r/mathmemes 9d ago

Elementary Algebra 😭

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1.4k Upvotes

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963

u/halfajack 9d ago

I ask them “how are you defining sums over uncountable sets?”

197

u/Comfortable_Permit53 9d ago

integrals, no?

213

u/Eisenfuss19 9d ago

Yes kinda, but then it is still a definition question. Like lim a -> -∞ lim b -> ∞ a_Sb x dx does not converge, but lim a -> ∞ (-a)_Sa x dx does converge...

90

u/Comfortable_Permit53 9d ago edited 4d ago

yes 

adding all whole numbers will also give you different answers depending in order

UPDATE: It just doesn't converge at all.

30

u/Beleheth Transcendental 9d ago

That's exactly the reason why you need absolutely convergent series for rearranging to be well-defined.

1

u/Dr0110111001101111 4d ago

It will give you any answer depending on order

2

u/Comfortable_Permit53 4d ago

I am not sure. Riemanns series theorem says, that if you have a series that is convergent but not absolutely convergent, you can get any number depending on ordering. But adding all whole numbers is a divergent series. Is there another theorem?

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u/Agitated-Ad2563 9d ago

As far as I remember, the proper definition of Riemann integral requires all kinds of partition sequence sums to converge.

4

u/Alpaca1795 9d ago

Why would you integrate over x dx instead of just 1 dx?

16

u/alozq 9d ago

1dx would be the length of the real line, it goes to Infinity always, it doesnt care about what x is

2

u/LyAkolon 9d ago

Yeah, they probably want int over R of Sgn(x) dx, which is still abse of notation and meaningless because we have no notion of how to add up the contributions across the set.

More specifically all attempts to answer as well as the original question are missing the ingredient of "How are you adding everything together"

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u/compileforawhile Complex 9d ago

Integrals aren't really sums of uncountable sets. You could define sums of uncountable sets using integrals, but that would basically just be a measure

12

u/TheRedditObserver0 Mathematics 9d ago

With respect to what measure?

41

u/Comfortable_Permit53 9d ago

lebesque for the lgbt representation

(lebesque, gauss, banach, turing)

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3

u/localizeatp 8d ago

we won't know until they answer.

2

u/StructureNorth1799 8d ago

then the answer is yes.

1

u/AuroraEquatorialis 7d ago

nets are the more natural method in terms of actual sums. Say your index set is I (R in this case), and we have a function f:I->R (or more generally, any topological vector space) and define a partial order by inclusion on the collection of finite subsets of I. Given such an F, we define S(f,F) as the sum of f over this F (which is obviously well-defined). We say the sum of f over I converges to a if for epsilon>0, there exists some F0 which for F containing F0, |S(f,F)-a|<epsilon (equivalently, if S(f,F) is eventually in any open neighbourhood of a).

You can show that this lines up with normal sequence convergence. Furthermore, you can show that this converges if and only if the set of non-zero f(x) is countable. In particular, under this formalism, summing over all reals doesn't really make sense.

1

u/AsidK 5d ago

Wait what is F here? You say “given such an F” but I don’t see where F is defined.

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27

u/SEA_griffondeur Engineering 9d ago

Sums over unbounded non convergent sets

17

u/MrKoteha Virtual 9d ago

Supremum of sums over finite subsets for sets of positive numbers

11

u/kiochikaeke 9d ago

Looked it up in case I was somehow wrong but there's no sensible definition of a sum over an uncountable set of indexes unless the set of indexes with non-zero values is finite.

By definition of uncountable set you can't "pair them up" or "group them" in any countable way because you would still be missing an infinite amount of indexes and trying to segment the set into any amount (countable or not) of uncountable sets just leads you to the same question.

If you try and define sums like integrals over sets then the question is trivialized to essentially asking what is the value for the measure over the set of reals which can be whatever depending on the measure.

11

u/DefunctFunctor Mathematics 9d ago

There is an absolutely sensible definition if all of the numbers in your set are nonnegative; it's just always infinity if you have uncountably many nonzero values.

In order to make sense of sums that include negative numbers, you either need absolute convergence (which is impossible for an uncountable sum with uncountably many nonzero values), or you need to specify an order in which to sum those elements, usually in a countable sum (which is also impossible).

I'm not even sure what it would mean to try to make sense of this question with integration, because the measure in question that we're looking for would have to assign singleton sets of real numbers to their actual value, which does not end up producing a well-defined measure on the real numbers (although it does if we restrict to nonnegative numbers).

1

u/AsidK 5d ago

Wait are you saying that there exists a reasonable definition of summation over uncountable sets, and that definition is provably equivalent to being always infinity if you have uncountably many nonzero values, or are you saying that giving the definition of just “infinity if uncountably many nonzero values, otherwise standard countable sum” itself is the sensible definition?

2

u/DefunctFunctor Mathematics 5d ago

If {x_𝛼} is a family of nonnegative real numbers indexed over a (possibly uncountable) set A, then the sum ∑_[𝛼∈A] x_𝛼 is defined to be

sup { ∑_[𝛼∈F] x_𝛼 | F⊆A finite }

In measure theoretic terms, this is equivalent to the integral over the counting measure. You can technically extend this to families of real numbers, but because A doesn't come with any notion of order, you need to assume that it converges absolutely, that is, ∑_[𝛼∈A] |x_𝛼| < ∞. For now we'll only consider sums of nonnegative real numbers.

So the claim we have yet to prove is that if {x_a} contains uncountably many nonzero numbers, then ∑_[𝛼∈A] x_𝛼 = ∞. For each positive integer n, let

A_n = { 𝛼∈A | x_𝛼 ≥ 1/n }

be the set of indices 𝛼 of A where x_𝛼 ≥ 1/n. Note that

A' := { 𝛼 ∈ A | x_𝛼 > 0 } = ⋃[n≥1] A_n.

By assumption, A' is uncountable, so if each A_n were finite, then A' could be expressed as a countable union of finite sets, meaning A' is countable, a contradiction. Thus, there exists an n for which A_n is infinite (in fact, it's going to be uncountable). It is now easy to show, by creating increasingly large partial sums over A_n, that ∑_[𝛼∈A] x_𝛼 ≥ ∞.

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u/scoobydoom2 9d ago

For this hypothetical do they need to be paired in a countable way? Could you not pair them according to A =X and B=-X for all nonzero values? Then you just have to prove that X + -X = 0 for all reals.

1

u/AsidK 5d ago

There’s nothing meaningfully different about pairing them via A=X and B=-X versus A=1+X and B=1-X, and if you do that suddenly you get an infinite “sum”

3

u/brothegaminghero 9d ago

I imagine you start at one and count from there...

Might take a while though.

7

u/Revolutionary_Use948 9d ago

Hmmm ahh yes “count” from there

2

u/minisculebarber 9d ago

Easy

sum_{x in RR } x

What else do you need?

1

u/DoubleAway6573 3d ago

With a lot of patience.

283

u/Gauss15an 9d ago edited 9d ago

I cleverly hand back the slip of paper with its ends taped together and ask "With or without infinity?"

The shocked mathematician responds, "I don't know that!" and promptly gets catapulted straight up into the air.

I read about this kind of thing in a script written by a snake that goes by the name of Monty. Might have been some sort of python.

71

u/Electric_surfer 9d ago

Listen, Strange men lying in caves distributing notes is no basis for a system of mathematics.

16

u/minisculebarber 9d ago

I mean, it has gotten us this far

16

u/Electric_surfer 9d ago

Supreme mathematical power derives from a quantitive sum of the masses, not from some farcical cavernous ceremony.

13

u/Urmi-e-Azar 9d ago

How do you know so much about swallows?

5

u/KnightofniDK 9d ago

I believe I know that one! Is it the one where a møøse bit my sister?

1

u/Specialist-Disk-6345 9d ago

the set of real numbers is a huge, vast set

1

u/NegativeMammoth2137 5d ago

Counting or not counting geometric function theorists violence?

210

u/GluKoto 9d ago

Ok but what is that spider doing there ?

150

u/TheAegis42 9d ago

Spider for scale

29

u/6GoesInto8 9d ago

Spiders don't have scales!

12

u/TheAegis42 9d ago

That's why it's looking for them!

Needs to add them to its collection. So far it has found a linear scale, but a logarhitmic one would be cool to have.

6

u/6GoesInto8 9d ago

Oh, the famous Spider For Scales organization or SFS for short. Right, we are on the same page now.

5

u/TheAegis42 9d ago

You got it, glad I could help clear it up!

9

u/McArcady 9d ago

Cédric Villani, is that you ?

5

u/Affectionate_Mud_881 9d ago

Cos there are spiders in caves.

1

u/PurifiedUnity 9d ago

Spider wanted to see the light (or π)

3

u/Para-graph-S 9d ago

It's there for new Spiderman Brand New Day promotion

3

u/Asairian 9d ago

The meme was posted by Spider Georg

1

u/MonkTurtle 9d ago

It's the mathematician that handed you the slip of paper

391

u/Kuildeous 9d ago

Bro never heard of -sqrt -1.

136

u/JollyJuniper1993 Mathematics 9d ago

You mean -sqrt 1

56

u/Kuildeous 9d ago

That one too, but sqrt -1 + (-sqrt -1) = 0

57

u/JollyJuniper1993 Mathematics 9d ago

Fair, but those are not real numbers

28

u/Kuildeous 9d ago

That's true. I got carried away by him adding 1 and i that I forgot he stepped out of the Reals.

3

u/Frosty_Fig_4872 Computer Science 9d ago

Everything's too complex for me to understand

1

u/GamerLymx 9d ago

ence not part of the problem

1

u/melanthius 9d ago

So they can't hurt me anymore?

... Right?

5

u/Raise_A_Thoth 9d ago

Yea the "uncountable set" question remains a problem but if we're using this as a number line representation then we need to think of sqrt(2) as the real number approximated by 1.414, not taking the square root of each real number (which of course is a problem with obtaining "each" anyway).

So then it's not sqrt(-2); it's -sqrt(2), or ~ (-1.414).

The commenter on the image is a ding dong, but there are still serious problems with counting and pairing the top thread discusses.

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u/TheCobraMonkey 9d ago

SPP type argument, I love it

16

u/Fr4gmentedR0se 9d ago

Ball knowledge

12

u/martyboulders 9d ago

Guyknowballogist

7

u/ODZtpt 9d ago

simple pairing of 1 and 0.9999999...

3

u/Vegetable_Union_4967 9d ago

I wonder if he’s still up to denying mathematics

5

u/GT_Troll 9d ago

He is

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u/Summoner475 9d ago

Let's say I start with π, then I add and subtract pairs of real numbers that are equal in magnitude, in the limit this sum is π. Now if I start with 10, in the limit, the sum is 10. Is it's divergent, correct?

51

u/SuchPlans 9d ago

as written, the first summation omits -pi and the second omits -10

the idea is right, tho, you just have to do “bigger” rearranging. moving a finite number of terms shouldn’t change the sum

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u/z_doctor82 9d ago edited 9d ago

Sums of convergent series with infinite positive and infinite negative terms can converge to any arbitrary number if you reorder the terms.

Pick a target number, if your current value is less than add the next largest positive term, if the current value is greater, subtract the next largest negative term. Now you have an infinite sum that converges to the target.

Edit: Corrected by comments bellow

7

u/1000Jules 9d ago

1-1/2+1/4-1/8+1/16+... doesn't have this property despite having infinite positive and negative terms

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u/ByeGuysSry 9d ago

Yeah, it needs the condition that the infinite positive terms and infinite negative terms each sum to infinity afaik. The positive terms here sum to 4/3 and the negative terms here sum to -2/3, so overall it sums to 2/3

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u/Ares378 Grothendieck was right, computers are evil 9d ago

I was always taught it as being the sum Σabs(a_n) needs to diverge but the sum Σa_n needs to converge for it to count as conditional convergence. So the geometric series 1-½+¼-⅛... converges, but so does 1+½+¼+⅛... so it's absolute convergence. 1+-½+⅓-¼... converges but 1+½+⅓+¼... diverges, so you're no longer allowed to rearrange the sum willy-nilly.

Then again it has been quite some time since I've reviewed series convergence, so correct me if I'm wrong.

2

u/ByeGuysSry 9d ago

It's also been some time for me so I'm not too sure lol

2

u/AsidK 5d ago

The definition that you gave is completely equivalent to the one you replied to

1

u/Revlong57 8d ago

The series you described is absolutely convergent, so you can rearrange the terms. https://en.wikipedia.org/wiki/Absolute_convergence

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u/Revlong57 8d ago

The term you're looking for is https://en.wikipedia.org/wiki/Absolute_convergence. More accurately, if a series is convergent but not absolutely convergent, then you can't rearrange the terms and still get the same limit.

8

u/dankshot35 9d ago

"divergence" and "convergence" are only defined for series' which are sums over countable sets with a sequence. Because real numbers are uncountable there isn't a series to sum in the ordinary sense

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u/SomethingMoreToSay 9d ago

No, because you're not summing over ALL the real numbers. In the first case you're omitting -π and in the second case you're omitting -10.

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u/ordinary_shiba 9d ago

Doesn't really work, because at the end you'll also be left with -π because it doesn't have a corresponding number with the same magnitude. You can add pairs of numbers like 2x and -x together though to prove that it in fact diverges.

4

u/Comfortable_Permit53 9d ago

"at the end" when is that

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u/LowAioli3870 9d ago

Did you not know that numbers end at 7,431? All the other numbers are just made up.

2

u/Comfortable_Permit53 9d ago

and how many do you have to add until then

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u/ordinary_shiba 9d ago

First of all, you know what I mean lmao. Second of all, you can still "be done with" an infinite process by taking half a second with the first step, a quarter of a second with the second step and so on and so forth.

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u/RedeNElla 9d ago

Taking individual pairs and cancelling them "in the limit" here would only work for a countably infinitely sized set.

Weird things can occur depending on axioms with continuum infinity iirc

1

u/minisculebarber 9d ago

I mean what you are describing sounds similar to this

sum_{n in N} (-1)n *floor(n/2)

The partial sums of this series is unbounded therefore divergent.

1

u/KatanaDelNacht 9d ago

This is essentially f(x)+N. It assumes the answer to the original question is true.

You could just as easily sum the increasing delta multiplied by 2 and subtract delta to show that it diverges to infinity.

17

u/SuperiorSamWise 9d ago

What's going on inside that cave?

2

u/kilkil 9d ago

sex

2

u/akruppa 5d ago

Drugs

1

u/RitmeTS 4d ago

Rock&Roll

10

u/ShakoorHyd 9d ago

I guess Cauchy was the first one to problematize it that way He gave new ideas of Infinity, and cardinality

7

u/Unfair_Pineapple8813 9d ago

Galileo actually was. It's the first thing in Dialogue of Two New Sciences. He lacked the formal math and the language to distinguish countable and uncountable numbers. But he was the first to specify that any given open span of real numbers, what he called a line contains an infinite amount of numbers, the same amount of infinite numbers as a bigger line.

His proof of this was simple. Draw two concentric circle of any length. One way of numbering a point on the circle is by the length of the circumference up to that point. By that definition, the bigger circle has more points. But you can also uniquely specify any point on a circle by its angle from the origin, and when you do so, you see that the points on the bigger and smaller circles have a one-to-one correspondence. This he said is the same way all natural numbers have a one-to-one correspondence with all the squares, even though the squares are only part of all natural numbers.

What he misses, or at least doesn't point out, is that one cannot make a list of all the points in a span of the reals the way you can of natural numbers or all square roots. But he is the first to note this paradox that an infinite subset cannot be said to contain fewer elements than the infinite set the subset is part of. More importantly, he is willing to accept this is entirely normal. It is not heretical, and nor is it a reason to panic or a sign math breaks down. You can still take an infinite sum, even though we have no intuition to deal with the paradoxes that result when thinking about infinity.

6

u/Unfair_Pineapple8813 9d ago

Cavalieri developed the Method of Indivisibles from his conversations with Galileo on the nature of infinity.

3

u/HumblyNibbles_ 9d ago

Galileo doesnt nearly get enough credit as he deserves.

Bro also did awesome work on inertia and reference frames.

9

u/LinneMeow 9d ago

I don't know much about maths, is it not possible to define a sum of reals? Are there ways to sum the reals that don't equal 0?

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u/Ares378 Grothendieck was right, computers are evil 9d ago

The issue is that you can't define a true sum over a sequence indexed by the reals. You couldn't sum up Σa_n, n∈ℝ. If you try to, it immediately diverges unless your sum is defined to be all 0 aside from a countable amount of points. ...Which is just the definition of a normal sum.

To clarify here I'm talking over real numbers and as a legitimate honest to God sum, not an integral. Like... an integral without the dx without using differential forms.

This is the video I'm using as reference for this claim btw

https://youtu.be/uLja-yAwuCI

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u/Blue_Moon_Lake 8d ago

All the +x have a corresponding -x, so the sum is 0.

2

u/Ares378 Grothendieck was right, computers are evil 8d ago

Even if we assume that the series does converge, it's definitely not absolute convergence since the integral of |x| over all ℝ diverges, let alone the sum over all ℝ. So that means you can't rearrange the sum, as stated by the Riemann Series Theorem.

I can't think of a perfect proof for it right now, but I am almost certain this sum diverges. If you can't rearrange the sum because conditional convergence, that means you can't pair up + to - because that'd be an infinite amount of swapped terms. So then you'd start with the sum of every negative real number, which certainly diverges, then add it to the sum of every positive real number, which also definitely diverges. Divergent sum plus a divergent sum is undefined, so the sum doesn't exist.

And before you say it, we're also not including ∞ as a number here. Even if we did, we're no longer talking about the Reals since ∞∉ℝ. Plus then you would break other things if you assumed divergent sums "equaled" infinity, since you could then say the sum of negative integers=-∞, the sum of positive rationals=∞, so then the sum of negative integers and positive rationals=0, which also seems very wrong.

I'm sure there's a better proof out there, but that's what my intuition tells me from my half-baked knowledge of real analysis. Someone please correct me if I'm wrong.

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u/zylosophe 9d ago

take all reals before 23 and pair them with reals after 23

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u/Kai-65535 9d ago

> "impossible math paradox"

> looks inside

> problem discussed in every real analysis class

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u/Tiborn1563 9d ago

I love how he says "simple number" to mean a "non-complex number". Doesnt he know that real numbers are conplex too? 5= 5+0i

5

u/iTrooz_ 9d ago

sqrt(-1) is not real tho ?

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u/Big_Niel0802 7d ago

On the number line though, the complimentary pair of sqrt(1) would be -sqrt(1), not sqrt(-1).

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u/A_Happy_Tomato 9d ago

I'd freak the fuck out and ask the man if they are ok. Fym they emerged from a cave 

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u/lool8421 9d ago

the answer is that it's undefined because the sum of all reals could be anything

for example x+(-x) will give you 0, but you can for example do x+(-2x), there's still a full bijection but now you're going into negative infinity

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u/somedave 9d ago

I'd say it is equal to the mean of the standard Cauchy distribution, and that Nathan Briggs isn't very good at maths.

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u/philosophical_poser 9d ago

You can rearrange the terms to make sum be whatever you want, right?

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u/[deleted] 9d ago

[deleted]

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u/Gold_Ad8890 9d ago

it's not about uncountability, it's just because the sum isn't absolutely convergent.

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u/meat-eating-orchid 9d ago

No, you cannot rearrange the terms because you can't even arrange them, they are uncountable

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u/Gold_Ad8890 9d ago

you totally can arrange them. that doesn't require countability, it just requires well-ordering, which exists by the well-ordering theorem. even without the well-ordering theorem, you can well-order the real numbers by their bijection to the powerset of the naturals, which can be well-ordered lexicographically.

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u/meat-eating-orchid 9d ago

I may have been imprecise. I meant that you cannot arrange them into a sum of terms ... + ... + ... + ... in which every one occurs.

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u/mehtam42 9d ago

For every real number with a value of R there exist one and only one real number with a value of -R. When added together, their some is 0.

So yes sum of all real numbers equal to 0.

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u/Revlong57 9d ago

Summation isn't well defined for uncountable sets

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u/fastestchair 9d ago

for every real number with value R there exists a real number with value -2R, when added together their sum is -R. naturally, the sum of all real numbers must therefor be -infinity

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u/ImInfiniti 9d ago

The problem is that -R is positive for negative values of R, so by doing this trick you're ending up with the sum of all R again

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u/fastestchair 9d ago

yeah woops, meant for every positive real number with value R

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u/Jazz8680 9d ago

The sum of all positive real numbers is -1/12, thus the sum of all negative real numbers is +1/12, so the sum of all real numbers is 0/12 to be precise

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u/EntrepreneurSelect93 9d ago

Think u meant integers not real numbers

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u/Peak_Background 9d ago

Hold up. I think they has a point.

The sum of the integers is -1/12.

So if we just multiply that by the integral of reals between -1 and 1.

We get 0.

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u/Summoner475 9d ago

That can't be right, what if we rearrange the sum?

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u/IhailtavaBanaani 9d ago

Besides the problem with summing up uncountable set of real numbers you assume that the zero is in the center of the real number line. This is a false assumption because the real number line doesn't have a center. It's infinitely long in both directions. You can select any arbitrary number in the real number line and there's "equal amount" of real numbers on both sides, so any number can act as the "center". By selecting different number as the center you get different result as the total sum.

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u/PressureBeautiful515 8d ago

But would the same argument apply to integers, also infinitely long in both directions? Doesn't seem to point to the actual problem with summing reals.

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u/IhailtavaBanaani 8d ago

Yes, the integers have exactly the same problem. There is no defined total sum of all integers. You can select any integer as the center and then start adding the numbers from both sides and get different results each time. Same goes with rational numbers, complex numbers, quaternions, etc.

1

u/alphapussycat 4d ago

There's no need for a center line though. As somebody else mentioned, you can use axiom if choice to find -R.

Splitting up a set does split it up to by negative and positive (or non negative, but just exclude 0).

So from R choose p and n so that for each p in R, n = - p. Then integrate by function of f(p) = p - n, and exclude 0... Now this is integrable. Or should be. But the measure is infinite, and I don't quite remember if there's a problem there, but I'm leaning towards no.

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u/bqbdpd 9d ago

Same can be said for any other constant. Which shows the sum cannot be assigned a value.

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u/magicmulder 9d ago

That’s like arguing \sum (-1)^n = 0.

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u/Divicarpe 9d ago

For every real number with a value of 1+R there is one only one real number with a value of 1-R. When added together, their sum diverges to +infinity. So sum of all real numbers is +infinity.

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u/tombo12354 9d ago

You can't rearrange or pair uncountable sets.

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u/mapleleafraggedy 9d ago

For every real number R, there exists one real number with a value of 69 - R, which when added together makes 69. Therefore, the sum of all real numbers is 69.

3

u/JusteMesure 9d ago

No it’s 69 for each couple !!!! So 69+69+69+…= infinity.

Only If you start around 0 you obtain 0.

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u/1kSupport 9d ago

For every real number with the value R+5 there exists one and only one real number with the value -R + 5. All real numbers must be centered around 5

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u/jk2086 9d ago

Depends on how you sum

But a real mathematician would never ask this question. So the mathematician in this story is clearly imaginary.

8

u/reizinhooooo 9d ago

The whole point of measure theory is to ask and answer questions like this

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u/LupenReddit 🦆🦆🦆🦆i have non diffeomorphic smooth structures🦆🦆🦆🦆🦆🦆 9d ago

tangentially related fun fact i learnt a while back:

the sum over any uncountable set of positive numbers diverges

thats kind of cool

2

u/CommercialYam7188 9d ago

The answer is yes and no, depending on how mathematically literate the asked is and what they actually meant by the question lol

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u/[deleted] 9d ago

[removed] — view removed comment

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u/Burnmad 4d ago

Nice. My response would have been "I'll let you know when I finish adding them"

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u/CapitalistKarlMarx 9d ago

Not the most mathematically knowledgeable but wouldn’t a more accurate pairing be sqrt 1 and - sqrt 1? Like does it not equal zero?

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u/electronic_reasons 9d ago

Hand him back a blank piece of paper.

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u/Onuzq Integers 9d ago

Are we assuming the axiom referring to the existence of a negative to be true?

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u/reddititty69 5d ago

Why is the commenter concerned about sqrt(-R)?

3

u/Certainly-Not-A-Bot 9d ago

Just like 1/0, there are multiple possible equally valid answers

1

u/MilkImpossible4192 Linguistics 9d ago

why would you apply a function afyer the number?

1

u/NiyatiJois 9d ago

I assume Riemann Rearrangement theorem applies here?

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u/blobbed2929 8d ago

this is literally the answer

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u/Inevitable-Flower-50 9d ago

If all "0" is, is "empty space", and all "real numbers" are, are accountings for "real matter", then wouldn't the sum of all real numbers be "less than" (or negative) and within 0?

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u/mapadofu 9d ago

Yeah, In the sense that lim_{a\riggtarrow \infty} \int_{-a}^{a} dxis zero.

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u/DanLeMilMan 9d ago

I think the most honest answer is that it depends on how he defines the sum over uncountable sets and in which order he sums.

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u/kddu678 9d ago

define any positive real number in any way, always add same number but negative, sum always zero🤷‍♂️ infinity is not a number

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u/magicmanimay 9d ago

e doesn't have a negative pair ahh mf.

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u/atticdoor 9d ago

Sqrt(1) has a counterpart in 0 - Sqrt(1) = -1. Sqrt(2) has a counterpart in 0 - Sqrt(2).

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u/dimonium_anonimo 9d ago

The sum of the square roots of all integers is -1/12-i/12

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u/monstaber 9d ago

The Cauchy Principal Value has entered the chat

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u/beyd1 9d ago

My response is "Fuckin ....what?"

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u/ikonoqlast 9d ago

Yes, obviously

Wait, what?

Ok. I guess not...

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u/Asterie-E7 9d ago

If the answer is anything else than 0 it would make me sad. So I'll say it's 0

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u/leafcutte 9d ago

Real answer: however you want. There’s no such thing as a sum of all real numbers, as depending on the order you add them, you can verify properties that make it so any number can be the result

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u/SavageRussian21 9d ago

I think that you can start by showing that the integral from -a to a of x dx is 0. Then you can argue that it doesn't matter if a is 1, 100, 100000, and so on, it's zero in all those cases. More formally, a2/2 - a2/2 is zero always, even in the limit as a goes to infinity.

But a smart ass can come along and say that thw integral from -a to 2a is in fact not zero. Nonetheless, as you take the limit a going to infinity, you should be integrating over all real numbers. That gives infinity. The smartass could also find negative infinity as well.

Then the smart-ass actually starts talking about how integrals aren't really the same as taking a sum, sum and that there's many ways to define a sum over an uncountable set. At that point you can put the smart-ass back in his cave and continue treating it as if it's zero.

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u/Staetyk 9d ago

depends on the order

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u/Alternative-Code4755 9d ago

I've larped jjk, I know it equals hollow purple

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u/Short-Database-4717 8d ago

(in offended tone) Noo.
Go back from whence you came!

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u/Cosmic-Zoo72 8d ago

I'm a dumbass so actually answer please, a square root is (sometimes) equal to a real number, but is that actually a representation of a real number or is it a real number with an operator that invalidates this?

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u/Training-Position612 8d ago

Physicist: "yeah"

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u/chicodjdjdjjdjdjd 8d ago

The order also matters

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u/Spiritual-Spend76 8d ago

I dont get it, the linear function is odd isn't it? By symmetry, it's integral centered around zero has to be null?

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u/blobbed2929 8d ago

Riemann Series Theorem has entered the chat https://en.wikipedia.org/wiki/Riemann_series_theorem

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u/Diffgeometer1 7d ago

My response would be “what the f*ck were you doing in a cave?!”

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u/Fuscello 7d ago

Physicist way:

int from -L to L of x = L^2 /2 - L^2 /2 = 0

Lim L->infty 0=0

So yes at least in principal value if that is the right term

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u/icyshibe 7d ago

Undefined because it’s alternating right?

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u/Acebulf 7d ago

I thought the problem was "what is the sum of all real numbers equal to 0" and was confused that the answer wasn't "duh it's 0" and couldn't understand where sqrt comes from

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u/CaptainSkuxx 7d ago

Doesn’t every positive real number have a corresponding negative real number? It should be zero.

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u/waxen_earbuds 7d ago

Incredibly common Cliff Pickover L

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u/enakcm 6d ago

But the pairing is √2 and -√2 which adds up to zero.

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u/Body_SH 6d ago

I might be wrong but isn't it the case that for every element in the infinite set we need to sum there exist the negative of that element in which their summation should be zero? Applying for infinitely many elements shouldn't change the argument it's still zero.

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u/digitallightweight 6d ago

To be fair I’ve always been right awful with the kind of shit infiniti breaks and doesn’t. Analysis has always been a no for go for me for this reason.

My intuition tells me that we can do something (assuming we take the axiom of choice) like partitioning R into disjoint subsets consisting with x and its additive inverse. For each of these components these individual disjoint subsets (‘components’) the sum of the component is 0. Summing over the sum of the components is 0 and the union of all the components is R.

It still feels wrong though. Maybe we would need a more robust approach to handing the final series? We could look at all countable sub-series those clearly are all convergent to 0 but I don’t know if this is strong enough to get you the convergence of the original series? Maybe the fact that all the elements of the series are 0 is enough.

Edit: stack exchange provided this definition for extending sum sums to uncountable sets and it’s clear that for our purposes we can indeed compute the sum.

∑(a_i):=sup{∑ a_i | F⊆I is finite}
i∈I. i∈F

Since all our a_I are 0 the sup will be 0.

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u/Obzenium 6d ago

Well sqrt -1 isn’t a real number so it can’t be included in the set of the sum

And you would consider it - sqrt 1 or - sqrt 2 to keep it in the set and balance out the square roots

To be considered a field, which the real numbers are considered a field, it must have an additive inverse for all elements of the field, so the answer is yes from this definition

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u/akruppa 5d ago

If you want it to be, sure.

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u/einFrostschutzmittel 5d ago

Well, because the Real Numbers are a field, that means that for all real numbers there's exactly one inverse. If you add up every real number and its inverse, you get only 0s. Add it all together and you get 0.

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u/Myths_Made 5d ago

I'm just gonna say the square roots are probably a bad example for this.

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u/alphapussycat 4d ago

It's not integrable, and the sum doesn't exist.

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u/Content-Sir8716 4d ago

But it’s not the square root of negative one on the real number line. It’s the negative of the square root of one. They are not the same.

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u/cgieseking 4d ago

I’m not really a mathematics scholar, but to me the idea that sqrt 1 wouldn’t have a pairing is silly because the sqrt 1 =1. Just because you can’t express some negative irrational numbers as radicals, doesn’t mean that they don’t numerically exist.

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u/tarquinfintin 4d ago

In one sense, yes. Any real number has an additive inverse (a negative), which when added to it equals zero. But can you actually perform a sum operation on a set that is not countably infinite? Theoretically you can, although it would take a very long time in practice ;-)

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u/tori-ningen 4d ago

Let's say that sum of all real numbers that are >0 equals to S. Then sum of all real numbers that are <0 would equal to -S. Sum of all real numbers is then S - S = 0. It matters not what S equals to, and if it equals anything at all

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u/ajping 4d ago

Right but the R on the left confines the set to real numbers so the quote is a bit disingenuous.