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Distractions and amusements, with a sandwich and coffee.

Sun is on my face ...a beautiful day without you.
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On March 14th celebrate `\pi` Day. Hug `\pi`—find a way to do it.

For those who favour `\tau=2\pi` will have to postpone celebrations until July 26th. That's what you get for thinking that `\pi` is wrong. I sympathize with this position and have `\tau` day art too!

If you're not into details, you may opt to party on July 22nd, which is `\pi` approximation day (`\pi` ≈ 22/7). It's 20% more accurate that the official `\pi` day!

Finally, if you believe that `\pi = 3`, you should read why `\pi` is not equal to 3.

How many ages hence

Shall this our lofty scene be acted over

In states unborn and accents yet unknown!

—Willian Shakespeare (Julius Caesar, Act III, Scene 1)

Welcome to this year's celebration of `\pi` and mathematics.

The theme this year is typographical and pure—in contrast to last year's extremely elaborate computational art. This year is also the first time I have made a special kids' edition!

This year's poem is by Viorica Hrincu and it is about complications.

This year's `\pi` day song is Tshinanu by Kashtin.

If you enjoy art based on type, explore my other typographical works.

Several teachers have reached out to me in the past and asked for art to hang in their classrooms. So I thought what better way to get kids excited and talking (and reading) about math than with an explosion of colors and phat fonts.

The international kids' poster uses the following languages: Afrikaans, Albanian, Azerbaijani, Basque, Catalan, Cebuano, Corsican, Croatian, Danish, Dothraki, Dutch, English, Esperanto, Estonian, Finnish, French, Frisian, Galician, German, Gongduk, Haitian/Creole, Hausa, Hawaiian, Huastec/Mayan, Icelandic, Indonesian, Irish, Italian, Khasi, Ladan, Latin, Latvian, Lithuanian, Luxembourgish, Malagasy, Maltese, Maori, Nao, Norwegian, Polish, Portuguese, Puyuma, Ro, Samoan, Scots/Gaelic, Sesotho, Shona, Slovak, Slovenian, Solon, Solresol, Somali, Spanish, Swahili, Swedish, Tagalog, Tenerife, Tetun, Turkish, Welsh, Xhosa, Yoruba, Zenaga, and Zulu.

Some of these languages are fictional or engineered—I'll leave it to you to find them.

The posters read out the digits of `\pi` in a variety of languages. For this version of the art, I've selected language digit words that have no diacritical marks—to fit the letters more tightly.

Adults shouldn't feel left out—they're free to enjoy the `\pi` Day kids' editions (I know I want to).

However, for the more discerning typographer in you (if Granby Elephant is too bloated for your eyes), I have prepared something slimmer.

These posters spell out the digits of `\pi` in a variety of languages and alphabets. I'll leave you to work out the rule for the red highlights.

The following languages are used: Afrikaans, Ainu, Albanian, Amharic, Arabic, Armenian, Avestan, Azerbaijani, Balinese, Banjar, Bengali, Biblical Arameic, Buginese, Bulgarian, Bumthang, Catalan, Cherokee, Chinese, Cia-cia, Coptic, Czech, Danish, Dothraki, English, Esperanto, Etruscan, Frisian, Galician, Gaulish, Georgian, Georgian Old, Gothic, Greek Old, Gujarati, Hebrew, Hindi, Hopi, Hungarian, Igbo, Inuktitut, Irish, Japanese Sino, Javanese, Kannada, Kazakh, Khmer, Klallam, Korean, Korean Sino, Kyrgyz, Ladan, Ladino, Lao, Latvian, Macedonian, Malagasy, Malay, Malayalam, Mandaic, Mongolian Classical, Myanmar, Norse, Norwegian, Old English, Old Turkic, Pashto, Persian, Phoenician, Proto Germanic, Punjabi, Ro, Romanian, Samaritan, Samoan, Sanskrit, Sesotho, Sindhi, Sinhala, Slavonic, Slovak, Slovenian, Spanish, Sundanese, Swahili, Swedish, Sylheti, Syriac, Tajik, Telugu, Tetun, Thai, Thompson, Turkish, Ugaritic, Urdu, Welsh, Wyandot, Xhosa, Yi, Yiddish, and Yonaguni.

The typeface is Helvetica Neue or Noto Sans.

In another version of the poster, instead of using words to spell out the digits, I use digit glyphs from various alphabets. Included in this version are also any single-glyph words for the digit.

The digit glyph poster uses Adlam, Ainu, Arabic, Balinese, Bengali, Brahmi, Burmese, Chakma, Cham, Chinese, Coptic, Ethiopic, Gujaranti, Gujarati, Gurmukhi, Hebrew, Japanese Sino, Javanese, Kannada, Kayahli, Kharoshthi, Khmer, Klingon, Korean, Koreansino, Lao, Lepcha, Limbu, Malayalam, Meetei Mayek, Mongolian, Nko, Odia, Olchiki, Oriya, Osmanya, Punjabi, Roman, Rumi, Sanskrit, Saurashtra, Shan, Sinhala, Sinhala Archaic, Sumerian, Sundanese, Tamil, Telugu, Thai, Tibetan, Vai, Yi, and Yonaguni.

Discover Cantor's transfinite numbers through my music video for the Aleph 2 track of Max Cooper's Yearning for the Infinite (album page, event page).

I discuss the math behind the video and the system I built to create the video.

*Everything we see hides another thing, we always want to see what is hidden by what we see.
—Rene Magritte*

A Hidden Markov Model extends a Markov chain to have hidden states. Hidden states are used to model aspects of the system that cannot be directly observed and themselves form a Markov chain and each state may emit one or more observed values.

Hidden states in HMMs do not have to have meaning—they can be used to account for measurement errors, compress multi-modal observational data, or to detect unobservable events.

In this column, we extend the cell growth model from our Markov Chain column to include two hidden states: normal and sedentary.

We show how to calculate forward probabilities that can predict the most likely path through the HMM given an observed sequence.

Grewal, J., Krzywinski, M. & Altman, N. (2019) Points of significance: Hidden Markov Models. *Nature Methods* **16**:795–796.

Altman, N. & Krzywinski, M. (2019) Points of significance: Markov Chains. *Nature Methods* **16**:663–664.

My cover design for Hola Mundo by Hannah Fry. Published by Blackie Books.

Curious how the design was created? Read the full details.

*You can look back there to explain things,
but the explanation disappears.
You'll never find it there.
Things are not explained by the past.
They're explained by what happens now.
—Alan Watts*

A Markov chain is a probabilistic model that is used to model how a system changes over time as a series of transitions between states. Each transition is assigned a probability that defines the chance of the system changing from one state to another.

Together with the states, these transitions probabilities define a stochastic model with the Markov property: transition probabilities only depend on the current stateâ€”the future is independent of the past if the present is known.

Once the transition probabilities are defined in matrix form, it is easy to predict the distribution of future states of the system. We cover concepts of aperiodicity, irreducibility, limiting and stationary distributions and absorption.

This column is the first part of a series and pairs particularly well with Alan Watts and Blond:ish.

Grewal, J., Krzywinski, M. & Altman, N. (2019) Points of significance: Markov Chains. *Nature Methods* **16**:663–664.

*Places to go and nobody to see.*

Exquisitely detailed maps of places on the Moon, comets and asteroids in the Solar System and stars, deep-sky objects and exoplanets in the northern and southern sky. All maps are zoomable.

Quantile regression explores the effect of one or more predictors on quantiles of the response. It can answer questions such as "What is the weight of 90% of individuals of a given height?"

Unlike in traditional mean regression methods, no assumptions about the distribution of the response are required, which makes it practical, robust and amenable to skewed distributions.

Quantile regression is also very useful when extremes are interesting or when the response variance varies with the predictors.

Das, K., Krzywinski, M. & Altman, N. (2019) Points of significance: Quantile regression. *Nature Methods* **16**:451–452.

Altman, N. & Krzywinski, M. (2015) Points of significance: Simple linear regression. *Nature Methods* **12**:999–1000.