University of Vienna

About: University of Vienna is a education organization based out in Vienna, Austria. It is known for research contribution in the topics: Population & Stars. The organization has 44686 authors who have published 95840 publications receiving 2907492 citations.

Social learning promotes institutions for governing the commons

Abstract: Cooperation in evolutionary games can be stabilized through punishment of non-cooperators, at a cost to those who do the punishing. Punishment can take different forms, in particular peer-punishment, in which individuals punish free-riders after the event, and pool-punishment, in which a fund for sanctioning is set up beforehand. These authors show that pool-punishment is superior to peer-punishment in dealing with second-order free-riders, who cooperate in the main game but refuse to contribute to punishment. Cooperation in evolutionary games can be stabilized by the punishment of non-cooperators, at a cost to those doing the punishing. It can take various forms, including peer punishment, where individuals punish free-riders after the event, and pool punishment, where a fund for sanctioning is set up beforehand. The former can be summed up as 'taking the law into your own hands', the latter as having police to do the job. Using a computational model, Sigmund et al. show that pool punishment has the edge over peer punishment in dealing with second-order free-riders — those who cooperate in the main game but refuse to contribute to punishment. The model shows that individuals can spontaneously adopt a self-governing institution to monitor contributions and sanction free-riders. It needs no top-down prescriptions or planning. Trial and error, and the imitation of successful examples, suffice to produce a social contract between individuals guided by self-interest. Theoretical and empirical research highlights the role of punishment in promoting collaborative efforts1,2,3,4,5. However, both the emergence and the stability of costly punishment are problematic issues. It is not clear how punishers can invade a society of defectors by social learning or natural selection, or how second-order free-riders (who contribute to the joint effort but not to the sanctions) can be prevented from drifting into a coercion-based regime and subverting cooperation. Here we compare the prevailing model of peer-punishment6,7,8 with pool-punishment, which consists in committing resources, before the collaborative effort, to prepare sanctions against free-riders. Pool-punishment facilitates the sanctioning of second-order free-riders, because these are exposed even if everyone contributes to the common good. In the absence of such second-order punishment, peer-punishers do better than pool-punishers; but with second-order punishment, the situation is reversed. Efficiency is traded for stability. Neither other-regarding tendencies or preferences for reciprocity and equity, nor group selection or prescriptions from higher authorities, are necessary for the emergence and stability of rudimentary forms of sanctioning institutions regulating common pool resources and enforcing collaborative efforts.

Histone H3 lysine 9 methylation is an epigenetic imprint of facultative heterochromatin.

Abstract: Post-translational modifications of histone amino termini are an important regulatory mechanism that induce transitions in chromatin structure, thereby contributing to epigenetic gene control and the assembly of specialized chromosomal subdomains. Methylation of histone H3 at lysine 9 (H3-Lys9) by site-specific histone methyltransferases (Suv39h HMTases) marks constitutive heterochromatin. Here, we show that H3-Lys9 methylation also occurs in facultative heterochromatin of the inactive X chromosome (Xi) in female mammals. H3-Lys9 methylation is retained through mitosis, indicating that it might provide an epigenetic imprint for the maintenance of the inactive state. Disruption of the two mouse Suv39h HMTases abolishes H3-Lys9 methylation of constitutive heterochromatin but not that of the Xi. In addition, HP1 proteins, which normally associate with heterochromatin, do not accumulate with the Xi. These observations suggest the existence of an Suv39h-HP1-independent pathway regulating H3-Lys9 methylation of facultative heterochromatin.

Unifying time evolution and optimization with matrix product states

Abstract: We show that the time-dependent variational principle provides a unifying framework for time-evolution methods and optimization methods in the context of matrix product states. In particular, we introduce a new integration scheme for studying time evolution, which can cope with arbitrary Hamiltonians, including those with long-range interactions. Rather than a Suzuki-Trotter splitting of the Hamiltonian, which is the idea behind the adaptive time-dependent density matrix renormalization group method or time-evolving block decimation, our method is based on splitting the projector onto the matrix product state tangent space as it appears in the Dirac-Frenkel time-dependent variational principle. We discuss how the resulting algorithm resembles the density matrix renormalization group (DMRG) algorithm for finding ground states so closely that it can be implemented by changing just a few lines of code and it inherits the same stability and efficiency. In particular, our method is compatible with any Hamiltonian for which ground-state DMRG can be implemented efficiently. In fact, DMRG is obtained as a special case of our scheme for imaginary time evolution with infinite time step.