#mathematical proof

[simplicius-simplicissimus]

Q.E.D. - quod erat demonstrandum (that which was to be demonstrated)

-Placed normally at the end of mathematical proofs or philosophical arguments

[catscomputerscreativewriting]

It's so funny to me that people think of Math/Mathematicians as being hyper-logical and rational. Like, have you seen some of the wild things hiding in the Math?

Did you know there are non-computable numbers?? (https://en.wikipedia.org/wiki/Chaitin%27s_constant)

Did you know that there are things that are true, but we can't prove them??? (https://en.wikipedia.org/wiki/G%C3%B6del%27s_incompleteness_theorems)

Did you know that we can prove that something exists, and yet never actually figure out what that thing is?? (https://mathworld.wolfram.com/NonconstructiveProof.html)

Math is crazy. Math is wild. Math hardly makes sense, and when you think you understand the weirdest parts of it, everyone who hears you explain it to thinks you're a gibbering lunatic.

"In mathematics you don’t understand things. You just get used to them." - von Neumann

(please share more unhinged math with me, i want to see more scary math)

[catboydan]

my very specific prediction for the upcoming AITA video: he'll open by explaining that last week on the gaming channel he had an 'AITA' moment himself [insert clips from the dan eye incident here]

then he'll lead into the vid by saying something like "and that inspired me to ask you all for your 'am I the hole' moments", and only then will he bring up that this time, he asked for specifically dating/love-related stories, leaving us to piece together that the intro is a dating/love related story via the maxim of relevance

[cuubism]

Math Dream and Comp Sci Hob in Complex Math verse could achieve infinite wealth doing this if they tried, unfortunately they're too busy fucking

[simplydnp]

who's hand is in this picture?

A Mathematically Rigorous Proof That I Spent Too Long Writing

welcome to university math: dnp hand edition

(no, don't leave, you'll be fine i promise)

to begin, we need a statement to prove. we have two options:

- the hand is dan's hand

- the hand is phil's hand

now, for most proofs in university math, you are told a true statement, and you must show why it is true using logic rules, definitions, and theorems. but, we do not know which of these statements are true, so we have to find out.

to prove that a statement is true, we must show that it is always true for the situation presented. to show a statement is false, we must present a single instance where the statement is false (also known as a counter example).

a quick not scary math example:

definition: a prime number is only divisible by 1 and itself.

statement: all prime numbers are odd

(this is false, because 2 is a prime number and it is even. you don't even need to check if there's any others, all you need is one single case where it isn't true to disprove it)

so now that we have a little background on proofs and how to prove and disprove them, we go back to our two statements.

the thing with this situation is, one of them must be true (unless you're gung-ho on someone else holding dan's face while phil takes a picture on his phone of dan in his glasses, in which case, i applaud your commitment, but in actuality this proof will cover that option too)

the full statement we have is: dan is touching his face or phil is touching dan's face

now, because this is Real Life and we have a picture where a hand is touching dan's face, we know already that one of these options is true (as mentioned above) but! using symbolic logic you could also come to this conclusion.

this type of statement is an 'or' statement, and if you're curious, you can look into 'truth-tables' and see why, but at least one of the options must be true.

back to the proof at hand (bah-duhm-tss)

okay. now, proofs also must be 'general' in order to mean anything, really. these are statements of truth of the universe, not just for individuals. so, we will prove this generally.

we have 2 people involved, so individual 1 (dan, the owner of the face and potential face toucher) will be labelled as 'D' , and individual 2 (phil, the possible face toucher who does not own the face) will be labelled as 'P'. thus, this can be true for any such D and any such P.

so with our 'or' statement, in order to prove it, we pick one of the options and say that it is not true, and we have to show then that the other is true.

step 1: let's assume this is not P's hand. (assumption)

step 2: thus, it must be D's hand. (what we take from our assumption)

step 3: now, if it is D's hand, we look at what a hand on one's own face is capable of appearing like. (a definition or true fact about step 2)

the position in the given photo shows the hand with a thumb on the cheek, and a finger on the forehead. so, we find an example of a person with their fingers in the same position (or close to) and see if this supports our claim.

consider:

now, with this image, you can clearly see how the subject's right hand has the thumb on the temple and index finger on the top of their head, however, it is a close enough position for our case.

from the view of the camera, the closest finger to the camera is the edge of the pinkie. in fact, it will always be the closest finger to the camera in this position, assuming the subject has all fingers and no additional appendages.

step 4: we now compare this to our photo (we verify if this holds to our claim or contradicts it)

in our photo, the closest appendage to the camera is the edge of the thumb.

step 5: thus, it cannot be the case that D is touching their own face. (what the evidence says)

step 6: as we assumed it was not P's hand and have shown it cannot be D's hand, and as this is an 'or' statement both of these claims cannot be false, we can therefore conclude it must be P's hand. (our conclusion: re-stating the statement and assumptions and conclusion)

step 7: we verify that P is true (optional step but in beginner proofs you generally show why your case works)

to do this, i will show a picture of a person touching another's face, and compare it to our image.

consider:

now, this image is not exactly the same, similar to above. however, P's left thumb is on the cheek, with their index on D's temple. the closest appendage to the camera (if it were in a similar perspective as our original) would be the edge of the thumb.

comparing it to our original:

our comparison holds.

thus, we can conclude that the true claim in this statement is that P must be touching D's face, which, in particular means that:

phil is touching dan's face in the image

thank you for partaking in phannie mathematics. we now know. i am not sorry.

bonus:

phil has a hitchhikers thumb and dan doesn't so why was this necessary at all 🤡

[pv1isalsoimportant]

(Semi-regularly updated) list of resources for (not only) young mathematicians interested in logic and all things related:

Igor Oliveira's survey article on the main results from complexity theory and bounded arithmetic is a good starting point if you're interested in these topics

Eitetsu Ken's list for resources on proof complexity, computational complexity, logic, graph theory, finite model theory, combinatorial game theory and type theory

The Complexity Zoo for information on complexity classes

The Proof Complexity Zoo for information on proof systems and relationships between them

Computational Complexity blog for opinions and interesting blog posts about computational complexity and bunch of other stuff

Student logic seminar's home page for worksheets on proof complexity and bounded arithmetic (great introduction for beginners)

Jan Krajíček's personal page is full of old teaching materials and resources for students (click past teaching). I also recommend checking out his books. They are basically the equivalent of a bible for this stuff, although they are a bit difficult to read.

Amir Akbar Tabatabai's page for materials on topos theory and categories including lecture notes and recording of lectures

Lean Game Server for learning the proof assistant Lean by playing fun games

[noosphe-re]

In 1931, the Austrian logician Kurt Gödel pulled off arguably one of the most stunning intellectual achievements in history. Mathematicians of the era sought a solid foundation for mathematics: a set of basic mathematical facts, or axioms, that was both consistent — never leading to contradictions — and complete, serving as the building blocks of all mathematical truths. But Gödel’s shocking incompleteness theorems, published when he was just 25, crushed that dream. He proved that any set of axioms you could posit as a possible foundation for math will inevitably be incomplete; there will always be true facts about numbers that cannot be proved by those axioms. He also showed that no candidate set of axioms can ever prove its own consistency. His incompleteness theorems meant there can be no mathematical theory of everything, no unification of what’s provable and what’s true. What mathematicians can prove depends on their starting assumptions, not on any fundamental ground truth from which all answers spring.

Natalie Wolchover, How Gödel’s Proof Works, Quanta Magazine, July 14, 2020

[rabbiteclair]

love to deploy something to production while telling the head of engineering "listen, when you guys built this thing about a decade ago you didn't make it capture any metrics, so I had to really guess about the throughput, and if I got it wrong then it will explode horribly in the morning, but the good news is that I'm the one on-call so it's ultimately my problem if that happens anyway"

[lipshits-continuous]

Pretty neat that for a topological group G with identity element e, we have that π₁(G,e) is abelian

[felixcloud6288]

Did you know if a number is divisible by 3, you can add all the digits of the number and the sum is divisible by 3? For example, 327 is divisible by 3 and 3+2+7 = 12, which is divisible by 3.

For the Proof on why this is, click Read More

Prerequisite Knowledge

Numbers

Yes, I need to explain how numbers work for this proof.

Let's go back to elementary school. You're being taught basic addition of multi-digit numbers. You're given the number 54,321. 54,321 = 50,000 + 4,000 + 300 + 20 + 1

Let's expand that a little.

54,321 = 5*10,000 + 4*1,000 + 3*100 + 2*10 + 1

Now I''m going to take an aside and mention exponents, just in case.

Normal notation for exponents are a base number with a smaller number floating on the top right corner of your base value. Since Tumblr can't support that notation, I'll use a^b as the notation.

When you have an exponent, you have to multiply the base number by itself however many times the exponent number is. For example, 3^4 = 3*3*3*3 = 81.

Also, if the exponent value is 0 or smaller, you start dividing by the base number instead. So 3^0 = 3/3 = 1.

Now that I've explained that, let's go back to earlier.

54,321 = 5*10,000 + 4*1,000 + 3*100 + 2*10 + 1

Each digit in 54,321 is beling multiplied by an exponential power of 10.

54,321 = 5*10^4 + 4*10^3 + 3*10^2 + 2*10^1 + 1*10^0

This expanded form of an arbitrary number is going to be necessary for the proof.

Modulo and Equivalence

Let's go back to elementary school again. Remember how in division, you'd give a remainder as part of your answer? 12 / 5 = 2 remainder 2.

The modulo operator returns the remainder when you divide two numbers. In programming, % is used for modulo so I'll use that for notation. So 12 % 5 = 2.

An expansion of modulo is equivalence. Two numbers are equivalent under a modulo value when both numbers have the same remainder when divided by the modulo value. Under modulo value 5, 7 and 12 are equivalent because 7 % 5 = 2 and 12 % 5 = 2.

The normal notation for equivalence is the equal sign with 3 lines instead of 2. I'll use == for the notation here.

Since 7 and 12 are equivalent under mod 5, the notation is 7 == 12 (mod 5)

If a number is evenly divisible by a modulo power, it's equivalent to 0. 5 / 5 = 1, therefore 5 % 5 = 0, therefore 5 == 0 (mod 5)

There are some special rules about numbers that are equivalent. We'll let a, b, c, and m be arbitrary numbers.

If a == b (mod m), then a+c == b+c (mod m) If a == b (mod m), then a*c == b*c (mod m) If a == b (mod m), then a^c == b^c (mod m)

The Proof

Now let's move onto the proof.

Let's say we have some arbitrary whole number X which is divisible by 3. That means there is some whole number c such that X = 3*c. So if X is 327, c would be 109 because 109*3 = 327.

Now let's rewrite X into the expanded form from the Numbers section. We'll let d(0) refer to the least significant digit (The one at the far right). d(1) will be the next least significant digit and so on until we reach d(n), the most significant digit.

X = d(n)*10^n + d(n-1)*10^(n-1) + ... + d(1)*10^1 + d(0)*10^0

Since X is divisible by 3, that means the long form of X is equal to 3c.

d(n)*10^n + d(n-1)*10^(n-1) + … + d(1)*10^1 + d(0)*10^0 = 3*c

Now let's start doing some modulo.

10 == 1 (mod 3)

By the earlier rules mentioned in the Modulo section, we can multiply and exponentiate each term (The parts between the +) and they're still equivalent. So now we'll replace each term with an equivalent term under mod 3.

d(n)*10^n + d(n-1)*10^(n-1) + ... + d(1)*10^1 + d(0)*10^0 == d(n)*1^n + d(n-1)*1^(n-1) + … + d(1)*1^1 + d(0)*1^0 (mod 3)

1 exponentiated to any power is 1 so

d(n)*10^n + d(n-1)*10^(n-1) + ... + d(1)*10^1 + d(0)*10^0 == d(n) + d(n-1) + … + d(1) + d(0) (mod 3)

Meanwhile 3*c == 0 (mod 3). Since 3*c = X, then anything equivalent to X is equivalent to 0.

d(n) + d(n-1) + … + d(1) + d(0) == 0 (mod 3)

At this point we can say the sum of all the digits of X can be divided by 3 because the sum would have no remainder.

So in conclusion, if the sum of all the digits of a number can be divided by 3, then the number can be divided by 3.

[humormehorny]

Yo math people, what was your first proof?

[lorei-writes]

... Isn't it funny that in order to gain confidence, you need to do things you don't feel confident about, the ones that push you simply force you to confront exactly how much you can do?

Ha. I tell myself to do things "in the ways I do not know". Maybe there is some merit to that, after all.

[catscomputerscreativewriting]

Listen, textbook, I admire the dedication to grifting my students into learning most of Constructive Logic before they've realized it. They are computer scientists after all. It's good for them. But... really? No explanation of formal proofs by contradiction in the undergraduate, Logic for CS textbook? You do include double negation elimination, which is an equivalent Classical axiom, but not, like, (A -> Bottom) -> ~A?

You mean I'm gonna have to derive that myself from the completely random and arbitrary Classical axioms you've decided to adopt and then neglected to prove the completeness of? Otherwise, all of these proofs will become extremely cumbersome and all of my students will swear off CS theory for the rest of their lives?

... ok fine, it'll be in the slides. T-T

[eulerseverything]

About to start a new semester, taking Foundations for Theoretical Mathematics and Differential Equations. Very excited to become even more annoying about math to my friends. Anyway does anyone on mathblr recommendations for either course cause I’m a freshman taking upper divs so I need the help

[simplydnp]

i love using my math degree to prove whose hand is in the dan-in-phils-glasses pic

[lonardmanaloto]

Blaise Pascal (born June 19, 1623, Clermont-Ferrand, France—died August 19, 1662, Paris)

a French mathematician, physicist, religious philosopher, and master of prose.

Pascal's Triangle

A pascal's triangle is a triangular pattern of numbers where each row starts and ends with a 1, while the inner numbers are the sum of the two numbers directly above them. This idea is commonly applied in probability, combinatorics, and algebra.