Archive for Reuven Rubinstein

X entropy for optimisation

Posted in Books, pictures, Statistics, Travel, University life with tags , , , , , , , , , , , on March 29, 2018 by xi'an

At Gregynog, with mounds of snow still visible in the surrounding hills, not to be confused with the many sheep dotting the fields(!), Owen Jones gave a three hour lecture on simulation for optimisation, which is a less travelled path when compared with simulation for integration. His second lecture covered cross entropy for optimisation purposes. (I had forgotten that Reuven Rubinstein and Dirk Kroese had put forward this aspect of their technique in the very title of their book. As “A Unified Approach to Combinatorial Optimization, Monte-Carlo Simulation and Machine Learning”.) The X entropy approaches pushes for simulations restricted to top values of the target function, iterating to find the best parameter in the parametric family used for the simulation. (Best to be understood in the Kullback sense.) Now, this is a wee bit like simulated annealing, where lots of artificial entities have to be calibrated in the algorithm, due to the original problem being unrelated to an specific stochastic framework. X entropy facilitates concentration on the highest values of the target, but requires a family of probability distributions that puts weight on the top region. This may be a damning issue in large dimensions. Owen illustrated the approach in the case of the travelling salesman problem, where the parameterised distribution is a Markov chain on the state space of city sequences. Further, if the optimal value of the target is unknown, avoiding getting stuck in a local optimum may be tricky. (Owen presented a proof of convergence for a temperature going to zero slowly enough that is equivalent to a sure exploration of the entire state space, in a discrete setting, which does not provide a reassurance in this respect, as the corresponding algorithm cannot be implemented.) This method falls into the range of methods that are doubly stochastic in that they rely on Monte Carlo approximations at each iteration of the exploration algorithm.

During a later talk, I tried to recycle one of my earlier R codes on simulated annealing for sudokus, but could not find a useful family of proposal distributions to reach the (unique) solution. Using a mere product of distributions on each of the free positions in the sudoku grid only led me to a penalty of 13 errors…

1    2    8    5    9    7    4    9    3
7    3    5    1    2    4    6    2    8
4    6    9    6    3    8    5    7    1
2    7    5    3    1    6    9    4    8
8    1    4    7    8    9    7    6    2
6    9    3    8    4    2    1    3    5
3    8    6    4    7    5    2    1    9
1    4    2    9    6    3    8    5    7
9    5    7    2    1    8    3    4    6

It is hard to consider a distribution on the space of permutations, 𝔖⁸¹.

Reuven Rubinstein (1938-2012)

Posted in Statistics with tags , , , , , , , , on December 10, 2012 by xi'an

I just learned last night that Professor Reuven Rubinstein passed away. While I was not a close collaborator of him, I met Reuven Rubinstein a few times at conferences and during a short visit to Paris, and each time learned from the encounter. I also appreciated his contributions to the field of simulation, esp. his cross-entropy method that culminated in the book The Cross-Entropy Method with Dirk Kroese. Reuven was involved in many aspects of simulation along his prolific career, he will be especially remembered for his 1981 book Simulation and the Monte Carlo Method that is arguably the very first book on simulation as a Monte Carlo method. This book had a recent second edition, co-authored with Dirk Kroese as well. It is thus quite a sad day to witness this immense contributor to the field leave us. (Here is a link to his webpage at Technion, including pictures of a trip to the Gulag camp where he spent most of his childhood.) I presume there will be testimonies about his influence at the WSC 2012 conference here in Berlin.

workshop in Columbia

Posted in Statistics, Travel, University life with tags , , , , , , , , , on September 24, 2011 by xi'an

The workshop in Columbia University on Computational Methods in Applied Sciences is quite diverse in its topics.  Reminding me of the conference on Efficient Monte Carlo in Sandbjerg Estate, Sønderborg in 2008, celebrating the 70th birthday of Reuven Rubinstein, incl. some colleagues I had not met since this meeting. Yesterday I thus heard (quite interesting) talks on domains somehow far from my own, from Robert Adler on cohomology (giving a second look  at the thing after the talk I head in Wharton last year), to José Blanchet on simulation for infinite server queues (with a link to perfect sampling I could not exactly trace but that was certainly there). Several of the talks made me think of our Brownian motion confidence band paper, with Wilfrid Kendall and Jean-Michel Marin, esp. Gennady Samorodnitsky’s on the maximum of stochastic processes (and wonder whether we could have gone further in that direction). Pierre Del Moral presented a broad overview of the Feynman-Kacs’ approaches to particle methods, in particular particle MCMC, with application to some financial objects. Paul Glasserman talked about robust MCMC, which I found quite an appealing concept in that it included uncertainties about the model itself. And linked with minimax concepts. And Paul Dupuis exposed a parallel tempering method linked with large deviations, whose paper I am definitely looking forward. Now it is more than time to work on my own talk! (On a very personal basis, I sadly lost my sturdy Canon camera in the taxi from the airport! Will need a new one for the ‘Og!)