake into BAT, suggesting that DIT is independent from BAT activity in humans (Clinicaltrials.gov Registration No. NCT03189511).NEW & NOTEWORTHY Cold-induced thermogenesis (CIT) was related to BAT activity as determined by FDG-PET/MRI after stimulation of BAT. Diet-induced thermogenesis (DIT) was not related to stimulated BAT activity. Supraclavicular skin temperature was related to CIT but not to DIT. DIT in humans is probably not a function of BAT.
  • A Micro-Foundation for Roles in Networks
    Item type: Doctoral Thesis
    Müller, Julian (2022)
    Networks have served as models for phenomena in many different disciplines, such as the Social Sciences, Biology, Linguistics, Medicine and Physics. In the study of such networks, research questions that pertain to network structure are often posed, and they are investigated by analyzing the networks in question with methods designed for this purpose. This thesis focuses on one of the recurring questions in network analysis, namely whether and if so which individuals are structurally similar in a network. Here, ‘structural similarity’ does not mean proximity, but rather whether individuals are involved in similar (but possibly quite distant) structures in the network. Many methods for investigating this problem were originally developed for the analysis of social relations, and were proposed as operationalizations of the concepts of social positions and roles from social theory. The underlying idea is that social roles played by individuals are expressed in their social relationships, and thus that roles can be inferred from the structure of social relations and individuals can be classified accordingly. The investigation of these kinds of questions is therefore also often called positional or role analysis. The concept of individual roles and the notion of structural similarity have often been operationalized in the shape of role equivalences, which express the idea that two individuals are equivalent if they are connected to the network similarly. However, all specific definitions of role equivalence rarely exhibit non-trivial instances in empirical contexts. In response, various relaxations of role equivalences have been proposed, but both role equivalences and relaxed approaches expose a number of limitations. They provide no explanation of how roles emerged and evolved within a network. Furthermore, adapting these methods to role definitions suitable for specific research questions often turns out to be difficult, and methods assume network models that are often too simple and inappropriate to represent networks in empirical research. At the core of this thesis is therefore a proposal for a novel formalization of the role concept, that unifies and generalizes previous definitions of role equivalences and provides a micro-foundation for the emergence of roles. Chapter 3 discusses differing interpretations of role in the social networks literature, and categorizes established methods for role analysis according to these interpretations. Subsequently, operationalizations of role in the form of role equivalences and their relaxations are introduced, and their limitations are argued. To highlight some of these, the structure of social relations among recruits at a US-American police academy is investigated using role analysis methods. Since cadets were divided into four squads, the aim of the analysis is to uncover the roles recruits play among the squads. The extreme density of social ties within squads, however, obscures the inter-squad relationship structure and established role analysis methods fail to partition recruits by their roles between squads. As the main contribution, a novel formalization of role is proposed in Chapter 4. The frequently self-referential definition of role equivalences is one of the reasons for the rarity of non-trivial instances. By dissolving this self-referentiality, a notion of relative roles is obtained which expresses that actors are role-equivalent relative to some given division of individuals if their incident relationships and neighborhoods are similar under some notion of substitutability. Repeated applications of such relative roles starting from some arbitrary initial state lead to deterministic processes of role evolution. Since these processes can be interpreted as idealized influence processes, the formalization offers a micro-foundation for the emergence of roles. Considering the underlying mechanism of role formation can provide support in explaining the emergence of a particular role structure, as well as help in selecting notions of role suitable for specific network studies. At the same time, the conceptualization unifies established definitions of role equivalence, as they form fixed points of role evolution processes, facilitating the investigation of common structural properties. Moreover, it also generalizes established notions, and allows to express nuanced notions of roles that incorporate application-specific knowledge and all aspects of network data. Chapter 5 discusses application to multivariate, directed and multiplex networks as well as indirect relations, and introduces generalizations from role equivalences to role dominances (quasiorders) and general role structures (binary relations). Furthermore, measures for role dissimilarity are described that quantify how distant actors are from role equivalence or dominance. The study about police recruits at an academy of a urban police department is taken up again in Chapter 6. Since established methods of role analysis as well as community detection and network embedding techniques did not succeed in identifying relevant structure, the proposed formalization is applied and its expressiveness exploited to derive a suitable role notion of integrator roles in a substantiated way. Previously unknown roles among recruits are identified and linked to racial tensions within the cohort. Towards the end, the computation of role structures is addressed. Chapter 7 discusses efficient algorithms for computing relative roles and following role evolution processes, as well as the hardness of related computational problems. Specifically, it is demonstrated that the enumeration of even a small number of fixed points is likely infeasible in general. Finally, Chapter 8 presents the design of a programming library that implements the proposed formalization of role and algorithms described previously.
  • A note on the complexity of manipulating weighted Schulze voting
    Item type: Journal Article
    Müller, Julian; Kosub, Sven (2020)
    Information Processing Letters
    We prove that the constructive weighted coalitional manipulation problem for the Schulze voting rule can be solved in polynomial time for an unbounded number of candidates and an unbounded number of manipulators. © 2020 Elsevier B.V.
  • The evolution of roles
    Item type: Journal Article
    Müller, Julian; Brandes, Ulrik (2022)
    Social Networks
    We propose a novel formalization of roles in social networks that unifies the most commonly used definitions of role equivalence. As one consequence, we obtain a single, straightforward proof that role equivalences form lattices. Our formalization focuses on the evolution of roles from arbitrary initial conditions and thereby generalizes notions of relative and iterated roles that have been suggested previously. In addition to the unified structure result this provides a potential micro-foundation for the emergence of roles. Considering the genesis of roles may explain, and help overcome, the problem that social networks rarely exhibit interesting role equivalences of the traditional kind. Finally, we discuss extensions of the role concept to multivariate networks with actor and tie attributes by example.
  • The evolution of roles
    Item type: Conference Paper
    Müller, Julian; Brandes, Ulrik (2019)
    Proceedings of the 2019 IEEE/ACM International Conference on Advances in Social Network Analysis and Mining
    We propose a novel formalization of roles in social networks that unifies the most commonly used definitions of role equivalence. As one consequence, we obtain a single, straightforward proof that role equivalences form lattices. Our formalization focuses on the evolution of roles from arbitrary initial conditions and thereby generalizes notions of relative and iterated roles that have been suggested previously. In addition to the unified structure result this provides a micro-foundation for the emergence of roles. Considering the genesis of roles may explain, and help overcome, the problem that social networks rarely exhibit interesting role equivalences of the traditional kind. Finally, we hint at ways to further generalize the role concept to multivariate networks.
  • Low-dose F-18-FDG TOF-PET/MR for accurate quantification of brown adipose tissue in healthy volunteers
    Item type: Journal Article
    ter Voert, Edwin E.G.W.; Svirydenka, Hanna; Müller, Julian; et al. (2020)
    EJNMMI Resarch
    Background Positron emission tomography (PET) is increasingly applied for in vivo brown adipose tissue (BAT) research in healthy volunteers. To limit the radiation exposure, the injected 18F-FDG tracer dose should be as low as possible. With simultaneous PET/MR imaging, the radiation exposure due to computed tomography (CT) can be avoided, but more importantly, the PET acquisition time can often be increased to match the more extensive magnetic resonance (MR) imaging protocol. The potential gain in detected coincidence counts, due to the longer acquisition time, can then be applied to decrease the injected tracer dose. The aim of this study was to investigate the minimal 18F-FDG dose for a 10-min time-of-flight (TOF) PET/MR acquisition that would still allow accurate quantification of supraclavicular BAT volume and activity. Methods Twenty datasets from 13 volunteers were retrospectively included from a prospective clinical study. PET emission datasets were modified to simulate step-wise reductions of the original 75 MBq injected dose. The resulting PET images were visually and quantitatively assessed and compared to a 4-min reference scan. For the visual assessment, the image quality and artifacts were scored using a 5-point and a 3-point Likert scale. For the quantitative analysis, image noise and artifacts, BAT metabolic activity, BAT metabolic volume (BMV), and total BAT glycolysis (TBG) were investigated. Results The visual assessment showed still good image quality for the 35%, 30%, and 25% activity reconstructions with no artifacts. Quantitatively, the background noise was similar to the reference for the 35% and 30% activity reconstructions and the artifacts started to increase significantly in the 25% and lower activity reconstructions. There was no significant difference in supraclavicular BAT metabolic activity, BMV, and TBG between the reference and the 35% to 20% activity reconstructions. Conclusions This study indicates that when the PET acquisition time is matched to the 10-min MRI protocol, the injected 18F-FDG tracer dose can be reduced to approximately 19 MBq (25%) while maintaining image quality and accurate supraclavicular BAT quantification. This could decrease the effective dose from 1.4 mSv to 0.36 mSv.
Publications1 - 7 of 7