I work with Statistical Mechanics and Information Theory applied to information processing systems such as Neural Networks and Societies of agents modeled by NN.I am a professor at the Physics Institute, Universidade de Sao Paulo andhave a B.Sc in Physics from UNICAMP and a PhD from Caltech.
Since 1982, starting with the work of Hopfield, theoretical physics is contributing to the theory... more Since 1982, starting with the work of Hopfield, theoretical physics is contributing to the theory of neural networks. In his pioneering work, Hopfield pointed out a relation between models of disordered magnets (spin glasses) and models of neurons interacting by competing synaptic couplings. This work started an extensive research effort: using models, methods and principles of statistical physics one has described the cooperative behavior of a large system of interacting neurons. Now, almost two decades later, much has been achieved in this field: associative memory, learning from examples, generalization from examples to an unknown rule, time series prediction, optimizing architectures and learning rules, all this has been expressed in a mathematical language which allows to calculate the cooperative properties of infinitely large systems being trained on infinitely many patterns [1, 2].
Empirical evidence suggests that social structure may have changed from hierarchical to egalitari... more Empirical evidence suggests that social structure may have changed from hierarchical to egalitarian and back along the evolutionary line of humans. We model a society subject to competing cognitive and social navigation constraints. The theory predicts that the degree of hierarchy decreases with encephalization and increases with group size. Hence hominin groups may have been driven from a phase with hierarchical order to a phase with egalitarian structures by the encephalization during the last two million years, and back to hierarchical due to fast demographical changes during the Neolithic. The dynamics in the perceived social network shows evidence in the egalitarian phase of the observed phenomenon of Reverse Dominance. The theory also predicts for modern hunter-gatherers in mild climates a trend towards an intermediate hierarchy degree and a phase transition for harder ecological conditions. In harsher climates societies would tend to bemore egalitarian if organized in small g...
Resumo Anedotas sobre Feynman são abundantes e cada um que teve contato com ele, das mais diversa... more Resumo Anedotas sobre Feynman são abundantes e cada um que teve contato com ele, das mais diversas formas, tem algo a contar. Este texto não inclui nenhuma física e nem o propósito de discutir algo técnico. Simplesmente traz algumas lembranças das aulas de física teórica dadas por Richard P. Feynman e do ambiente no California Institute of Technology no começo da década de oitenta. Mais do que técnicas, a mensagem transmitida era sobre o prazer de fazer ciência, sobre a coragem frente à dúvida e sobre a honestidade intelectual. As citações são traduações do original na minha memória.
Interacting NN are used to model US Appellate Court three judge panels. Agents, whose initial sta... more Interacting NN are used to model US Appellate Court three judge panels. Agents, whose initial states have three contributions derived from common knowledge of the law, political affiliation and personality, learn by exchange of opinions, updating their state and trust about other agents. The model replicates data patterns only if initially the agents trust each other and are certain about their trust independently of party affiliation, showing evidence of ideological voting, dampening and amplification. Absence of law or party contribution destroys the theoretical-empirical agreement. We identify quantitative signatures for different levels of the law, ideological or idiosyncratic contributions.
As proteinas com atividade enzimatica podem ser divididas em seis grandes classes de acordo com s... more As proteinas com atividade enzimatica podem ser divididas em seis grandes classes de acordo com suas funcoes especificas, que sao: oxirredutases, hidrolases, transferases, lyases, isomerases e ligases. Utilizando algoritmo de agrupamento nao-parametrico, este trabalho tem como objetivo predizer diferentes classes enzimaticas usando parâmetros estruturais e fisico-quimicos.
We model a society of agents that interact in pairs by exchanging for/against opinions about issu... more We model a society of agents that interact in pairs by exchanging for/against opinions about issues using an algorithm obtained with methods of Bayesian inference and maximum entropy. The agents gauge the incoming information with respect to the mistrust attributed to the other agents. There is no underlying lattice and all agents interact among themselves. The interaction pair can be described as a dynamics along the gradient of the logarithm of the evidence. By using a symmetric version of the two-body interactions we introduce a Hamiltonian for the whole society. Knowledge of the expected value of the Hamiltonian is relevant information for the state of the society. In the case of uniform mistrust, independent of the pair of agents, the phase diagram of the society in a mean field approximation shows a phase transition that separates an ordered phase where opinions are to a large extent shared by the agents and a disordered phase of dissension of opinions.
While social interactions tend to decrease differences in opinions, multiplicity of groups and in... more While social interactions tend to decrease differences in opinions, multiplicity of groups and individual opinion differences persist in human societies. Axelrod identified homophily and social conformity seeking as basic interactions that can lead to multiculturalism in spatial scenarios in models under certain special conditions. We follow another route, where the social interaction between any two agents is given by the descent along the gradient of a cost function deduced from a Bayesian learning formalism. The cost function depends on a hyperparameter that estimates the trust of one agent in the information provided by the other. If the expected value of the total cost function is relevant information, Maximum Entropy permits characterizing the state of the society. Furthermore we introduce a dynamics on the trust parameters, which increases when agents concur and decreases otherwise. We study the resulting phase diagram in the case of large number of interacting agents on a complete social graph, hence under sympatric conditions. Simulations show that there is evolution of assortative distrust in rich cultural environments measured by the diversity of the set of issues under discussions. High distrust leads to antilearning which leads to multiple groups which hold different opinions on the set of issues. We simulate conditions of political pressure and interaction that describe the House of Congress of Brazil and are able to qualitatively replicate voting patterns through four presidential cycles during the years of 1994 to 2010.
Since 1982, starting with the work of Hopfield, theoretical physics is contributing to the theory... more Since 1982, starting with the work of Hopfield, theoretical physics is contributing to the theory of neural networks. In his pioneering work, Hopfield pointed out a relation between models of disordered magnets (spin glasses) and models of neurons interacting by competing synaptic couplings. This work started an extensive research effort: using models, methods and principles of statistical physics one has described the cooperative behavior of a large system of interacting neurons. Now, almost two decades later, much has been achieved in this field: associative memory, learning from examples, generalization from examples to an unknown rule, time series prediction, optimizing architectures and learning rules, all this has been expressed in a mathematical language which allows to calculate the cooperative properties of infinitely large systems being trained on infinitely many patterns [1, 2].
Empirical evidence suggests that social structure may have changed from hierarchical to egalitari... more Empirical evidence suggests that social structure may have changed from hierarchical to egalitarian and back along the evolutionary line of humans. We model a society subject to competing cognitive and social navigation constraints. The theory predicts that the degree of hierarchy decreases with encephalization and increases with group size. Hence hominin groups may have been driven from a phase with hierarchical order to a phase with egalitarian structures by the encephalization during the last two million years, and back to hierarchical due to fast demographical changes during the Neolithic. The dynamics in the perceived social network shows evidence in the egalitarian phase of the observed phenomenon of Reverse Dominance. The theory also predicts for modern hunter-gatherers in mild climates a trend towards an intermediate hierarchy degree and a phase transition for harder ecological conditions. In harsher climates societies would tend to bemore egalitarian if organized in small g...
Resumo Anedotas sobre Feynman são abundantes e cada um que teve contato com ele, das mais diversa... more Resumo Anedotas sobre Feynman são abundantes e cada um que teve contato com ele, das mais diversas formas, tem algo a contar. Este texto não inclui nenhuma física e nem o propósito de discutir algo técnico. Simplesmente traz algumas lembranças das aulas de física teórica dadas por Richard P. Feynman e do ambiente no California Institute of Technology no começo da década de oitenta. Mais do que técnicas, a mensagem transmitida era sobre o prazer de fazer ciência, sobre a coragem frente à dúvida e sobre a honestidade intelectual. As citações são traduações do original na minha memória.
Interacting NN are used to model US Appellate Court three judge panels. Agents, whose initial sta... more Interacting NN are used to model US Appellate Court three judge panels. Agents, whose initial states have three contributions derived from common knowledge of the law, political affiliation and personality, learn by exchange of opinions, updating their state and trust about other agents. The model replicates data patterns only if initially the agents trust each other and are certain about their trust independently of party affiliation, showing evidence of ideological voting, dampening and amplification. Absence of law or party contribution destroys the theoretical-empirical agreement. We identify quantitative signatures for different levels of the law, ideological or idiosyncratic contributions.
As proteinas com atividade enzimatica podem ser divididas em seis grandes classes de acordo com s... more As proteinas com atividade enzimatica podem ser divididas em seis grandes classes de acordo com suas funcoes especificas, que sao: oxirredutases, hidrolases, transferases, lyases, isomerases e ligases. Utilizando algoritmo de agrupamento nao-parametrico, este trabalho tem como objetivo predizer diferentes classes enzimaticas usando parâmetros estruturais e fisico-quimicos.
We model a society of agents that interact in pairs by exchanging for/against opinions about issu... more We model a society of agents that interact in pairs by exchanging for/against opinions about issues using an algorithm obtained with methods of Bayesian inference and maximum entropy. The agents gauge the incoming information with respect to the mistrust attributed to the other agents. There is no underlying lattice and all agents interact among themselves. The interaction pair can be described as a dynamics along the gradient of the logarithm of the evidence. By using a symmetric version of the two-body interactions we introduce a Hamiltonian for the whole society. Knowledge of the expected value of the Hamiltonian is relevant information for the state of the society. In the case of uniform mistrust, independent of the pair of agents, the phase diagram of the society in a mean field approximation shows a phase transition that separates an ordered phase where opinions are to a large extent shared by the agents and a disordered phase of dissension of opinions.
While social interactions tend to decrease differences in opinions, multiplicity of groups and in... more While social interactions tend to decrease differences in opinions, multiplicity of groups and individual opinion differences persist in human societies. Axelrod identified homophily and social conformity seeking as basic interactions that can lead to multiculturalism in spatial scenarios in models under certain special conditions. We follow another route, where the social interaction between any two agents is given by the descent along the gradient of a cost function deduced from a Bayesian learning formalism. The cost function depends on a hyperparameter that estimates the trust of one agent in the information provided by the other. If the expected value of the total cost function is relevant information, Maximum Entropy permits characterizing the state of the society. Furthermore we introduce a dynamics on the trust parameters, which increases when agents concur and decreases otherwise. We study the resulting phase diagram in the case of large number of interacting agents on a complete social graph, hence under sympatric conditions. Simulations show that there is evolution of assortative distrust in rich cultural environments measured by the diversity of the set of issues under discussions. High distrust leads to antilearning which leads to multiple groups which hold different opinions on the set of issues. We simulate conditions of political pressure and interaction that describe the House of Congress of Brazil and are able to qualitatively replicate voting patterns through four presidential cycles during the years of 1994 to 2010.
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Papers by Nestor Caticha