Archive for blog
what is the caption?!
Posted in Books with tags blog, cartoon, competition, maths humour, The New Yorker, Xi'an's Og on July 27, 2023 by xi'an
the alpinist [film review]
Posted in Books, Kids, Mountains, pictures, Running with tags Alaska, alpinism, Alpinist, blog, Canada, Cerro Torre, Dawn Wall, El Capitan, film review, free climbing, ice climbing, ice-axe, Jeddah, Juneau, Marc-André Leclerc, Mendenhall Glacier, Mount Robson, Patagonia, solo climbing, Squamish, Vancouver on June 1, 2023 by xi'an
Watched (with supplementary oxygen) The Alpinist in the plane to Jeddah. It is a documentary (made by the same filmmakers who filmed the Dawn Wall) about the amazing Canadian alpinist Marc-André Leclerc, who died in 2018 on the Mendenhall Glacier, Alaska, in an avalanche, after achieving extraordinary complex solo climbs as eg on Mount Robbson, Cerro Torre in Patagonia. These are winter climbs, partly ice climbing, where no repetition is possible and where the objective conditions (hence dangers) may vary considerably. In that regard, these achievements could be argued to go even beyond Alex Honnold’s free solo climb of El Capitan, where Honnold practiced the route over and over before making his successful free solo attempt. (Obviously an inhuman achievement when considering the hardest bits are at least 7c!) Watching Marc-André Leclerc when mixed climbing is just as heart stopping as watching Honnold rock climbing. He must have been incredibly strong to master these monstruous icy walls and maintain his absolute vigilance in each crampon move, in each ice-pick placement. Sadly it ended up with an avalanche… I obviously enjoyed X’ing many places I had visited, like the approach walk to Robson (in 1991!), the Three Sisters of Canmore [below], and routes of Squamish [above]. (I wonder who filmed during these non-advertised climbs. For instance, he told no one except his partner when he summited Mount Robson. In some cases he was clearly self-filing at lower intensity points, but in others it could have been an helicopter (or a drone?). In this respect, but by far not only in this respect, his blog is definitely worth the read.)
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a perfectly normally distributed sample
Posted in R, Statistics with tags bayestestR, blog, R, R-bloggers, random number generator, rnorm(), rnorm_perfect(), sample, sampling on May 9, 2019 by xi'anWhen I saw this title on R-bloggers, I was wondering how “more perfect” a Normal sample could be when compared with the outcome of rnorm(n). Hence went checking the original blog on bayestestR in search of more information. Which was stating nothing more than how to generate a sample is perfectly normal by using the rnorm_perfect function. Still unsure of the meaning, I contacted one of the contributors who replied very quickly
…that’s actually a good question. I would say an empirical sample having characteristics as close as possible to a cannonic gaussian distribution.
and again leaving me hungering for more details. I thus downloaded the package
bayestestR and opened the rnorm_perfect function. Which is simply the sequence of n-quantilesstats::qnorm(seq(1/n, 1 – 1/n, length.out = n), mean, sd)
which I would definitely not call a sample as it has nothing random. And perfect?! Not really, unless one associates randomness and imperfection.
multiplying a Gaussian matrix and a Gaussian vector
Posted in Books with tags Bessel functions, blog, cross validated, Gaussian matrix, Laplace distribution, Wishart distribution on March 2, 2017 by xi'anThis arXived note by Pierre-Alexandre Mattei was actually inspired by one of my blog entries, itself written from a resolution of a question on X validated. The original result about the Laplace distribution actually dates at least to 1932 and a paper by Wishart and Bartlett!I am not sure the construct has clear statistical implications, but it is nonetheless a good calculus exercise.
The note produces an extension to the multivariate case. Where the Laplace distribution is harder to define, in that multiple constructions are possible. The current paper opts for a definition based on the characteristic function. Which leads to a rather unsavoury density with Bessel functions. It however satisfies the constructive definition of being a multivariate Normal multiplied by a χ variate plus a constant vector multiplied by the same squared χ variate. It can also be derived as the distribution of
Wy+||y||²μ
when W is a (p,q) matrix with iid Gaussian columns and y is a Gaussian vector with independent components. And μ is a vector of the proper dimension. When μ=0 the marginals remain Laplace.
a new Editor for Series B
Posted in Statistics with tags blog, JRSSB, Royal Statistical Society, Series B on January 16, 2017 by xi'an
As every odd year, the Royal Statistical Society is seeking a new joint editor for Series B! After four years of dedication to the (The!) journal, Piotr Fryzlewicz is indeed going to retire from this duty by the end of 2017. Many thanks to Piotr for his unfailing involvement in Series B and the preservation of its uncompromising selection of papers! The call thus open for candidates for the next round of editorship, from 2018 to 2021, with a deadline of 31 January, 2017. Interested candidates should contact Martin Owen, at the Society’s address or by email at rss.org.uk with journal as recipient (local-part). The new editor will work with the current joint editor, David Dunson, whose term runs till December 2019. (I am also looking forward working with Piotr’s successor in developing the Series B blog, Series’ Blog!)
Xi'an's Og 