 Oct 2020

zhuanlan.zhihu.com zhuanlan.zhihu.com向量的内积与外积1

两个向量的外积，又叫向量积、叉乘等
叉乘代表外积


www.amazon.com www.amazon.com

The Visual Display of Quantitative Information 2nd Edition by Edward R. Tufte



Alexs Adventures In Numberland


www.amazon.com www.amazon.com

How Not to Be Wrong: The Power of Mathematical Thinking Paperback – Illustrated, May 26, 2015 by Jordan Ellenberg


www.amazon.com www.amazon.com

A Mind for Numbers: How to Excel at Math and Science (Even If You Flunked Algebra) Audible Audiobook – Unabridged Barbara Oakley (Author), Grover Gardner (Narrator), Gildan Media, LLC (Publisher) & 2 more


www.linkedin.com www.linkedin.com

Espen Slettnes 3rd degree connection3rd Espen has a account BMCUpper Monthly contest designer and local coordinator at Berkeley Math Circle
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theintercept.com theintercept.com

But these lookalike audiences aren’t just potential new customers — they can also be used to exclude unwanted customers in the future, creating a sort of ad targeting demographic blacklist.

How consumers would be expected to navigate this invisible, unofficial creditscoring process, given that they’re never informed of its existence, remains an open question.

“It sure smells like the prescreening provisions of the FCRA,” Reidenberg told The Intercept. “From a functional point of view, what they’re doing is filtering Facebook users on creditworthiness criteria and potentially escaping the application of the FCRA.”

In an initial conversation with a Facebook spokesperson, they stated that the company does “not provide creditworthiness services, nor is that a feature of Actionable Insights.” When asked if Actionable Insights facilitates the targeting of ads on the basis of creditworthiness, the spokesperson replied, “No, there isn’t an instance where this is used.” It’s difficult to reconcile this claim with the fact that Facebook’s own promotional materials tout how Actionable Insights can enable a company to do exactly this. Asked about this apparent inconsistency between what Facebook tells advertising partners and what it told The Intercept, the company declined to discuss the matter on the record,


www.bloomberg.com www.bloomberg.com

YouTube doesn’t give an exact recipe for virality. But in the race to one billion hours, a formula emerged: Outrage equals attention.
Talk radio has had this formula for years and they've almost had to use it to drive any listenership as people left radio for television and other media.
I can still remember the different "loudness" level of talk between Bill O'Reilly's primetime show on Fox News and the louder level on his radio show.

A 2015 clip about vaccination from iHealthTube.com, a “natural health” YouTube channel, is one of the videos that now sports a small gray box.
Does this box appear on the video itself? Apparently not...
Examples:
But nothing on the embedded version:
A screengrab of what this looks like:

When Wojcicki took over, in 2014, YouTube was a third of the way to the goal, she recalled in investor John Doerr’s 2018 book Measure What Matters.“They thought it would break the internet! But it seemed to me that such a clear and measurable objective would energize people, and I cheered them on,” Wojcicki told Doerr. “The billion hours of daily watch time gave our tech people a North Star.” By October, 2016, YouTube hit its goal.
Obviously they took the easy route. You may need to measure what matters, but getting to that goal by any means necessary or using indefensible shortcuts is the fallacy here. They could have had that North Star, but it's the means they used by which to reach it that were wrong.
This is another great example of tech ignoring basic ethics to get to a monetary goal. (Another good one is Marc Zuckerberg's "connecting people" mantra when what he should be is "connecting people for good" or "creating positive connections".

The conundrum isn’t just that videos questioning the moon landing or the efficacy of vaccines are on YouTube. The massive “library,” generated by users with little editorial oversight, is bound to have untrue nonsense. Instead, YouTube’s problem is that it allows the nonsense to flourish. And, in some cases, through its powerful artificial intelligence system, it even provides the fuel that lets it spread.#lazyimg336042387:before{paddingtop:66.68334167083543%;}
This is a great summation of the issue.

Somewhere along the last decade, he added, YouTube prioritized chasing profits over the safety of its users. “We may have been hemorrhaging money,” he said. “But at least dogs riding skateboards never killed anyone.”


dancohen.org dancohen.org

A more active stance by librarians, journalists, educators, and others who convey truthseeking habits is essential.
In some sense these people can also be viewed as aggregators and curators of sorts. How can their work be aggregated and be used to compete with the poor algorithms of social media?


techcrunch.com techcrunch.com

Meta cofounder and CEO Sam Molyneux writes that “Going forward, our intent is not to profit from Meta’s data and capabilities; instead we aim to ensure they get to those who need them most, across sectors and as quickly as possible, for the benefit of the world.”
Odd statement from a company that was just acquired by Facebook founder's CVI.


knightcolumbia.org knightcolumbia.org

Meanwhile, politicians from the two major political parties have been hammering these companies, albeit for completely different reasons. Some have been complaining about how these platforms have potentially allowed for foreign interference in our elections.3 3. A Conversation with Mark Warner: Russia, Facebook and the Trump Campaign, Radio IQWVTF Music (Apr. 6, 2018), https://www.wvtf.org/post/conversationmarkwarnerrussiafacebookandtrumpcampaign#stream/0 (statement of Sen. Mark Warner (DVa.): “I first called out Facebook and some of the social media platforms in December of 2016. For the first six months, the companies just kind of blew off these allegations, but these proved to be true; that Russia used their social media platforms with fake accounts to spread false information, they paid for political advertising on their platforms. Facebook says those tactics are no longer allowed—that they've kicked this firm off their site, but I think they've got a lot of explaining to do.”). Others have complained about how they’ve been used to spread disinformation and propaganda.4 4. Nicholas Confessore & Matthew Rosenberg, Facebook Fallout Ruptures Democrats’ Longtime Alliance with Silicon Valley, N.Y. Times (Nov. 17, 2018), https://www.nytimes.com/2018/11/17/technology/facebookdemocratscongress.html (referencing statement by Sen. Jon Tester (DMont.): “Mr. Tester, the departing chief of the Senate Democrats’ campaign arm, looked at social media companies like Facebook and saw propaganda platforms that could cost his party the 2018 elections, according to two congressional aides. If Russian agents mounted a disinformation campaign like the one that had just helped elect Mr. Trump, he told Mr. Schumer, ‘we will lose every seat.’”). Some have charged that the platforms are just too powerful.5 5. Julia Carrie Wong, #Breaking Up Big Tech: Elizabeth Warren Says Facebook Just Proved Her Point, The Guardian (Mar. 11, 2019), https://www.theguardian.com/usnews/2019/mar/11/elizabethwarrenfacebookadsbreakupbigtech (statement of Sen. Elizabeth Warren (DMass.)) (“Curious why I think FB has too much power? Let's start with their ability to shut down a debate over whether FB has too much power. Thanks for restoring my posts. But I want a social media marketplace that isn't dominated by a single censor. #BreakUpBigTech.”). Others have called attention to inappropriate account and content takedowns,6 6. Jessica Guynn, Ted Cruz Threatens to Regulate Facebook, Google and Twitter Over Charges of AntiConservative Bias, USA Today (Apr. 10, 2019), https://www.usatoday.com/story/news/2019/04/10/tedcruzthreatensregulatefacebooktwitteroverallegedbias/3423095002/ (statement of Sen. Ted Cruz (RTex.)) (“What makes the threat of political censorship so problematic is the lack of transparency, the invisibility, the ability for a handful of giant tech companies to decide if a particular speaker is disfavored.”). while some have argued that the attempts to moderate discriminate against certain political viewpoints.
Most of these problems can all fall under the subheading of the problems that result when social media platforms algorithmically push or accelerate content on their platforms. An individual with an extreme view can publish a piece of vile or disruptive content and because it's inflammatory the silos promote it which provides even more eyeballs and the acceleration becomes a positive feedback loop. As a result the social silo benefits from engagement for advertising purposes, but the community and the commons are irreparably harmed.
If this one piece were removed, then the commons would be much healthier, fringe ideas and abuse that are abhorrent to most would be removed, and the broader democratic views of the "masses" (good or bad) would prevail. Without the algorithmic push of fringe ideas, that sort of content would be marginalized in the same way we want our inane content like this morning's coffee or today's lunch marginalized.
To analogize it, we've provided social media machine guns to the most vile and fringe members of our society and the social platforms are helping them drag the rest of us down.
If all ideas and content were provided the same linear, nonpromotion we would all be much better off, and we wouldn't have the need for as much human curation.

It would allow end users to determine their own tolerances for different types of speech but make it much easier for most people to avoid the most problematic speech, without silencing anyone entirely or having the platforms themselves make the decisions about who is allowed to speak.
But platforms are making huge decisions about who is allowed to speak. While they're generally allowing everyone to have a voice, they're also very subtly privileging many voices over others. While they're providing space for even the least among us to have a voice, they're making far too many of the worst and most powerful among us logarithmically louder.
It's not broadly obvious, but their algorithms are plainly handing massive megaphones to people who society broadly thinks shouldn't have a voice at all. These megaphones come in the algorithmic amplification of fringe ideas which accelerate them into the broader public discourse toward the aim of these platforms getting more engagement and therefore more eyeballs for their advertising and surveillance capitalism ends.
The issue we ought to be looking at is the dynamic range between people and the messages they're able to send through social platforms.
We could also analogize this to the voting situation in the United States. When we disadvantage the poor, disabled, differently abled, or marginalized people from voting while simultaneously giving the uberrich outsized influence because of what they're able to buy, we're imposing the same sorts of problems. Social media is just able to do this at an even larger scale and magnify the effects to make their harms more obvious.
If I follow 5,000 people on social media and one of them is a racistpolicysupporting, white nationalist president, those messages will get drowned out because I can only consume so much content. But when the algorithm consistently pushes that content to the top of my feed and attention, it is only going to accelerate it and create more harm. If I get a linear presentation of the content, then I'd have to actively search that content out for it to cause me that sort of harm.


www.newyorker.com www.newyorker.com

A spokeswoman for Summit said in an email, “We only use information for educational purposes. There are no exceptions to this.” She added, “Facebook plays no role in the Summit Learning Program and has no access to any student data.”
As if Facebook needed it. The fact that this statement is made sort of goes to papering over the idea that Summit itself wouldn't necessarily do something as nefarious or worse with it than Facebook might.



With Alphabet Inc.’s Google, and Facebook Inc. and its WhatsApp messaging service used by hundreds of millions of Indians, India is examining methods China has used to protect domestic startups and take control of citizens’ data.
Governments owning citizens' data directly?? Why not have the government empower citizens to own their own data?


www.buzzfeed.com www.buzzfeed.com

Having low scores posted for all coworkers to see was “very embarrassing,” said Steph Buja, who recently left her job as a server at a Chili’s in Massachusetts. But that’s not the only way customers — perhaps inadvertently — use the tablets to humiliate waitstaff. One diner at Buja’s Chili’s used Ziosk to comment, “our waitress has small boobs.”According to other servers working in Ziosk environments, this isn’t a rare occurrence.
This is outright sexual harrassment and appears to be actively creating a hostile work environment. I could easily see a class action against large chains and/or against the app maker themselves. Aggregating the data and using it in a smart way is fine, but I suspect no one in the chain is actively thinking about what they're doing, they're just selling an idea down the line.
The maker of the app should be doing a far better job of filtering this kind of crap out and aggregating the data in a smarter way and providing a better output since the major chains they're selling it to don't seem to be capable of processing and disseminating what they're collecting.

Systems like Ziosk and Presto allow customers to channel frustrations that would otherwise end up on public platforms like Yelp — which can make or break a restaurant — into a closed system that the restaurant controls.
I like that they're trying to own and control their own data, but it seems like they've relied on a third party company to do most of the thinking for them and they're not actually using the data they're gathering in the proper ways. This is just painfully deplorable.


daily.jstor.org daily.jstor.org

I literally couldn’t remember when I’d last looked at my RSS subscriptions. On the surface, that might seem like a win: Instead of painstakingly curating my own incoming news, I can effortlessly find an endless supply of interesting, worthwhile content that the algorithm finds for me. The problem, of course, is that the algorithm isn’t neutral: It’s the embodiment of Facebook and Twitter’s technology, data analysis, and most crucial, business model. By relying on the algorithm, instead of on tags and RSS, I’m letting an army of web developers, business strategists, data scientists, and advertisers determine what gets my attention. I’m leaving myself vulnerable to misinformation, and manipulation, and giving up my power of selfdetermination.


bookbook.pubpub.org bookbook.pubpub.org

Safiya Noble, Algorithms of Oppression (New York: New York University Press, 2018). See also Mozilla’s 2019 Internet Health Report at https://internethealthreport.org/2019/letsaskmoreofai/.


www.economist.com www.economist.com

eight years after release, men are 43% more likely to be taken back under arrest than women; AfricanAmericans are 42% more likely than whites, and highschool dropouts are three times more likely to be rearrested than college graduates.
but are these possibly the result of external factors (like racism?)


www.shelovesmath.com www.shelovesmath.com

MATH LESSONS SheLovesMath.com is a free math website that explains math in a simple way, and includes lots of examples, from Counting through Calculus. Here are the topics that She Loves Math covers, as expanded below: Basic Math, PreAlgebra, Beginning Algebra, Intermediate Algebra, Advanced Algebra, PreCalculus, Trigonometry, and Calculus.
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mathproblemsolvingskills.wordpress.com mathproblemsolvingskills.wordpress.com

Math Problem Solving Skills

I was just helping a student with his AoPS homework, when I came across the following related problems: Eight people, including Fred, are in a club. They decide to form a 3 person committee. How many possible committees can be formed? So we are choosing 3 people out of 8 or . How many possible committees include Fred? Since Fred is taking one committee seat, that means we need to choose 2 more people from the remaining seven, or . How many possible committees do not include Fred? Since we can’t choose Fred, we need to choose 3 members from the remaining 7 or . Since the total number of committees is equal to the number of of committees with Fred plus the number of committees without Fred, then we can say
Pascal's rule and is Fred on the committee
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mathproblemsolvingskills.wordpress.com mathproblemsolvingskills.wordpress.com

What Everyone Should Know About Math Contests
Reasons for writing math contests

 Sep 2020

www.washingtonpost.com www.washingtonpost.com

4.… 9.7
You know, they say, all Supreme Courts are created equal, but you look at the court now, and you look at the court in 1868, and you can see that statement is NOT TRUE. See, normally, you propose something like an Expanded Supreme Court, you got a fiftyfifty chance of passing it. But Majority Leader McConnell is a genetic freak, and the Congress as it stands is not normal.
So, right now, you got a 25% chance AT BEST at beating this donothing Congress and passing something. But then you add Election Day to the mix? You the chances of passing the expanded court drastically go up. See, at the three way on Election Day (where the Senate will either be Republican Majority, Democratic Majority, or tied, because of course that can happen), you got a 33 and a third chance of the Republicans winning! But Democrats! Democrats got a 66 and two thirds chance of winning, 'cause President Trump KNOOOOOWS he can't win fairly, and he's not even gonna try, and he might take some of his party's senators down with him. So, DAVID, you take Republicans 33 and a 3rd percent chance of winning minus Democrats 25% chance of passing something through the Senate (if they was to go oneonone right now) and you've got an 8 1/3 percent chance of Republicans defeating court expansion! But THEN, you take Democrats 75% chance of passing something through the House, plus the 66 and 2/3 percents, they got a 141 2/3 chance of expanding the court!
Senator McConnell! The numbers don't lie, and they spell 17 1 5 4 5 7 3 12 for you at Sacrifice!

 Aug 2020

mgonzalezm.keybase.pub mgonzalezm.keybase.pubUntitled36

(a?—a + 1) (a*—a? + 1) (a% + a + 1). Comprobar el resultado ha ciendo a = 2.

(a2—ab+b2+a+b+1)(a+b—1).

(a* + a% + a®b? +ab® + b*) (a—b).

(x2—x—1)2(x2 + x + 1).

(2 + xy—6)*) + (x+29).

2a*mx?y) + 6a?nyz?) + (20”y)
Typo: Sobra el pimer parentesis ')'

(4abx — 8b%x?y) + (2bx?).

(2 + 322 + 5) (x*—1 + 4x).

(m3 —m? + m—1) (—m3 + m—m +1).

(x2 4 y2 + 22— ay — az — yz) (x + y + 2).

(a? — 2ab + 4b?) (a + 2b). Comprobar el resultado haciendo a = 2 3.

(x2—3xy + y*) (2x— 3y + 2).

(a? + 2ab — 2b*) (3a—7b).

(2x2—5y) (4x + 2y*).

xy(x2— 2y + 4).

(—ab2c) (3a2bc) (2abc?).

(8a2b) (—2ab2).

Demostrar que la suma de cualquier número negativo con su valor abso luto es igual a cero.

Demostrar que la suma de todas las expresiones en los ejercicios 1115 es igual a la expresión en el ejercicio 11.

Calcular 4 —B—C.

Calcular B— A4 —C.

Calcular 4 —B + C.

Calcular B—4 + C.

Calcular 4 + B—C.

m* + 6m$ — 7m? + 8m — 9, 2m? + Im?—4m — 3.

2a + 4by — 2ey? + dys, 2dy> — 2by —a + 30y”.

a — 3a”b + 3ab? —b, a5 — 4a*b + 2ab? + b.

x3—4x? + 2x—5, —a3 + 2x2—3x —3.

3a—2b + 4c — d, 20 + b—3c—d.

E2 + 2cd — 2d, 3c — 3cd — 2d?, e? 4+ 4d — 2c + 2d?.

3x3 — 8x? + 9x, —a3 + 3x2—8, 2x3 — 2x?—7x 5.

x2—4xy + 3y, 2x? + 2xy—2y?, 2xy — y? —22.

4m? — 3mn + 2n?, Gmn—2n? + 5, 3n2 — 3—2m.

2a> — 2a?b + 2b5, Yu?b — 4ab? — 4b3, dab* —m.

(x+a)(y +a)(z +a).

Demostrar el Teorema 3 del Art. 2.4.

 Jul 2020

mgonzalezm.keybase.pub mgonzalezm.keybase.pub

f.real numbers.
f. real numbers. \( \{ 9, 1.3, 0, 0.\overline{3}, \frac{\pi}{2}, \sqrt{9}, \sqrt{10} \} \)

e.irrational numbers
e. irrational numbers. \( \{ \frac{\pi}{2} , \sqrt{10} \} \)

d.rational numbers
d. rational numbers. \( \{ 9, 1.3, 0, 0.\overline{3}, \sqrt{9} \} \)
\( 1.3 = \frac{13}{10} \)
\( 0.\overline{3} = \frac{1}{3} \)
\( 0 = \frac{0}{1} \)
\( \sqrt{9} = \frac{3}{1} \)

c.integers.
c. integers \( \{ 9, 0, \sqrt{9} \} \)

b.whole numbers
b. whole numbers. \( \{ 0, \sqrt{9} \} \)
0 is a whole number

a.natural numbers
a. natural numbers. \( \{ \sqrt{9} \} \)
\( \{ \sqrt{9} \} = 3 \)

Find the union:{3, 4, 5, 6, 7}∪{3, 7, 8, 9}.
\( \{7,8,9,10,11\}\cup\{6,7,8,9,10,11,12\}=\{7,8,9,10,11,6,12\} \)

Find the intersection:{3, 4, 5, 6, 7} ̈{3, 7, 8, 9}.
\( \{3,4,5,6,7\}\cap\{3,7,8,9\}=\{3,7\} \)

b.By how much does the formula underestimate or overestimate the actual cost shown in Figure P.1?
\( 8893  8714 = 179 \)
It underestimates by 179

a.Use the formula T=4x2+330x+3310, described in Example 2, to find the average cost of tuition and fees at public U.S. colleges for the school year ending in 2014.
\( T=4x^2 +330x +3310 \)
Because 2014 is 14 years after 2000, we substitute 14 for x in the given formula.
\( T=4(14)^2 + 330(14) +3310 \)
\( T=4(196) +4620 +3310 \)
\( T=784 +7930 \)
\( T=8714 \)

Evaluate 8+6(x3)2 for x=13.
Evaluate \( 8+6(x3)^2 \) for \( x=13 \)
\( 8 + 6 (13  3)^2 \)
\( 8 + 6 (10)^2 \)
\( 8 + 6 * 100 \)
\( 8 + 600 \)
\( 608 \)


en.wikipedia.org en.wikipedia.org

In logic, functions or relations A and B are considered dual if A(¬x) = ¬B(x), where ¬ is logical negation. The basic duality of this type is the duality of the ∃ and ∀ quantifiers in classical logic. These are dual because ∃x.¬P(x) and ¬∀x.P(x) are equivalent for all predicates P in classical logic

the ∧ and ∨ operators are dual in this sense, because (¬x ∧ ¬y) and ¬(x ∨ y) are equivalent. This means that for every theorem of classical logic there is an equivalent dual theorem. De Morgan's laws are examples


en.wikipedia.org en.wikipedia.org

Dual (mathematics), a notion of paired concepts that mirror one another
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 May 2020

en.wikipedia.org en.wikipedia.org

Related concepts in other fields are: In natural language, the coordinating conjunction "and". In programming languages, the shortcircuit and control structure. In set theory, intersection. In predicate logic, universal quantification.
Strictly speaking, are these examples of dualities (https://en.wikipedia.org/wiki/Duality_(mathematics))? Or can I only, at strongest, say they are analogous (a looser coonection)?


en.wikipedia.org en.wikipedia.org
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en.wikipedia.org en.wikipedia.org

Mathematically speaking, necessity and sufficiency are dual to one another. For any statements S and N, the assertion that "N is necessary for S" is equivalent to the assertion that "S is sufficient for N".


en.wikipedia.org en.wikipedia.org
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en.wikipedia.org en.wikipedia.org

This is an abstract form of De Morgan's laws, or of duality applied to lattices.


en.wikipedia.org en.wikipedia.org

A plane graph is said to be selfdual if it is isomorphic to its dual graph.



en.wikipedia.org en.wikipedia.org

In mathematical contexts, duality has numerous meanings[1] although it is "a very pervasive and important concept in (modern) mathematics"[2] and "an important general theme that has manifestations in almost every area of mathematics".[3]


www.shelltips.com www.shelltips.com

echo "scale=2; 2/3"  bc .
the right way to do math in bash
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 Apr 2020

en.wikipedia.org en.wikipedia.org

the phrase up to is used to convey the idea that some objects in the same class — while distinct — may be considered to be equivalent under some condition or transformation

"a and b are equivalent up to X" means that a and b are equivalent, if criterion X, such as rotation or permutation, is ignored



en.wikipedia.org en.wikipedia.org

If solutions that differ only by the symmetry operations of rotation and reflection of the board are counted as one, the puzzle has 12 solutions. These are called fundamental solutions; representatives of each are shown below


0.30000000000000004.com 0.30000000000000004.com

Computers can only natively store integers, so they need some way of representing decimal numbers. This representation comes with some degree of inaccuracy. That's why, more often than not, .1 + .2 != .3
Computers make up their way to store decimal numbers


math.stackexchange.com math.stackexchange.com

Suppose you have only two rolls of dice. then your best strategy would be to take the first roll if its outcome is more than its expected value (ie 3.5) and to roll again if it is less.
Expected payoff of a dice game:
Description: You have the option to throw a die up to three times. You will earn the face value of the die. You have the option to stop after each throw and walk away with the money earned. The earnings are not additive. What is the expected payoff of this game?
Rolling twice: $$\frac{1}{6}(6+5+4) + \frac{1}{2}3.5 = 4.25.$$
Rolling three times: $$\frac{1}{6}(6+5) + \frac{2}{3}4.25 = 4 + \frac{2}{3}$$


math.stackexchange.com math.stackexchange.com

Therefore, En=2n+1−2=2(2n−1)
Simplified formula for the expected number of tosses (e) to get
n
consecutive heads(n≥1)
:$$e_n=2(2^n1)$$
For example, to get 5 consecutive heads, we've to toss the coin 62 times:
$$e_n=2(2^51)=62$$
We can also start with the longer analysis of the 5 scenarios:
 If we get a tail immediately (probability 1/2) then the expected number is e+1.
 If we get a head then a tail (probability 1/4), then the expected number is e+2.
 If we get two head then a tail (probability 1/8), then the expected number is e+2.
 If we get three head then a tail (probability 1/16), then the expected number is e+4.
 If we get four heads then a tail (probability 1/32), then the expected number is e+5.
 Finally, if our first 5 tosses are heads, then the expected number is 5.
Thus:
$$e=\frac{1}{2}(e+1)+\frac{1}{4}(e+2)+\frac{1}{8}(e+3)+\frac{1}{16}\\(e+4)+\frac{1}{32}(e+5)+\frac{1}{32}(5)=62$$
We can also generalise the formula to:
$$e_n=\frac{1}{2}(e_n+1)+\frac{1}{4}(e_n+2)+\frac{1}{8}(e_n+3)+\frac{1}{16}\\(e_n+4)+\cdots +\frac{1}{2^n}(e_n+n)+\frac{1}{2^n}(n) $$


blog.jessitron.com blog.jessitron.com

In math, idempotence describes only unary functions that you can call on their own output. Mathidempotence is, “If you take the absolute value of a number, and then you take the absolute value of that, the result doesn’t change on the second (or subsequent) operations.” Math.abs is mathidempotent. Mathidempotence only applies to functions of one parameter where the parameter type and return type are the same. Not so useful in programming.

Programmingidempotence is about side effects. It’s about stuff that happens to the outside world when you call a function. Idempotence says “If you’ve called me once, it doesn’t matter whether you called me again.”


tauday.com tauday.com

Tau Day is an annual celebration of the circle constant τ=6.283185…\tau = 6.283185\ldots, which takes place every June 28 (6/28 in the American calendar system).
Tau (τ) = 2π radians = 360°
It's simply a more intuitive way of representing a full circular rotation. Possibly Euler could've changed it 100 years ago, but too many textbooks already applied the concept of 2π radians instead

 Mar 2020

amitness.com amitness.com

This denotes the factorial of a number. It is the product of numbers starting from 1 to that number.
Exclamation in Python: $$x!$$ is written as:
x = 5 fact = 1 for i in range(x, 0, 1): fact = fact * i print(fact)
it can be shortened as:
import math math.factorial(x)
and the output is:
# 5*4*3*2*1 120

The hat gives the unit vector. This means dividing each component in a vector by it’s length(norm).
Hat in Python: $$\hat{x}$$ is written as:
x = [1, 2, 3] length = math.sqrt(sum([e**2 for e in x])) x_hat = [e/length for e in x]
This makes the magnitude of the vector 1 and only keeps the direction:
math.sqrt(sum([e**2 for e in x_hat])) # 1.0

It gives the sum of the products of the corresponding entries of the two sequences of numbers.
Dot Product in Python: $$X.Y$$ is written as:
X = [1, 2, 3] Y = [4, 5, 6] dot = sum([i*j for i, j in zip(X, Y)]) # 1*4 + 2*5 + 3*6 # 32

It means multiplying the corresponding elements in two tensors. In Python, this would be equivalent to multiplying the corresponding elements in two lists.
Element wise multiplication in Python: $$z=x\odot y$$ is written as:
x = [[1, 2], [3, 4]] y = [[2, 2], [2, 2]] z = np.multiply(x, y)
and results in:
[[2, 4]], [[6, 8]]

This is basically exchanging the rows and columns.
Transpose in Python: $$X^T$$ is written as:
X = [[1, 2, 3], [4, 5, 6]] np.transpose(X)
and results in:
[[1, 4], [2, 5], [3, 6]]

This denotes a function which takes a domain X and maps it to range Y. In Python, it’s equivalent to taking a pool of values X, doing some operation on it to calculate pool of values Y.
Function in Python: $$f:X \rightarrow Y$$ is written as:
def f(X): Y = ... return Y
Using
R
instead ofX
orY
means we're dealing with real numbers: $$f:R \rightarrow R$$ then,R^2
means we're dealing with ddimensional vector of real numbers (in this case, example ofd=2
isX = [1,2]

This symbol checks if an element is part of a set.
Epilson in Python: $$3 \epsilon X$$ is written as:
X = {1,2,3} 3 in X

The norm is used to calculate the magnitude of a vector. In Python, this means squaring each element of an array, summing them and then taking the square root.
Norm of vector in Python (it's like Pythagorean theorem): $$ x$$ is written as:
x = [1, 2, 3] math.sqrt(x[0]**2 + x[1]**2 + x[2]**2)

This symbol denotes the absolute value of a number i.e. without a sign.
Absolute value in Python: $$x$$ is written as:
x = 10 y = 20 abs(x) #10 abs(y) #20

In Python, it is equivalent to looping over a vector from index 0 to index N1 and multiplying them.
PI in Python is the same as sigma, but you multiply (*) the numbers inside the for loop. $$\prod_{i=1}^Nx^i$$

we reuse the sigma notation and divide by the number of elements to get an average.
Average in Python: $$\frac{1}{N}\sum_{i=1}^Nx_i$$ is written as:
x = [1, 2, 3, 4, 5] result = 0 N = len(x) for i in range(n): result = result + x[i] average = result / N print(average)
or it can be shortened:
x = [1, 2, 3, 4, 5] result = sum(x) / len(x)

In Python, it is equivalent to looping over a vector from index 0 to index N1
Sigma in Python: $$\sum_{i=1}^Nx_i$$ is written as:
x = [1, 2, 3, 4, 5] result = 0 N = len(x) for i in range(N): result = result + x[i] print(result)
or it can be shortened:
x = [1, 2, 3, 4, 5] result = sum(x)

2D vectors
2D vectors in Python: $$x_{ij}$$ are written as:
x = [ [10, 20, 30], [40, 50, 60] ] i = 0 j = 1 print(x[i][j]) # 20
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datagenetics.com datagenetics.com

This volume of paper should be the same as the coaxial plug of paper on the roll.
Calculating volume of the paper roll: $$\mathbf{Lwt = \pi w(R^2  r^2)} \~\ L = \text{length of the paper} \ w = \text{width of the paper} \ t = \text{thickness} \ R = \text{outer radius} \ r = \text{inner radius}$$ And that simplifies into a formula for R: $$\color{red} {\bf R = \sqrt{\frac{Lt}{\pi}+r^2}}$$
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 Feb 2020


During our lives we learn that numbers are linear, that the spaces between them are all the same. If you count from one to nine, each number is one more than the previous one. If you ask someone what number is halfway between one and nine, they will say five—but only because they have been taught to. Wake up, sheeple! Humans instinctively perceive numbers logarithmically, not linearly. A young child or someone who has not been indoctrinated by education will place three halfway between one and nine.
Logarithmic middle: middle with respect to multiplication rather than addition > why do we do this by default

 Jan 2020

www.youtube.com www.youtube.com

Good interview. not about AI. ideas in math
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 Sep 2019

theconversation.com theconversation.com

P vs NP completion problems

 Jul 2019

mmlbook.github.io mmlbook.github.io

One major idea in mathematics is the idea of “closure”. This is the question: What is the set of all things that can result from my proposed operations? In the case of vectors: What is the set of vectors that can result bystarting with a small set of vectors, and adding them to each other andscaling them? This results in a vector space
closure in mathematics. sounds similar to domain of a function
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en.wikipedia.org en.wikipedia.org

In Hardy's words, "Exposition, criticism, appreciation, is work for secondrate minds. [...] It is a melancholy experience for a professional mathematician to find himself writing about mathematics. The function of a mathematician is to do something, to prove new theorems, to add to mathematics, and not to talk about what he or other mathematicians have done."
similar to Nassim Taleb's "History is written by losers"
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 May 2019

www.businessinsider.com www.businessinsider.com

If you do the math
Here is the math: 20202023 4 years (Jan 1Dec 31). 1 million annual is 4 million. That is way more than the 1.7 million

four
3 million vehicles divided by 4 years divided by 500,000 per factory makes less than two factories in my book.

 Mar 2019

sprott.physics.wisc.edu sprott.physics.wisc.edu

pi = 3.1415 ... e = 2.718 ... Euler's constant, gamma = 0.577215 ... = lim n > infinity > (1 + 1/2 + 1/3 + 1/4 + ... + 1/n  ln(n)) (Not proven to be transcendental, but generally believed to be by mathematicians.) Catalan's constant, G = sum (1)^k / (2k + 1 )^2 = 1  1/9 + 1/25  1/49 + ... (Not proven to be transcendental, but generally believed to be by mathematicians.) Liouville's number 0.110001000000000000000001000 ... which has a one in the 1st, 2nd, 6th, 24th, etc. places and zeros elsewhere. Chaitin's "constant", the probability that a random algorithm halts. (Noam Elkies of Harvard notes that not only is this number transcendental but it is also incomputable.) Chapernowne's number, 0.12345678910111213141516171819202122232425... This is constructed by concatenating the digits of the positive integers. (Can you see the pattern?) Special values of the zeta function, such as zeta (3). (Transcendental functions can usually be expected to give transcendental results at rational points.) ln(2). Hilbert's number, 2(sqrt 2 ). (This is called Hilbert's number because the proof of whether or not it is transcendental was one of Hilbert's famous problems. In fact, according to the GelfondSchneider theorem, any number of the form ab is transcendental where a and b are algebraic (a ne 0, a ne 1 ) and b is not a rational number. Many trigonometric or hyperbolic functions of nonzero algebraic numbers are transcendental.) epi pie (Not proven to be transcendental, but generally believed to be by mathematicians.) MorseThue's number, 0.01101001 ... ii = 0.207879576... (Here i is the imaginary number sqrt(1). Isn't this a real beauty? How many people have actually considered rasing i to the i power? If a is algebraic and b is algebraic but irrational then ab is transcendental. Since i is algebraic but irrational, the theorem applies. Note also: ii is equal to e( pi / 2 ) and several other values. Consider ii = e(i log i ) = e( i times i pi / 2 ) . Since log is multivalued, there are other possible values for ii. Here is how you can compute the value of ii = 0.207879576... 1. Since e^(ix) = Cos x + i Sin x, then let x = Pi/2. 2. Then e^(iPi/2) = i = Cos Pi/2 + i Sin Pi/2; since Cos Pi/2 = Cos 90 deg. = 0. But Sin 90 = 1 and i Sin 90 deg. = (i)*(1) = i. 3. Therefore e^(iPi/2) = i. 4. Take the ith power of both sides, the right side being i^i and the left side = [e^(iPi/2)]^i = e^(Pi/2). 5. Therefore i^i = e^(Pi/2) = .207879576... Feigenbaum numbers, e.g. 4.669 ... . (These are related to properties of dynamical systems with perioddoubling. The ratio of successive differences between perioddoubling bifurcation parameters approaches the number 4.669 ... , and it has been discovered in many physical systems before they enter the chaotic regime. It has not been proven to be transcendental, but is generally believed to be.)
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 Jan 2019

en.wikipedia.org en.wikipedia.org

A rooted binary tree is full if every vertex has either two children or no children.
Example of Catalan Numbers use case.
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en.wikipedia.org en.wikipedia.org

Catalan numbers notation and short explanation of it.
Use  LookUP: Combinatorics (non crossing combinations) ex: ((())), ()(()), ()()(), (())(), (()())
ref: https://www.geeksforgeeks.org/programnthcatalannumber/

 Dec 2018

www.npr.org www.npr.org

computer owned by Patrick Laroche of Ocala, Fla., discovered the number on Dec. 7,
Newest Mersenne Prime was discovered in Ocala, FL, very close to us.
Interesting tidbit :)
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 Nov 2018

www.coursera.org www.coursera.org

Basic Statistics
part of specialization
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www.coursera.org www.coursera.org

Mathematics for Machine Learning
specialization


meduza.io meduza.io

Начните с нескольких основных книг, которые изменят ваши представления о математикеЛучше всего «ставит голову» книга «Начала теории множеств» Николая Верещагина и Александра Шеня. Она даст основу для понимания (а в дальнейшем и самостоятельного построения!) логических рассуждений. С нее же начнется и понимание теории множеств, лежащей в основе современной математики.Охватить больше разделов математики поможет книга Рихарда Куранта и Герберта Роббинса «Что такое математика?». Как и книгу Верещагина и Шеня, эту книгу нужно читать внимательно, делая все упражнения. Если первые две книги окажутся сложными, можете начать с моей «Математики для гуманитариев». Ее также следует читать с самого начала, страницу за страницей, не стоит браться за чтение с середины. Она не очень простая, но предварительных сведений и математической культуры не предполагает.Чтобы понять, каким образом математика входит в нашу жизнь, можно прочитать «Кому нужна математика?» Андрея Райгородского и Нелли Литвак или «Математическую составляющую» (сборник сюжетов под редакцией Николая Андреева).
nice books

 Sep 2018

blog.emacsen.net blog.emacsen.net

Learning Math Earlier With Computers

 Apr 2018

www.tandfonline.com www.tandfonline.com

we found advanced math and science courses advertised but not actually offered and specialized STEM programs eroded or discontinued within a few years of inception.
Some interesting data points in this article.


www.edweek.org www.edweek.org

Yet, across the country, 2 in 5 high schools don't offer physics, according to an Education Week Research Center analysis of data from the U.S. Department of Education's office for civil rights.
How widely known is this figure?



n this note we determine thetorsion subgroup of the additive group of (1.1) in the case whereLhas rank 2, thatisXis a set of two elements, andcis a prime number
Wow!
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link.springer.com link.springer.com

Of particular interest are the lower central series: G1=GG1=GG_1 =G, Gi+1=[Gi,G]Gi+1=[Gi,G]G_{i+1} = [G_i, G], i≥1i≥1i \ge 1, and, for a fixed prime number p, the Zassenhaus series (see [7, 8]).
This is interesting.


www.exploratorium.edu www.exploratorium.edu

Leonhard Euler, who adopted it in 1737
Actually, he first used π in 1727, and it meant 6.28.., not 3.14..
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 Mar 2018

www.wiasberlin.de www.wiasberlin.de

 Gummel mapping/method
 the part about machineexecuted proofs sounds like https://media.ccc.de/v/34c39105coming_soon_machinechecked_mathematical_proofs_in_everyday_software_and_hardware_development
 "semantic TeX" => CTAN package?
 OMDoc
 sTeX
 MMT
 TGView
 MPD Viewer
 OpenDreamKit = domainspecific language?
 MaMoReD
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www.wiasberlin.de www.wiasberlin.de

 R/(gg)plots visualise => linguists detect interesting phenomena to analyse further
 Wang2Vec: train backprogagation network
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 Feb 2018

ytproject.github.io ytproject.github.io

Let’s use some common units as examples: gram (g), erg (erg), and solar mass per cubic megaparsec (Msun / Mpc33^3). g is an atomic, CGS base unit, erg is an atomic unit in CGS, but is not a base unit, and Msun/Mpc33^3 is a combination of atomic units, which are not in CGS, and one of them even has an SI prefix. The dimensions of g are mass and the cgs factor is 1. The dimensions of erg are mass * length$^2$ * time−2−2^{2} and the cgs factor is 1. The dimensions of Msun/Mpc33^3 are mass / length33^3 and the cgs factor is about 6.8e41.
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 Jul 2017

www.degruyter.com www.degruyter.com

satisfy the two usual distributive laws. Singlesemirings as well as classes of semirings form important structures in Automataand Formal Languages Theories [5
Like so.

 May 2017

coolwanglu.github.io coolwanglu.github.io

1st order Eulerian numbers:Permutations
Annotate math with math!
$$\varepsilon = \frac{2}{h^3} \int_0^{p_F} \sqrt{p^2 c^2 + m^2 c^4} \cdot 4 \pi p^2 dp=$$
$$\frac{8 \pi}{h^3} \frac{m c^2}{\lambda^3} \int_0^x \sqrt{1+y^2} \cdot y^2 dy $$
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www.annualreviews.org www.annualreviews.org

The first review, by C. Hendricks Brown et al., poses the issues raised by the growingrecognition
$$\varepsilon = \frac{2}{h^3} \int_0^{p_F} \sqrt{p^2 c^2 + m^2 c^4} \cdot 4 \pi p^2 dp=$$
$$\frac{8 \pi}{h^3} \frac{m c^2}{\lambda^3} \int_0^x \sqrt{1+y^2} \cdot y^2 dy$$

 Apr 2017

www.cogentoa.com www.cogentoa.com

For detailed study of various properties, generalization and application of Wrightfunction and generalized Wright function,

 Jan 2017

betterexplained.com betterexplained.com

Whether you're a student, parent, or teacher, this book is your key to unlocking the aha! moments that make math click  and learning enjoyable.
You had me already at the Coffee Cup picture over the equations! :)


academic.evergreen.edu academic.evergreen.edu

LSB (least significant bit): The rightmost bit MSB (most significant bit): The leftmost bit
This only applies to Littleendian architecture. Bigendian is reversed.

 Nov 2016

www.knewton.com www.knewton.com

x2
maybe it should be \(x^3\)

 Oct 2016

bullshit.ist bullshit.ist

Sunil Singh asks us to stop promoting mathematics based on its current applications in business and science. Math is an art that should be enjoyed for its own sake.
This reminded me of A Mathematician's Lament by Paul Lockhart. This is a 25page essay which was later worked into a 140page book. (And Sunil Singh has read at least one of them. He credits Lockhart in one of the replies.)
It also reminds me of this article on the history of Gaussian elimination and the birth of matrix algebra. Newton's algebra text included instructions for solving systems of equations  but it didn't have much practical use until later. (Silly word problems are as old as mathematics.)

 Sep 2016

en.wikipedia.org en.wikipedia.org

A forest is just a collection of trees. The main difference is that a forest does not necessarily need to be connected.

 Jul 2016

www.quantamagazine.org www.quantamagazine.org

hyperuniform distribution  Appears random at smaller scales, but more predictable at larger scales.


steinhardt.nyu.edu steinhardt.nyu.edu

The first set, called Math Instructional, was for apps that would make math relevant for students by linking it to their lives and enabling students at different ability levels to work together
This has real potential because of the research base that supports the instructional approach.

 Mar 2016

www.quantamagazine.org www.quantamagazine.org

New property of prime numbers discovered. Primes greater than 5 can end with 1, 3, 7, or 9. The next prime is less likely to end with the same digit, and biased toward one of the remaining three. For instance, a prime ending in 3 is most likely to be followed by a prime ending in 9. The bias evens out as the primes get larger, but only very slowly.

 Feb 2016

www.mathcs.gordon.edu www.mathcs.gordon.edu

Number Theory: In Context and Interactive, KarlDieter Crisman (online textbook)


linear.pugetsound.edu linear.pugetsound.edu

A First Course in Linear Algebra, Robert A. Beezer<br> Free as PDF or online.
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www.complexityexplorer.org www.complexityexplorer.org

Free courses and tutorials from the Santa Fe Institute on subjects related to complex systems science.

 Jan 2016

blogs.scientificamerican.com blogs.scientificamerican.com

P(BE) = P(B) X P(EB) / P(E), with P standing for probability, B for belief and E for evidence. P(B) is the probability that B is true, and P(E) is the probability that E is true. P(BE) means the probability of B if E is true, and P(EB) is the probability of E if B is true.

The probability that a belief is true given new evidence equals the probability that the belief is true regardless of that evidence times the probability that the evidence is true given that the belief is true divided by the probability that the evidence is true regardless of whether the belief is true. Got that?


www.gutenberg.org www.gutenberg.org

I would like to see an accurate array of photographs of these tasty lunch options. What does a a "Princess Sandwich" even look like? Is a "Celery Sandwich" satisfying? I'd be pleased to see precise measurements of the ideal "Tea Biscuit" Sandwich.


categorytheory.mitpress.mit.edu categorytheory.mitpress.mit.edu

Category Theory for the Sciences by David I. Spivak<br> Creative Commons AttributionNonCommercialShareAlike 4.0<br> MIT Press.



Open source textbooks approved by the American Institute of Mathematics.
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 Mar 2015

arxiv.org arxiv.org

θ dμ ≥ p 16 π  Σ 
Qual a relação dessa desigualdade com a dita desigualdade de Penrose Riemanniana provada por HuiskenIlmanen e Bray?

GIBBONSPENROSE INEQUALITY
Qual a relação dessa desigualdade com a dita desigualdade de Penrose Riemanniana provada por HuiskenIlmanen e Bray?

θ dμ ≥ p 16 π  Σ 
Isso significa que a taxa expansão nula futura (para fora) \( \theta \) é no mínimo $$ \sqrt{16 \pi \Sigma} $$

 Jan 2015

mathinsight.org mathinsight.org

A function like f(x,y)=x+y is a function of two variables. It takes an element of R2, like (2,1), and gives a value that is a real number (i.e., an element of R), like f(2,1)=3. Since f maps R2 to R, we write f:R2→R. We can also use this “mapping” notation to define the actual function. We could define the above f(x,y) by writing f:(x,y)↦x+y. To contrast a simple real number with a vector, we refer to the real number as a scalar. Hence, we can refer to f:R2→R as a scalarvalued function of two variables or even just say it is a realvalued function of two variables. Everything works the same for scalar valued functions of three or more variables. For example, f(x,y,z), which we can write f:R3→R, is a scalarvalued function of three variables.
f:R^2 \rightarrow R demek f(x,y)=z  SkalarDeğerli f f:R \rightarrow R^2 demek f(x)=(y,z)  VektörelDeğ f

f:R→R as a shorthand way of expressing that f is a function from R onto R.
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 Sep 2013

en.wikipedia.org en.wikipedia.org

A computable Dedekind cut is a computable function which when provided with a rational number as input returns or ,
This definition of computable Dedekind cut is wrong. The correct definition is that the lower and the upper cut be computably enumerable.
