# Xah Math Blog

O, math, my true love, how i have alienated thee, and you being quite difficult.

Math, Algebra: on the Phraseology of X Over K, and What's Group Theory?

the supreme mystery of the universe, is math. note, not physics, quantum or blackhole crap.

u can write scifi about blackhole or quantum physics stuff, but u cant for math.

[A Course In Universal Algebra by Stanley N Burris, H P Sankappanavar. At http://www.math.uwaterloo.ca/~snburris/htdocs/ualg.html]

### cauchy sequence defines calculus and complete metric space

this sequence, defines analysis (aka calculus)

and here's complete metric space.

### smooth map

been slacking in the math department. Going to pick up again. Each day, i spend 1 hour reading math, and post whatever. Today, let's learn about “smooth map”.

xah talk show 2019-05-06 geometric inversion, stereographic projection, their relations

this is logic, part of proof theory. this is what intent to learn in next 5 years, as opposed to category theory. (i got asked about the latter often, from programer idiots) https://twitter.com/johncarlosbaez/status/1122976661132021760

Tennenbaum's theoremTennenbaum's theorem is a result in mathematical logic that states that no countable nonstandard model of first-order Peano arithmetic (PA) can be recursive (Kaye 1991:153ff).

In mathematical logic, the compactness theorem states that a set of first-order sentences has a model if and only if every finite subset of it has a model. This theorem is an important tool in model theory, as it provides a useful method for constructing models of any set of sentences that is finitely consistent.

Finite model theory (FMT) is a subarea of model theory (MT). MT is the branch of mathematical logic which deals with the relation between a formal language (syntax) and its interpretations (semantics). FMT is a restriction of MT to interpretations on finite structures, which have a finite universe.

An interpretation is an assignment of meaning to the symbols of a formal language. Many formal languages used in mathematics, logic, and theoretical computer science are defined in solely syntactic terms, and as such do not have any meaning until they are given some interpretation. The general study of interpretations of formal languages is called formal semantics.

The most commonly studied formal logics are propositional logic, predicate logic and their modal analogs, and for these there are standard ways of presenting an interpretation. In these contexts an interpretation is a function that provides the extension of symbols and strings of symbols of an object language. For example, an interpretation function could take the predicate T (for “tall”) and assign it the extension {a} (for “Abraham Lincoln”). Note that all our interpretation does is assign the extension {a} to the non-logical constant T, and does not make a claim about whether T is to stand for tall and ‘a’ for Abraham Lincoln. Nor does logical interpretation have anything to say about logical connectives like ‘and’, ‘or’ and ‘not'. Though we may take these symbols to stand for certain things or concepts, this is not determined by the interpretation function.

An interpretation often (but not always) provides a way to determine the truth values of sentences in a language. If a given interpretation assigns the value True to a sentence or theory, the interpretation is called a model of that sentence or theory.

In mathematical logic, Lindström's theorem (named after Swedish logician Per Lindström, who published it in 1969) states that first-order logic is the strongest logic[1] (satisfying certain conditions, e.g. closure under classical negation) having both the (countable) compactness property and the (downward) Löwenheim–Skolem property.[2]

these shapes form hex stars. #geometry https://twitter.com/CGTNOfficial/status/1115445476537450496

[Too good to be Truchet By Colin Beveridge. At http://chalkdustmagazine.com/features/too-good-to-be-truchet/ ]

[Truchet By Cameron Browne. At http://cambolbro.com/games/truchet/ ]

[Truchet, Braille and Euler By Peter Rowlett. At https://aperiodical.com/2010/02/truchet-braille-and-euler/ ]

todo

#### Math and Illustrations

- 1:53 emacs xah html mode as org babel
- 6:46 python 3 iterator, OOP language vs functional language
- 15:50 starts on group theory intro

### Geometric algebra

todo read

[Let's remove Quaternions from every 3D Engine (An Interactive Introduction to Rotors from Geometric Algebra) By Marc Ten Bosch. At http://marctenbosch.com/quaternions/ ]

i no unstand.

help:

[Linear and Geometric Algebra By Alan Macdonald. At Buy at amazon ]

https://enkimute.github.io/ganja.js/examples/coffeeshop.html

[Geometric Algebra for Computer Science By Leo Dorst , Daniel Fontijne , Stephen Mann. At Buy at amazon ]

some old articles.

- Mandelbrot Set Explained (no complex number needed)
- How Computing Science created a new mathematical style
- The TeX Pestilence (Why TeX/LaTeX Sucks)
- English/Chinese Math Terminology 中/英 数学术语
- What is the Difference of Russell's Logicism, Hilbert's Formalism, Axiomatic System?
- Math Notation, Computer Language Syntax, and the “Form” in Formalism
- Math Notation, Proof System, Computer Algebra, in One Language
- The Codification of Mathematics
- Math Terminology and Naming of Things
- Mathematical Notation: Past and Future
- Pattern Matching vs Grammar Specification
- A Notation for Plane Geometry
- State of Theorem Proving Systems 2008
- The Problems of Traditional Math Notation
- The Geometric Significance of Complex Conjugate
- Notes On Plane Curves and Proofs
- Math Insight: Multiplication and Multiplicative Identity

Hyperboloid of Two Sheet http://VirtualMathMuseum.org/Surface/hyperboloid2/hyperboloid2.html

Dirac Belt Trick http://VirtualMathMuseum.org/Surface/dirac-belt/DiracBelt.html

### Mathematician Gaston Julia

this guy, is the first to study julia set. #math #geometry

### Mathematician Pierre Wantzel

Pierre Laurent Wantzel (5 June 1814 in Paris – 21 May 1848 in Paris) was a French mathematician who proved that several ancient geometric problems were impossible to solve using only compass and straightedge.[1]

In a paper from 1837,[2] Wantzel proved that the problems of

- doubling the cube, and
- trisecting the angle
are impossible to solve if one uses only compass and straightedge. In the same paper he also solved the problem of determining which regular polygons are constructible:

a regular polygon is constructible if and only if the number of its sides is the product of a power of two and any number of distinct Fermat primes (i.e. that the sufficient conditions given by Carl Friedrich Gauss are also necessary) The solution to these problems had been sought for thousands of years, particularly by the ancient Greeks. However, Wantzel's work was neglected by his contemporaries and essentially forgotten. Indeed, it was only 50 years after its publication that Wantzel's article was mentioned either in a journal article[3] or in a textbook.[4] Before that, it seems to have been mentioned only once, by Julius Petersen, in his doctoral thesis of 1871. It was probably due to an article published about Wantzel by Florian Cajori more than 80 years after the publication of Wantzel's article[1] that his name started to be well-known among mathematicians.[5]

Wantzel was also the first person who proved, in 1843,[6] that when a cubic polynomial with rational coefficients has three real roots but it is irreducible in Q[x] (the so-called casus irreducibilis), then the roots cannot be expressed from the coefficients using real radicals alone, that is, complex non-real numbers must be involved if one expresses the roots from the coefficients using radicals. This theorem would be rediscovered decades later by (and sometimes attributed to) Vincenzo Mollame and Otto Hölder.

“Ordinarily he worked evenings, not lying down until late; then he read, and took only a few hours of troubled sleep, making alternately wrong use of coffee and opium, and taking his meals at irregular hours until he was married. He put unlimited trust in his constitution, very strong by nature, which he taunted at pleasure by all sorts of abuse. He brought sadness to those who mourn his premature death.” — Adhémar Jean Claude Barré de Saint-Venant on the occasion of Wantzel's death.[1]

### Mathematician Pierre Fatou and Julia Set

this guy, is the first to study Julia set.

the math work of Pierre Fatou. Analysis, analytical functions, dynamical systems, chaos theory.

### Mathematician Adrien Douadly, and Mandelbrot Set

this guy started research on Mandelbrot set. one major result is that the Mandelbrot set is connected. #math #geometry

the Mandelbrot set, is truly one of the most incredible thing in math. Incredible is the word, and amazing, cosmetic, and in a visual way too. It shows the deep mystery of math.

See also: Mandelbrot Set Explained (no complex number needed)

here's the deepest zoom. Zoom to: 3.4 * 10^1091, and the video plays for 70 minutes.

here's the deepest zoom of Mandelbrot set, Zoomed to 3.4 * 10^1091. If you zoom this much on a atom, it'd be 10 followed by 109 zeros times larger than the screen. it's 70 minutes video.

and i always thought, if you zoom to a particular point deep enough, the face of god would suddenly appear, and the universe would blow up. It's, like, playing lottery, you wouldn't know there's no jackpot untill you played all the possible numbers. #math

### annulus, math

### differential geometry site

been helping mat professors build differential geometry site.

latest are Soliton Surface and others, see

- http://VirtualMathMuseum.org/Surface/hyperbolic_k1_sor/hyperbolic_k1_sor.html
- http://VirtualMathMuseum.org/Surface/two-soliton/two-soliton.html
- http://VirtualMathMuseum.org/Surface/three-soliton/three-soliton.html
- http://VirtualMathMuseum.org/Surface/four-soliton/four-soliton.html

visit the whole gallery at http://virtualmathmuseum.org/index.html

we've been working on it in past year.

See also: Wikipedia

### graduate level differential geometry

math Three-Soliton Surface http://VirtualMathMuseum.org/Surface/three-soliton/three-soliton.htmlBreather Surface http://VirtualMathMuseum.org/Surface/breather/breather.html

#math Chaitin's constant Chaitin's constant this is when, computer science enters the twilight zone. if you understand it, let me know. i want to kiss your ass.

### Calculus, Gradient

### Derivative and Jacobian Matrix

Jacobian matrix and determinant

#math derivative. if you are rusty with calculus, or programer who never learned it, it's good time to revisit. join my journey. read my daily snippet, and lookup Wikipedia.

Wikipedia is usually chaotic, rambling on and on, touching highschool stuff to research stuff. if calculus is new to you, read textbooks. Here's Free Math Textbooks i verified quality. Free Math Textbooks

#math programers, if u haven't seen Conway's Game of Life yet, look into. it's eye opening. In 1990s, i spent years “playing” it. 1st deep theory you learn: deterministic system can be unpredictable. https://twitter.com/icm7216/status/1080209582041907200

### monty hall problem

#math #probability. The famous Monty Hall problem, Marilyn vos Savant, one with highest IQ. Am still trying to resolve this paradox from philosophical aspect.

Say we are told that a woman has two children. If we ask whether either of them is a girl, and are told yes, what is the probability that the other child is also a girl? Considering this new child independently, one might expect the probability that the other child is female is 1/2. But by building a probability space (illustrating all possible outcomes), we see that the probability is actually only 1/3. This is because the possibility space illustrates 4 ways of having these two children: boy-boy, girl-boy, boy-girl, and girl-girl. But we were given more information. Once we are told that one of the children is a female, we use this new information to eliminate the boy-boy scenario. Thus the probability space reveals that there are still 3 ways to have two children where one is a female: boy-girl, girl-boy, girl-girl. Only ⅓ of these scenarios would have the other child also be a girl.[17] Using a probability space, we are less likely to miss one of the possible scenarios, or to neglect the importance of new information. For further information, see Boy or girl paradox.

This technique provides insights in other situations such as the Monty Hall problem, a game show scenario in which a car is hidden behind one of three doors, and two goats are hidden as booby prizes behind the others. Once the contestant has chosen a door, the host opens one of the remaining doors to reveal a goat, eliminating that door as an option. With only two doors left (one with the car, the other with another goat), the player must decide to either keep their decision, or switch and select the other door. Intuitively, one might think the player is choosing between two doors with equal probability, and that the opportunity to choose another door makes no difference. But probability spaces reveal that the contestant has received new information, and can increase their chances of winning by changing to the other door.[17]

2018-12-30 Randomness

super beautiful. #math #geometry https://twitter.com/TilingBot/status/1079505991048282112

Why Are Textbook So Expensive?

### Mathematica Logo History

[the story of spikey By Stephen Wolfram. At https://blog.stephenwolfram.com/2018/12/the-story-of-spikey/ ]

added to Mathematica Logo History

### idiotic math jargons

2 idiotic math jargon are: Probability Mass Function and Probability Density Function. They should be called Discrete Probability Distribution Function and Continuous Probability Distribution Function.

idiotic math jargon: Stochastic Process. It has no mathematical meaning, opaque, and also misleading. Stochastic Process force math to be viewed as a “process”, but the very nature of math is inert and static.

another idiotic math jargon is Monte Carlos method. It has no mathematical meaning, and is opaque. Better is random element method.

This is the so-called Monte Carlos method of computing π. Area of 1/4 circle is (pi r^2)/4. Area of the square is r^2. Ratio is π/4. Throw darts on it, then count dots inside / total, times 4, = π. Silly? why not just use grid points instead of random dots?

### Introduction to Probability

#math 2 dices. throw 24 times. what is the probability it turns out at least 1 double 6? Programers, go write code to simulate it. But you can never be sure. Mathematician can. Introduction to Probability, by Charles M Grinstead, J Laurie Snell Shoup Introduction to Probability, by Charles M Grinstead, J Laurie Snell Shoup

### Meromorphic function and Holomorphic function

#math #geometry Meromorphic function should be called Pinholed Analytic function, or just Pinholed function. the term “Holomorphic function” should be ban'd, replaced by Analytic Function.

### #physics Wave–particle duality, still not understood

how many beatings does it take? Wave–particle duality

Wave–particle duality is the concept in quantum mechanics that every particle or quantum entity may be partly described in terms not only of particles, but also of waves. It expresses the inability of the classical concepts “particle” or “wave” to fully describe the behavior of quantum-scale objects. As Albert Einstein wrote:[1]

It seems as though we must use sometimes the one theory and sometimes the other, while at times we may use either. We are faced with a new kind of difficulty. We have two contradictory pictures of reality; separately neither of them fully explains the phenomena of light, but together they do.

Through the work of Max Planck, Albert Einstein, Louis de Broglie, Arthur Compton, Niels Bohr and many others, current scientific theory holds that all particles exhibit a wave nature and vice versa.[2] This phenomenon has been verified not only for elementary particles, but also for compound particles like atoms and even molecules. For macroscopic particles, because of their extremely short wavelengths, wave properties usually cannot be detected.[3]

Although the use of the wave-particle duality has worked well in physics, the meaning or interpretation has not been satisfactorily resolved; see Interpretations of quantum mechanics.

Bohr regarded the “duality paradox” as a fundamental or metaphysical fact of nature. A given kind of quantum object will exhibit sometimes wave, sometimes particle, character, in respectively different physical settings. He saw such duality as one aspect of the concept of complementarity.[4] Bohr regarded renunciation of the cause-effect relation, or complementarity, of the space-time picture, as essential to the quantum mechanical account.[5]

Werner Heisenberg considered the question further. He saw the duality as present for all quantic entities, but not quite in the usual quantum mechanical account considered by Bohr. He saw it in what is called second quantization, which generates an entirely new concept of fields which exist in ordinary space-time, causality still being visualizable. Classical field values (example: the electric and magnetic field strengths of Maxwell) are replaced by an entirely new kind of field value, as considered in quantum field theory. Turning the reasoning around, ordinary quantum mechanics can be deduced as a specialized consequence of quantum field theory.[6][7]

### is 0 a natural number?

https://twitter.com/divbyzero/status/1063441410811199488

saw this late. 0 is the most unnatural, by definition! afaik, 0 didn't come into being until thousands of years of humanity. Between 0 and ∞, they are the 1st artificial number! ☺

### Robert Langlands won 2018 Abel Prize

#math Robert Langlands won 2018 Abel Prize ($740k) for his work that “relate Galois groups in algebraic number theory to automorphic forms and representation theory of algebraic groups over local fields and adeles.” Langlands program

### Synthetic geometry, absolute geometry, ordered geometry, incident geometry, tarski's axioms

Geometry, as presented by Euclid in the Elements, is the quintessential example of the use of the synthetic method. It was the favoured method of Isaac Newton for the solution of geometric problems.

Synthetic methods were most prominent during 1800s when geometers rejected coordinate methods in establishing the foundations of projective geometry and non-Euclidean geometries. For example the geometer Jakob Steiner (1796 to 1863) hated analytic geometry, and always gave preference to synthetic methods.

Karl von Staudt showed that algebraic axioms, such as commutativity and associativity of addition and multiplication, were in fact consequences of incidence of lines in geometric configurations. David Hilbert showed[8] that the Desargues configuration played a special role. Further work was done by Ruth Moufang and her students. The concepts have been one of the motivators of incidence geometry.

Tarski's axioms Tarski's axioms that turns synthetic #geometry to its actual nature, formal logic

### magic of dot product

#math dot product https://en.wikipedia.org/wiki/Dot_product a magic operation that formalize vector space with angle n length: inner product space

- A:=(a1,a2)
- B:=(b1,b2)
- A.B:=a1*b1+b2*b2

the magic is this: a1*b1+b2*b2 “happens to” == √(a1^2+a2^2)*√(b1^2+b2^2)*cos[θ] by trig

See also: Free Math Textbooks

so, can anyone explain what's multilinear algebra? #math differential #geometry

last year (2017) i spend few hours per day reading math. gonna start to do that again.

here's several Free Math Textbooks i've verified for quality Free Math Textbooks if you not familiar, refresh your calculus, then linear algebra. They are the most fundamental

### smooth function

In mathematical analysis, the smoothness of a function is a property measured by the number of derivatives it has that are continuous. A smooth function is a function that has derivatives of all orders everywhere in its domain.

### continuous function

In mathematics, a continuous function is a function for which sufficiently small changes in the input result in arbitrarily small changes in the output. A continuous function with a continuous inverse function is called a homeomorphism.

### measure theory and modern math

measure theory. basically, a modern version of the concept of length, area, volume Measure (mathematics)

the writing style of #math since 1990s, even its body, is about make it logical statements from set theory

it's all great, until you learned, there are a few math foundations, and different logic systems, and logic used in most traditional math is so bad that it cannot be computerized. thus Homotopy Type Theory only began in ~2010

### Lebesgue's Universal Covering Problem

just read john baez's fascinating article on Lebesgue's Universal Covering Problem https://johncarlosbaez.wordpress.com/2015/02/03/lebesgues-universal-covering-problem-part-2/

### Alex's Adventures in Numberland

found this math book Alex's Adventures in Numberland by Alex Bellos Buy at amazon year 2011. good reviews

### Algorithmic Mathematical Art, hits hackernews

https://news.ycombinator.com/item?id=18397804

i think that article hit hackernews for the 3rd times in past decade+.

### math, programing, proof systems

i haven't done any math in 2018. sad. but i hope to start again. and in next 2 months, create js code lib plotting plane curves. in 2017, was spending like 4 hours a day reading math. differential geometry and others. really missed that.

1 interesting thing about math is that, ever since 1990s, i tried to write math by programing code. always got frustrated. lots problems, between syntax, programing code, and math semantics (i.e. set theory). Turns out, this is a major prob, only in recently years there's solution. e.g. homotopy type theory.

in computer code, we can define functions or variable. But try to define definition of group, or even a set. you have problems. One gist of problem to illustrate is: there exist. This phrase, is a concrete example core problem of defining math (set theory) by algorithm.

when you try to say write math learning notes as computer programs, e.g. you just learned linear function. Now, try to express that concept in programing language. you start to get a sense of the probles. and this is what i mean, was what i was trying to do in 1990s.

the problem here, is a inter-mesh of so called foundation of math, logic, algorithm, and notation/syntax, as well as programing language features, such as pattern matching, logic oriented languages, etc. In 1 word, it's what automated/computer-aid proof system researh since 60s.

math is about what is. programing is how to. it's definition vs algorithm. On the surface, you'd think both are utmost precise and logic. One should easily turn entire body of math into code, or vice versa. In reality, it's proof system/math foundation research in past decades.

it's only in past decade or so, there's a glimmer of hope that math/code can be unified. e.g. look into howard curry isomorphism, homotopy type theory. (btw, if u r programer or haskell fan, lol. it's not what u think. shuddaFU. i no wanna hear what u have to say.)

it's only in past decade or so, there's a glimer of hope that math/code can be unified. e.g. look into howard curry isomorphism, homotopy type theory. (by the way, if you are programer, or haskell fan, lol, no u. it's not what you think. and your wishes are not true. STFU. i no wanna hear what u have to say.)

old article. Introduction to Symmetry and wallpaper groups to laymen Introduction to Symmetry

### Non-Archimedean geometry

[Yifeng Liu wins prestigious award in mathematics By Maeve Forti. At https://news.yale.edu/2018/10/25/yifeng-liu-wins-prestigious-award-mathematics ]

his publications https://gauss.math.yale.edu/~yl2269/

Chinese nuclear physicist 何泽慧 He Zehui (1914–2011)

### iff or not to iff

Math copyediting question: is it “if and only if” or “if, and only if,” (i.e., commas or no commas?) (I know I could avoid the issue by using iff…)

2018-09-24 from mathematician Dave Richeson https://twitter.com/divbyzero/status/1044209941064617984

“if and only if” of course. That's one unit.

i used to like iff when i was student ~92. now i find that annoying, as if if typo. am with @henryseg, prefer the symmetric ⇔. But i find that i prefer it too much, that am rly going for total computerized math by logical symbols. as i learned, has lots of its own problems. ☺

See Also: John Carlos Baez's discussion https://twitter.com/johncarlosbaez/status/1043975994246291456

geometry Steiner chain

### Roller Coaster = Calculus

when i see a roller-coaster, i see calculus.

I rode one last week, at Santa Cruz. haven't done so for some 20 years.

i didn't want to get on a roller coaster, fearing me too old and will flop limp. but my roomate pushed me into. now, ha, a child's play. when i was young, i ride jet fighters. lol.

when 40, u felt meaning of age. 45, friends died left n right. 50, u wit the reaper not far away.

the death reaper, he's right there, do you see? well, if you are young, some things you are incapable of seeing.

the calculus of roller coaster. what if a derivative is wrong? death.

apparently, in the old days, roller coaster is height based. u chain pull seats high, and let it drop. modern, is powered. Technically, u no longer coast. you are driven into sinuousness.

[Card Shuffling Visualizations By Roger Antonsen. At http://archive.bridgesmathart.org/2018/bridges2018-451.pdf ]

### math John Conway Free will theorem

“if we have a free will in the sense that our choices are not a function of the past, then, subject to certain assumptions, so must some elementary particles”

[John Conway – discovering free will (part I) By Rachel Thomas. At https://plus.maths.org/content/john-conway-discovering-free-will-part-i ]

Great Math Software: Polyhedrons and Polytopes

minor update. If you have a old Mac from 1990s to 2000s, check out that old math software page.

Great software for Tilings, Patterns, Symmetry

added a new tiling app for Mac

### Knit Brain Hat

brain hat now on amazon. brain knitted hat

Magic Polyhedrons (updated)

Great Software for Plane Geometry (updated)

## obsolete math software, 1990s to 2005

revisiting and updating my page on list of great math software, mostly from 1990s. Many, are from hobbyists, truly great programs. Times flies. People get old. Many are not updated. Many websites gone. And many Java applets, as relics of tech progress.

It's like, right now you are hot into programing, machine learning, this or that. 10 or 20 years later, you might be something else, or puffed off. You look into the ebb and flow of each human animal life activity as cellular automata, it'd be something.

for nostalgia, here's list of obsolete math software. 1990s to ~2005. In Mac classic, Java, Flash. Old Math Software

Great software for Tilings, Patterns, Symmetry (major update)

### preorder, partial order, total order

The original concept of totally ordered set or order, still dominant today, obeys a bunch of rules:

- reflexivity: x≤x
- transitivity: x≤y and y≤z imply x≤z
- antisymmetry: if x≤y and y≤x then x=y
- trichotomy: for all x,y we either have x≤y or y≤x.
The real numbers with the usual ≤ obeys all these. Then people discovered many situations where rule 4 does not apply. If only rules 1-3 hold they called it a partially ordered set or poset. Then people discovered many situations where rule 3 does not hold either! If only rules 1-2 hold they called it a preordered set or preorder.

Category theory teaches us that preorders are the fundamental thing: see Lecture 3. But we backed our way into this concept, so it has an awkward name. Fong and Spivak try to remedy this by calling them posets, but that's gonna confuse everyone even more! If they wanted to save the day they should have made up a beautiful brand new term.

2018-03-29 by John Baez from https://forum.azimuthproject.org/discussion/comment/16083/#Comment_16083

### category theory online course

So, am taking a online course of category theory, lead by famous mathematician and theoretical physicist John Baez.

the home page is at https://forum.azimuthproject.org/discussion/1717/welcome-to-the-applied-category-theory-course

the textbook is: https://arxiv.org/abs/1803.05316

local mirror category_theory_brendan_fong_david_spivak_2018-03.pdf

some great math related JavaScript site added.

### Using Neural Net to Create New Knitting Patterns

Quite amazing.

https://twitter.com/JanelleCShane/status/964945688613199872

by Janelle Shane

### random sequence, not mathematically defined

it's interesting, that the concept of random number, or random sequence, is actually undefined in math, because it is impossible to define mathematically.

The concept of a random sequence is essential in probability theory and statistics. The concept generally relies on the notion of a sequence of random variables and many statistical discussions begin with the words “let X1,…,Xn be independent random variables…”. Yet as D. H. Lehmer stated in 1951: “A random sequence is a vague notion… in which each term is unpredictable to the uninitiated and whose digits pass a certain number of tests traditional with statisticians”.[1]

Axiomatic probability theory deliberately avoids a definition of a random sequence.[2] Traditional probability theory does not state if a specific sequence is random, but generally proceeds to discuss the properties of random variables and stochastic sequences assuming some definition of randomness. The Bourbaki school considered the statement “let us consider a random sequence” an abuse of language.[3]

[2018-03-19 Random sequence]

### follow xah

follow me on my new mastodon account at https://mstdn.io/@xahlee and on reddit https://www.reddit.com/user/xah

### Philosophy of Math: Logicism, Formalism, Intuitionism, and Category Theory

the 3 philosophy of math of 1900s are: ① logicism. Math is pure derivation of logic. ② Hilbert's formalism. Math is just bunch of fomulaic symbols. No meaning. ③ intuitionism. Math is mind's construction.

these 3 schools are the basis of foundations of math. Each has its problems. logic became formal (i.e. symbolic) logic, which is basically formalism. They in turn, became constructivism (a variation of intuitionism), so it can run by computer.

comment at ~~https://plus.google.com/+XahLee/posts/LZJPcj4KGRB~~

here's my comment to mathematician john baez:

thanks for correction.

i've always wondered what's the difference of logicism and formalism. Back in 1990s, I was introduced to them by Russell. (my top 3 fav author) Tried to read about them now and then. In 2000s, wikipedia is still not great, but in past year, i read a lot math again, and i seem to get some understanding. Is my characterization roughly correct?

i never cared about intuitionism, until now, and now i very much appreciate constructivism. As i understand, it is what mechanical manipulation can actually build.

PS few days ago i tried to read wikipedia about category theory again. Again, came away as incomprehensible abstraction. Meant to write a rant about it… but here's a gist…

so i spend half hour thinking, what does abstraction mean? After all, numbers 1 2 3 ... are abstraction to begin with. But then we have equations, such as 2*x+3 = 4, which is abstraction of description of math problems. Then, abstract algebra and 1800s's math, are the 2nd stage of abstraction. e.g. abstract algebra came from the systematic formal maniputlation of equations. And likewise stuff in algebraic geometry e.g. variety, and differential geometry geometry e.g. manifold, and so on in other branches of math.

Then, what's the next level abstraction? I'm thinking, scheme and sheaf etc (which i have no idea what they are), or, the category theory stuff.

but anyway, so i was reading Wikipedia on category theory. Category theory

in other math articles, say, holomorphic function, homotopy, hilbert space, riemann sphere, homomorphism, etc i've recently read, there's a cold definition. Most of the time, i can at least understand the definition, and go on from there. (in the above examples, i also appreciate what they mean, why they are there, etc, except hilbert space.)

But the category theory, it begin with pages of pages of meta description. And am at loss. But perhaps, to appreciate it, one must first have solid understanding of various branches graduate level math?

but then, my first exposure to it is

Conceptual Mathematics: A First Introduction to Categories by F William Lawvere, Stephen Hoel Schanuel. [Buy at amazon]

in 1990s. Which claimed to be written for laymen, and useful even outside of math. I recall, after reading 1 chapter, it's going too slow, and i was feeling, “what's the point?”. I never continued after the 1 chapter. (i don't have the book now. must gave away or something long ago.)

### Math, Naturalness of a Field's Conditions

So, in algebra, you have a field. It's a set and 2 functions f and g, of the form f(a,b) and g(a,b), and X nesting properties of f and g (called commutativity, associativity, distributive, invertible, etc).

So, what happens if we have more than 2 functions, 3 parameters each? So, such study is called universal algebra. (i haven't studied, but i wonder what happens there, in general. The mix of nesting of function, i gather, would create more complex concept similar to associativity and distributive, involving 3 functions, but we don't have a name.)

But, WHAT is the fundamental nature, that real number (a field) is this specific X nesting properties? What is it, that real numbers, which we consider as naturally occurring or developed, form this “field” with such specific nesting properties?

am thinking, there must be some logical answer.

to describe my questions further, for example, real number developed because, first we have counting, 1, 2, 3, then naturally we developed 0, then we have rational, which is ratio, e.g. 1/2 as cutting a pie, then we have negative numbers (from, say, I OWE YOU). And from rational we discovered irrational, as in pythagorean. So there, we have real numbers. And, addition came from simple counting. Multiplication can be considered as a short for repeated additions.

so, am guessing, addition, and multiplication (repeated addition) necessitates the commutativity, associativity, distributive, properties?

now, having written this out, it seems obvious and is the answer to my own question.

... because, by looking at the definition of field, i've always thought, it's somehow arbitrary and complicated. I'd be interested, in a systematic approach of studying structures, e.g. a set, with n operation of m-arity, starting with n=1 and m=1. Then, we develop, all possible ways of nesting n such functions of m arity, so that associativity, commutativity (order of arg), distributivity, are just 3 of the possible properties.

but i gather that, universal algebra may began like this, but actually has become a bit something else.

#### similar situation is group theory

Group, is much simpler than field. But if you look at definition of group, you see that, it seems also arbitrary and complex.

But then, if you look at symmetry, such as symmetry of polyhedra, you see that all the requirement of group is necessary, and no more, no less.

but the question remain, why is the group definition seems arbitrary?

i mean, is there some point of view, so the associativity
`(a • b) • c = a • (b • c)`

requirement, dissolved as if it is natural?

again, i really like to see, a combinatorial exploration of all such possible condition of n functions of m arity. (as in universal algebra)

perhaps after seeing that, then one can judge, comparatively, whether the associativity condition is natural.

now, to be sure, function with 1 or 2 arity is actually perhaps the most natural. And 1 or 2 functions is also pretty bare. As opposed to, a structure with function that has 3 arity, or more than 2 functions.

If you just have 1 function, or just function wit 1 arity, than it may become too simple to have interesting things going on.

but again, would like to see a systematic combinatorial list of the conditions that may arise of n functions and m arity.

comment at
~~https://plus.google.com/+XahLee/posts/6tiAGpLiQ5z~~

Introduction to Probability, by Charles M Grinstead, J Laurie Snell Shoup (free book)

for those of you programers doing big data or AI, understand probability is essential.

### major issues of writing 2D math plotter

when writing 2D math plotter, major issues are

- ① adoptive sampling. Else, you get kinks at sharp turns.
- ② divide by zero or ∞. e.g. plot 1/x.
- ③ asymptotes. e.g. in hyperbola
- ④ auto canvas range. (find max/min and/or find point cluster)

### Chinese number names

- 10^52 恒河沙 (sads of eternal river)
- 10^56 阿僧祇 (asamkhya)
- 10^60 那由他 (let it go)
- 10^64 不可思議 (unfathomable)
- 10^68 無量大數 (unmeasurable big number)

Quality Free and Legal Math Textbooks https://www.patreon.com/posts/17392806

added a number theory book.

### ideal of ring theory

For an arbitrary ring ( R , + , ⋅ ), let (R,+) be its additive group.

A subset I is called a two-sided ideal (or simply an ideal) of R if it is an additive subgroup of R that “absorbs multiplication by elements of R.”

i.e.

- (I,+) is a subgroup of ( R , + )
- ∀ x ∈ I , ∀ r ∈ R : x ⋅ r , r ⋅ x ∈ I

digging out my 3 Volumes classic 〈Mathematical Thought From Ancient to Modern Times〉. I haven't read for 20 years. To read about how sin exp were extended to complex plane.

Buy Mathematical Thought From Ancient to Modern Times, Volume 2 by Morris Kline

See also: Math and Geometry Books

### origami simulator

Origami Simulator, by Amanda Ghassaei, at http://www.amandaghassaei.com/projects/origami_simulator/

added more logic book. Free Math Textbooks

### The sound of space-filling curves: examples

this is incredibly beautiful, not just the novelty, but MUSICALLY!

[The sound of space-filling curves: examples By Herman Haverkort. At http://www.win.tue.nl/~hermanh/doku.php?id=sound_of_space-filling_curves ]

listen to the first one, the hilbert curve (the one “without intro”)

that's the best one. The other ones are not so good.

The author, Herman Haverkort, is a mathematician specializing on algorithms.

Free Math Textbooks (added books on logic)

If you have a question, put $5 at patreon and message me.