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Tight Localizations of Feedback Sets
(2021) Hecht, Michael; Gonciarz, Krzysztof; Horvát, Szabolcs; Department of Computer Science
The classical NP-hard feedback arc set problem (FASP) and feedback vertex set problem (FVSP) ask for a minimum set of arcs ϵ ⊆ E or vertices ν ⊆ V whose removal G ∖ ϵ, G ∖ ν makes a given multi-digraph G=(V, E) acyclic, respectively. Though both problems are known to be APX-hard, constant ratio approximations or proofs of inapproximability are unknown. We propose a new universal O(|V||E|4)-heuristic for the directed FASP. While a ratio of r ≈ 1.3606 is known to be a lower bound for the APX-hardness, at least by empirical validation we achieve an approximation of r ≤ 2. Most of the relevant applications, such as circuit testing, ask for solving the FASP on large sparse graphs, which can be done efficiently within tight error bounds with our approach.
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Impossible ecologies : Interaction networks and stability of coexistence in ecological communities
(2025-07-16) Meng, Yu; Horvát, Szabolcs; Modes, Carl D.; Haas, Pierre A.; Department of Computer Science
Does an ecological community allow stable species coexistence? Identifying the general effects of competitive, mutualistic, and predator-prey interactions on stability remains a central problem of systems ecology because established approaches cannot account for the full network arrangement of these interactions. Here, we therefore analyze all interaction networks of N≤5 species with Lotka-Volterra dynamics by combining exact results and numerical exploration. We find that a very small subset of these networks is “impossible ecologies,” in which stable coexistence is non-trivially impossible. We prove that the possibility of stable coexistence is determined by similarly rare “irreducible ecologies.” Statistical sampling shows that this probability varies over orders of magnitude even in ecologies that differ only in the network arrangement of identical interactions. Thus, our approach reveals that the full network structure of interactions can influence stability of coexistence more than the established effect of interaction-type proportions. A record of this paper's transparent peer review process is included in the supplemental information.
Verk
Patient Generated Data And Affordances: Conversation Cues In Clinical Decision Making
(2018) Grisot, Miria; Lindroth, Thomas; Islind, Anna Sigridur; Department of Computer Science
Verk
Balancing the social media seesaw in public sector: a sociomaterial perspective
(2017) Norström, Livia; Islind, Anna Sigridur; Vallo Hult, Helena; Department of Computer Science
Verk
Spatial Embedding and Wiring Cost Constrain the Functional Layout of the Cortical Network of Rodents and Primates
(2016-07-21) Horvát, Szabolcs; Gămănuț, Răzvan; Ercsey-Ravasz, Mária; Magrou, Loïc; Gămănuț, Bianca; Van Essen, David C.; Burkhalter, Andreas; Knoblauch, Kenneth; Toroczkai, Zoltán; Kennedy, Henry; Department of Computer Science
Mammals show a wide range of brain sizes, reflecting adaptation to diverse habitats. Comparing interareal cortical networks across brains of different sizes and mammalian orders provides robust information on evolutionarily preserved features and species-specific processing modalities. However, these networks are spatially embedded, directed, and weighted, making comparisons challenging. Using tract tracing data from macaque and mouse, we show the existence of a general organizational principle based on an exponential distance rule (EDR) and cortical geometry, enabling network comparisons within the same model framework. These comparisons reveal the existence of network invariants between mouse and macaque, exemplified in graph motif profiles and connection similarity indices, but also significant differences, such as fractionally smaller and much weaker long-distance connections in the macaque than in mouse. The latter lends credence to the prediction that long-distance cortico-cortical connections could be very weak in the much-expanded human cortex, implying an increased susceptibility to disconnection syndromes such as Alzheimer disease and schizophrenia. Finally, our data from tracer experiments involving only gray matter connections in the primary visual areas of both species show that an EDR holds at local scales as well (within 1.5 mm), supporting the hypothesis that it is a universally valid property across all scales and, possibly, across the mammalian class.

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