Archive for biking
no mountain, no matter [blue wave#2]
Posted in Mountains, pictures, Running with tags Élancourt, bike crash, biking, France, gold medal, mountain biking, mountains, MTB, Paris, Paris 2024 Olympics, Pauline Ferrand-Prévot on July 30, 2024 by xi'an
simulation as optimization [by kernel gradient descent]
Posted in Books, pictures, Statistics, University life with tags ABC, biking, Charles Stein, CREST, diffusions, discrepancies, Edo, Gare de Lyon, gradient descent, Hiroshige, INRIA, kernel Stein discrepancy descent, Kullback-Leibler divergence, maximum mean discrepancy, MCMC, Mokaplan, mollified discrepancy, New York city, One Hundred Famous Views of Edo, optimal transport, optimisation, Paris, simulation, SMC, Stein kernel on April 13, 2024 by xi'an
Yesterday, which proved an unseasonal bright, warm, day, I biked (with a new wheel!) to the east of Paris—in the Gare de Lyon district where I lived for three years in the 1980’s—to attend a Mokaplan seminar at INRIA Paris, where Anna Korba (CREST, to which I am also affiliated) talked about sampling through optimization of discrepancies.
This proved a most formative hour as I had not seen this perspective earlier (or possibly had forgotten about it). Except through some of the talks at the Flatiron Institute on Transport, Diffusions, and Sampling last year. Incl. Marilou Gabrié’s and Arnaud Doucet’s.
The concept behind remains attractive to me, at least conceptually, since it consists in approximating the target distribution, known up to a constant (a setting I have always felt standard simulation techniques was not exploiting to the maximum) or through a sample (a setting less convincing since the sample from the target is already there), via a sequence of (particle approximated) distributions when using the discrepancy between the current distribution and the target or gradient thereof to move the particles. (With no randomness in the Kernel Stein Discrepancy Descent algorithm.)
Ana Korba spoke about practically running the algorithm, as well as about convexity properties and some convergence results (with mixed performances for the Stein kernel, as opposed to SVGD). I remain definitely curious about the method like the (ergodic) distribution of the endpoints, the actual gain against an MCMC sample when accounting for computing time, the improvement above the empirical distribution when using a sample from π and its ecdf as the substitute for π, and the meaning of an error estimation in this context.
“exponential convergence (of the KL) for the SVGD gradient flow does not hold whenever π has exponential tails and the derivatives of ∇ log π and k grow at most at a polynomial rate”
double-parked [cover]
Posted in Books, Kids, pictures, Running, Travel with tags art, beach, bicycle, biking, cover, Fort Tilden beach, New York city, Rockaways, summer, The New Yorker on August 24, 2022 by xi'an
`Paris is in anarchy’ [cycle woes]
Posted in Travel with tags bicycle, biking, cycle path, Eiffel Tower, New York, NYT, Paris, puncture, repair, Seine, Université Paris Dauphine on October 7, 2021 by xi'an
An overblown view of the cycling war in Paris, from New York! I read with amusement the report on how Xing a Parisian street is a matter of life or death, when anarclists go through red lights while shouting at pedestrians… Actually, the figures show that the number of accidents involving cyclists (as victims or culprits) has only gone up by 30% when the traffic has increased by 70%. And I could not find an online trace of a pedestrian killed by a cyclist over the past years. Based on my weekly 130 kilometer biking average, mostly to and from Paris Dauphine, I do not perceive a major tension between pedestrians and cyclists, maybe because I am not entering the centre of town (and give priority to pedestrians at both green and red lights). The danger in my experience comes rather from other cyclists’ unpredictable paths, (psychopath) mopeds that run on cycle paths, and cars turning right without checking for bicycles. But I concur with the point made in this article of a poor network of cycle paths, with too many discontinuities, bad surface, inexistent maintenance (esp. in winter months when wet leaves accumulate there and all year long for broken glass and metal parts), and the deadly pavés! Which are unpleasant for road bikes (ask the Paris-Roubaix runners!), slippery, esp. when frosted (speaking from experience), and damaging to tubes and ties. As it happens, I have had thee tube punctures over the three past weeks, two of which were due to running over a particularly uneven pavé or entering a cycle path with a very high step. (And a total of six since April. Making me reconsider using an heavier mountain bike instead. After switching unsuccessfully to anti-puncture road tyres…)
ring of fire [jatp]
Posted in Mountains, pictures, Travel, University life with tags biking, Institut de Physique du Globe, Jardin des Plantes, jatp, Jussieu, neon, Pacific Rim, Paris, Ring of Fire, sculpture, Université de Paris, volcanoes on September 4, 2021 by xi'anWhile leisurely biking in Paris the previous (grey) weekend, my wife and I stopped by the entrance to the Jardin des Plantes, in front of a (grey) renovated building of l’Institut de physique du globe, where we both attended (maths) lectures in the 1980’s. I had noticed the renovation years ago and in particular the red neon fractured line going over the faces of the building, but never considered it had a specific meaning.
In fact, the apparent random walk has nothing random as it reproduces the Ring of Fire, a Fuller projection of the continuum of volcanoes that ring the Pacific Ocean. (This sculpture was created by Angela Detanico et Rafael Lain.)
