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## Recap Lecture 11

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**Recap Lecture 11**• Simulate a particle of systems at equilibrium • Know particle positions and energy function • At each time step have a snap shot of particle positions and energy • Particles correspond to random variables • P(s)=1/Z.Exp(-E(s) / T), Z=Sum_s exp(-E(s) / T • Free energy F = -T.log(Z) = <E> - T.H • Three core problems: inference, learning, model class selection. CSI 661 - Uncertainty in A.I. Lecture 12**Base Problems To Solve**• Calculating expectation of a function • Inference • Optimal Bayesian Classifier • Calculating maximum function value • Finding mode • Multiple modes • Figure 3.1 Neal CSI 661 - Uncertainty in A.I. Lecture 12**Stochastic Algorithmic Solutions**• Numerical Methods: Exact solutions • Rejection sampling • Importance sampling • Methods to find the posterior mode • Markov Chain Monte Carlo Sampling CSI 661 - Uncertainty in A.I. Lecture 12**Markov Chains**• Invariant/stationary distributions • Ergodic chains • Irreducible, aperiodic, one stationary distribution. • Computational perspective: • how quickly moves between states can be generated, • how quickly equilibrium is reached from any initial state, • the number of moves required to move from state x to a state y which is independent of x. CSI 661 - Uncertainty in A.I. Lecture 12**Markov Chains**• A Markov chain consists of a series of random variables X(0), X(1), X(2), X(3), … X(t) • P(x(t+1) | x(t), x(t-1),x(t-2) … x(0)) = P(x(t+1) | x(t)). • X(0) is known as the initial distribution and the distribution for X(t+1) • We begin in a randomly drawn state from the initial distribution and move around the state space according to the state transition probabilities • Homogenous versus heterogeneous CSI 661 - Uncertainty in A.I. Lecture 12**Markov Chain Properties**CSI 661 - Uncertainty in A.I. Lecture 12**Markov Chain Properties II**• It will converge to the equilibrium distribution as t regardless of X(0) CSI 661 - Uncertainty in A.I. Lecture 12**Metropolis Algorithm**CSI 661 - Uncertainty in A.I. Lecture 12**Hastings Generalization**CSI 661 - Uncertainty in A.I. Lecture 12**BBN Learning Example**CSI 661 - Uncertainty in A.I. Lecture 12**Statistical Physics - Background**• Micro-state versus Macro-state • Micro-state unknown, partial knowledge • P(s) = 1/Z . Exp(-E(s) / T), Z = Sum_s exp(-E(s) / T • Known as the Gibbs, Boltzmann, canonical, equilibrium distribution • Equivalent to what? • Intensive versus Extensive quantities (grow linearly with system size) • Extensive quantities per unit/particle reach constant limits • Interested in systems at thermodynamic equilibrium • Macroscopic properties can be expressed as expectations CSI 661 - Uncertainty in A.I. Lecture 12**Ising Spin Glass Model**Phase transitions CSI 661 - Uncertainty in A.I. Lecture 12**Free Energy of a SystemRelationship to A.I.**• Free energy F = -T.log(Z) = <E> - T.H • Z is the partition function, T temperature • H the system entropy • F and H are extensive values • (cf. Slide 1) What are the analagous particles for: • Parameter estimation (learning) • Inference • Model class selection CSI 661 - Uncertainty in A.I. Lecture 12**Next Lecture**• Read, Neal, Chapter 2 CSI 661 - Uncertainty in A.I. Lecture 12