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r9m
12:01 AM
@robjohn you can say so (in a way yes) .. but this time I have managed to get closed forms of $\sum\limits_{n=1}^{\infty} \frac{H_n^3}{n^3}, \sum\limits_{n=1}^{\infty} \frac{H_nH_n^{(3)}}{n^2}, \sum\limits_{n=1}^{\infty} \frac{H_nH_n^{(2)}}{n^3}, \sum\limits_{n=1}^{\infty} \frac{H_n^2H_n^{(2)}}{n^2}, \sum\limits_{n=1}^{\infty} \frac{H_n^4}{n^2}$ with series manipulation alone .. no integrals, no polylogarithms :-)
ah and $\sum\limits_{n=1}^{\infty} \frac{H_n^2}{n^4}$ and $\sum\limits_{n=1}^{\infty} \frac{H_n^{(2)}}{n^4}$ could be mentioned as well .. but I think I did them b4 ..
 
@r9m the ones that I did were with only series manipulations.
 
r9m
@robjohn yes!! :D all my inspirations come from your answers here! That's one irrefutable truth!
 
@r9m Cool! I am glad to have been of assistance.
 
r9m
@robjohn It's more like you are the wise Gandalf! I am a hobbit :P
@robjohn is there a mail address I can mail to you (for when I wish to share some stuff that I can't share here? :) .. )
 
 
1 hour later…
1:13 AM
Hello
 
 
1 hour later…
2:29 AM
@TedShifrin I finished your first semester course. It was a delight. I am really grateful you put it on youtube. I enjoyed it a lot and some of its beauty.
 
Anyone know of any online calculators that convert propositonal logic expressions to ones using only the NAND operator?
 
 
4 hours later…
user227867
6:08 AM
@r9m He didn't give me his address before, so I would be jealous if he did give you.
 
6:20 AM
moment recorded: room without moderators
 
 
1 hour later…
7:27 AM
@Agawa001 Ha!
 
7:46 AM
@robjohn that was shortlived
 
8:03 AM
@DanielFischer i couldnt really get ur proof in my perception maybe you could detail it in mathematical terms/symbols
it is too literary
 
Does anyone have experience with CrossRef and OrcID? It seems to keep updating the same piece of work on my profile without actually changing anything.
 
or maybe i should pay more attention to it
 
@Agawa001 Take a piece of paper, sketch (part of) a grid, with arrows showing the direction of the outer edges, and see what happens when you give the inner edges directions (respecting the "two in, two out" condition), trying to avoid a square with oriented boundary.
 
r9m
@robjohn (+1) Awesome!! :D
 
8:14 AM
@r9m are you on Dropbox? That is how Chris's sis and I transfer things.
 
r9m
@robjohn oh! I don't have dropbox (actually don't know how to use it) .. I use google docs to share stuff ..
 
@DanielFischer so from a small grid, things can be extrapolated by induction
 
r9m
@DanielFischer I might plan on cloning your brain when the technology arrives that is =P .. It was a very simple but elegant proof. Thanks!
bbl .. classes ryt now
 
@r9m unfornunately for me i cant read proofs where numbers and symbols are barely present
 
 
1 hour later…
9:31 AM
@r9m All these series I've finished a long time ago. And my initial intention was to keep my tool only for my book, but well, I had to change my mind a bit.
By series manipulations only.
 
@user1618033 i have done tools for auto-scripting and intelligent converters, i didnt share em eithet they took me much time
my coworker requested them for money, he couldnt even find em in internet
 
@Agawa001 I see.
@robjohn about r9m posts. All these will be elementarily done in my book.
@robjohn together with advanced forms never done before.
@r9m it's good to say your inspiration comes from robjohn answers but don't miss the fact that without me you probably never did any of the series of weight 6 you claimed above you did.
 
r9m
10:17 AM
@user1618033 Your solutions are inspiration too :) I never forget that .. but you never showed me your series manipulation solutions to these problems and I did the weight 6 ones without using any of the tools you showed me ..
 
I simply don't wanna sound as a bragging or anything like that, but the simple thing is that I'm the first that ever did all series elementarily using an incredibly powerful tool not known to the present date (according to my knowledge).
@r9m You used the disguised form of Au-Yeung ...(it's the same story)
 
r9m
@user1618033 and extended it further (which is no longer the same story .. I've only showed you a part of it .. there's more that is required to get the other weight 6 sums)
 
@r9m Yeah, but that is a critical point also for the extention.
@r9m Just wait a second ...
\begin{equation*}
\sum_{n=1}^{\infty} \left(\frac{H_n}{n}\right)^2 \left(1+\frac{1}{2^2}+\cdots+\frac{1}{n^2}\right)=\frac{41}{12}\zeta(6)+2\zeta^2(3),
\end{equation*}
@r9m ^^^ an example
Which needs an extention, indeed. To get my point well. ;)
Anyway, this discussion leads nowhere. I'm back to my work, and promise in the future to be thousands times more careful about sharing stuff.
BBL
 
r9m
@user1618033 in that spirit the whole of mathematical development should be duely credited to those who laid the founding stones. If you claim that such development 'might' not be possible without those foundations .. that is a valid claim .. but don't say it'd be impossible otherwise :P
 
Is there really any way user 1618033 could be more careful about sharing stuff? I don't think he/she has so far shared any of these amazing tools.
 
r9m
10:25 AM
@user1618033 nah ,.. no extension for that one ;) .. but $\sum\limits_{n=1}^{\infty} \frac{H_n^4}{n^2}$? yes .. atleast I needed to tinker and upgrade it a bit ,, :) (when I say upgrade you'll probably know what I mean .. it's just a recursive application of the previous au-yeung lemma right? )
 
@r9m $$\sum_{n=1}^{\infty} \frac{H_n^4}{n^2}=\frac{979}{24}\zeta(6)+3 \zeta^2(3)$$
BBL
For the record
I finalized \begin{equation*} \sum_{n=1}^{\infty} \left(\frac{H_n}{n}\right)^2 \left(1+\frac{1}{2^2}+\cdots+\frac{1}{n^2}\right)=\frac{41}{12}\zeta(6)+2\zeta^2‌​(3), \end{equation*} in 3 different ways!
BBL (I'm out to buy some food for my dogs)
 
10:50 AM
@r9m Back. That lemma is indeed very powerful.
 
user227867
@user1618033 You misspelled extension.
 
@TobiasKildetoft If I showed you anything about my tools do you think you would be capable of properly assessing anything?
 
Probably not, no
 
@JasperLoy You're right. Thanks.
@JasperLoy How are you doing?
 
user227867
@user1618033 I am OK. I did not make any videos today. I want to write some blog posts soon on my blog. It is currently empty.
 
10:53 AM
@JasperLoy Sing some more.
 
user227867
@user1618033 Yes, I certainly will. Now 4 videos. I will go to at least 20, maybe 30 eventually. On my blog, I would like to do book recommendations on various topics, non-math and math.
 
@JasperLoy awesome. Hope you'll add my book there too. :-)
 
user227867
@user1618033 Nobody would read my blog, LOL.
 
@JasperLoy I would read it.
 
user227867
@user1618033 OK, we will see about it when your book comes out.
 
10:59 AM
@r9m sorry if I seemed a bit tough, but so many things happened, and I simply worked like hell for all I got. That's why I'm talking with so much affection .
Nobody gave me any bit of help in all I did (related to the stuff in the book).
 
user227867
@user1618033 I won't talk to or talk about the user I mentioned yesterday anymore, but it suffices to say that he/she is the most evil person I have met online, abusing me and this site's users in the past and in the present in various ways.
 
@JasperLoy Maybe he/she is in love with you?
 
user227867
@user1618033 That is not relevant at all. And whether someone is mentally ill or not is also irrelevant. Abuse is abuse. Of course, I considered carefully before using the word abuse.
 
@JasperLoy maybe @robjohn can help?
 
user227867
@user1618033 Nope, this matter has got nothing to do with the moderators of this site, so let's not bother them.
 
11:04 AM
@JasperLoy OK
 
user227867
@user1618033 Anyway, this will be the last time I talk about this person, so let's forget about him/her right now!
 
@JasperLoy As you wish then!
 
user227867
@user1618033 In the words of Chris the Great, no more 'negative energy', LOL.
 
@JasperLoy :-) I'm actually an ordinary person, no gain to lie myself. ;)
 
user227867
@user1618033 Yeah, just like Ramanujan, LOL.
 
11:09 AM
oooo, that would be amazing to happen to me!!!
 
user227867
I was wondering if you might be Ramanujan reborn, or his Indian goddess reborn.
 
r9m
@user1618033 I understand :-) :-) .. You have all my affection. :)
 
@r9m Thanks! :-)
I'm entirely with all my being into this. Maybe it's not a good idea, but well, this is the way I am with mathematics.
 
r9m
very mentally pressurizing classes today :P I learnt tensors (still head spinning) .. bbl I need a smoke :P
 
user227867
@r9m Tensors make you tense.
 
11:13 AM
@JasperLoy lollll :-) How to answer back to this? :-)
 
@r9m Tensors as in algebra? Good stuff.
 
@BalarkaSen Mostly when people say tensors, rather than tenor products, they are speaking of geometry or mathematical physics.
 
Hmm, indeed.
 
r9m
@BalarkaSen ya .. it's difficult for me to grasp it in the first go .. maybe I need more time to wrap my head round it ..
 
user227867
Tensors is just multilinear algebra, just like vectors is just linear algebra.
 
11:25 AM
What is a tensor anyway?
 
More precisely, making multilinear algebra into linear algebra.
 
user227867
Just remove multi with an eraser.
 
@TobiasKildetoft Probably an element of $\bigotimes^n V$.
 
@r9m If someone else here would complain about it I would be worried, but I'm definitely not worried knowing (I'd say) pretty well how capable you are. ;)
@r9m I guess people here don't know you well.
 
user227867
I am going to eat, later.
 
11:28 AM
@JasperLoy r9m has a very powerful mathematical mind. Don't miss to add him besides the other people you usually mention.
BBL (trying to finish some proof)
 
@TobiasKildetoft How's life?
 
@BalarkaSen Good. Just got back from vacation and slowly getting back into gear.
 
Nice.
I have been trying to prove something but not getting any workable ideas.
 
r9m
@BalarkaSen still our prof has been generously slow with the explanations so far .. which is a lot helpful :-) quotienting off (or identifying) multilinear expressions from a formal linear space ..
 
Slow is good. It's worth understanding why one would quotient the multilinear expressions off though.
What's the rationale behind doing that? Why not quotient by weirder things?
Yikes, I gotta go.
 
r9m
11:38 AM
@BalarkaSen of course we need the map to be linear .. (which justifies quotienting off just those multilinear expressions) .. :)
@user1618033 nah .. I'm a turtle with new stuff .. first I have a panic attack .. then I run and hit the wall and faint :P
 
@r9m hehe. Well, I couldn't learn such stuff in a class, but only alone. Learning in a class is pretty hellish to me.
The comfort of learning alone is absolutely divine! The best way here!
 
it is because minds differ in terms of translating and processing an information, people usually dont get used to other interpretations of an arbitrary problem when they receive it once
 
r9m
@user1618033 having a professor explain stuff is really helpful to me .. somehow that 'humanizes' the crazy stuff and gives me confidence that it's humanly achievable :P (of course it's some human's work .. but reading it from a book or print somehow makes it look like 'divine judgement' to me :P )
 
@r9m I need a terrific silence before learning, and in a class this si the thing you never meet. I prefer books with good explanations, and mind solves the rest during the time.
 
r9m
@Agawa001 indeed! :) ordinary humans like myself often need a human medium to do this translation for us ..
 
11:45 AM
and some fresh wind coming from the sea
or forest
 
I'm definitely functioning differently.
 
r9m
@user1618033 ya .. I don't think all the information consolidates during the lecture .. it's after the lecture is over when I think it on my own that I see the pieces fit together .. (atleast for me)
 
@r9m as einstein said, judging a fish from a criterion of the ability of climbing a tree s worthless, everyone has its own guise of moving around : walking/diving/flying etc
 
r9m
:)
 
so i thought my self very mentally regressed when i couldnt receive informations in class, after awhile of deeply focusing on those problems (alone), i found my trump, and exploited it
 
11:49 AM
I don't say it's better, students learn in a class usually. Of course it couldn't be better this way, since you need isolation and all the stuff like that, deprived in a way by social activities.
 
so all those hours spent in class were a loss of time
and as a sidepoint, i see myself more adapted to understand the machine than a human
or a book or any logical principle expressed in mathematical symbols
 
@r9m The map $A \times B \to C$ which is linear on it's two components, yes. That's it.
 
@r9m Yeah, I know. Eventually learning is a process we accomplish alone, no matter how many professors there are around.
 
Such a map descends down to a linear map $A \otimes B \to C$ by construction.
 
r9m
@BalarkaSen yes :)
@user1618033 indeed! :-)
 
11:54 AM
@user1618033 by intuition, for a logician or an innate mathematician, he could contact a book/a machine directly without human medium
contrarily, humans can make it worse for them
i said "can"
 
@Agawa001 like Ramanujan
 
yes, and many people with this exceptional virtue
@r9m is this refered to the latest puzzle, well noone could pretend to have a clue from a first glance
 
@Agawa001 In the past I searched on google for some professional articles about why some find comfort for learning only in isolation. I couldn't find more on this topic.
I'm pretty interested in this topic.
 
r9m
@Agawa001 I was referring to my Linear algebra classes today.
 
@user1618033 yes, sometimes it is disturbing because one needs peers to rub elbows with
 
12:01 PM
@Agawa001 :-)
OK
I need to finish a proof.
BBL
 
tc and dont let the proof finish you
 
12:40 PM
Is there a name for the PDE that looks like the heat equation, except the Laplacian is replaced with the standard spin-1/2 Dirac operator in the plane? That is: is there a name for this equation du(x,t)/dt=Du(x,t) where $D=-i(\sigma_x\partial_x + \sigma_y\partial_y)$ is the Dirac operator.
and the $\sigma_x,\sigma_y$ are Pauli matrices.
 
Hi
What annual instalment will discharge a debt of 1092 due in 3 years at 12% simple interest?
I have tried :Simple Interest is Means Calculating the Interest for every Year, the above Problem is Little Different Can you Guide Me how to approach
 
1:40 PM
I will add that the above message is related to this post: math.stackexchange.com/questions/1907314/…
 
2:03 PM
@0celo7 Tell me something interesting.
 
2:52 PM
@r9m I have a magnificent relation with $$\sum_{n=1}^{\infty} (-1)^{n+1} \left(\frac{H_n}{n}\right)^3$$
(most probably unknown)
(perhaps one of the most mysterious identities I ever derived - however not the most mysterious one - hard to even imagine that)
 
(refresh)
 
3:24 PM
Hi @JuanFran, @Adeek.
 
hi @BalarkaSen
 
hi
 
 
1 hour later…
4:47 PM
@BalarkaSen Uhh
Hydrofluoric acid dissolves glass
 
@0celo7 Hydrofluoric acid means HF?
 
yes
also you can die with 2.5% skin exposure
the pain will make you go insane
but the pain can be delayed by 24 hours
it replaces calcium and magnesium in your bones
they usually amputate limbs that get HF on them
 
Urk.
 
(assuming you don't get it washed and put the counteragent on it)
 
Hopefully our school-lab doesn't have it.
 
4:52 PM
there's a special gel
they don't, guaranteed
I'm being trained to use it...pretty scared tbh
it's nearly indistinguishable from water
one way to tell if it is HF is if the fumes burn your face ;_;
 
lol
 
but that's only in really high concentrations
below <50% there are no fumes
but usually there's no confusion
if you have a beaker with water-looking stuff in it, it's not HF
HF would have burned out the bottom of the beaker
 
Heh.
 
they store in in special plastics that incorporate fluoride
 
I thought glass was pretty strong (SiO2, right?). HF must be terrifying shit.
 
4:56 PM
HF is pretty bad.
I have a list of stuff NOT to store it in or near
> Store in a cool, dry place away from
incompatible materials. HF reacts with many
materials therefore avoid contact with glass,
concrete, metals, water, other acids,
oxidizers, reducers, alkalis, combustibles,
organics and ceramics.
> Store in containers made of polyethylene or
fluorocarbon plastic, lead, or platinum. Place
storage bottles in polyethylene secondary
containment trays.
@BalarkaSen do you know anything about holonomy groups?
 
Hmm, the fumes are rather curious. HF hydrogen bonds right (H --- F hydrogen bonds would be pretty strong, as F is highly electronegative)? I thought that'd bind the molecules together.
@0celo7 Vaguely.
 
@BalarkaSen I've had two books say that the property $\mathrm{Hol}_{(p,q)}(M_1\times M_2,g_1\oplus g_2)\cong \mathrm{Hol}_p(M_1,g_1)\times \mathrm{Hol}_q(M_2,g_2)$ is obvious, but when I sit down to do it I get lost
 
I don't really know the definition of Hol_p(M, g) so not the ideal person to ask
 
@BalarkaSen set of isometries of $T_pM$ induced by parallel transport
set/group
I was wondering if there's an easy representation theory argument
in The h Bar, 8 mins ago, by 0celo7
@ACuriousMind Suppose I have a rep $G\to V$ of a group. Suppose the rep is reducible, and each irrep is an irrep for groups $G_1$ and $G_2$. Is $G=G_1\times G_2$?
 
Dunno
Ask Ted when he's here
 
5:07 PM
For some reason Kobayashi-Nomizu do not even state this
Maybe Michor has it...
nope, just holonomy of principal bundles
@BalarkaSen I think conceptually it is similar to $T_{(p,q)}M_1\times M_2\cong T_pM_1\oplus T_qM_2$.
 
Perhaps.
 
I almost see a proof using something called the "factorization lemma" but it's a very nontrivial statement about parallel transport around contractible product loops
 
is $(Z,|.|)$ is complete ?
 
what is $Z$
 
the set of integer
 
5:14 PM
it's much better to write that as $\Bbb Z$.
any Cauchy sequence has to eventually become constant
so they all converge
 
can we use the fact that $\mathbb{Z}$ is closed ?
 
for?
 
to say that it is a complete set
 
just take $\epsilon=1/2$ or something and convince yourself that a Cauchy sequence has to become constant
beacuse you're in $\Bbb Z$, the distances between your points can only be 0, 1, 2, ...
 
closed doesn't even make sense as an adjective for a topological space
 
5:17 PM
so if you pick $\epsilon=1/2$ you're gonna need the distances to be $0$
@BalarkaSen I think he means closed as a subset of $\Bbb R$ w/ the standard metric
 
r9m
@user1618033 Nice!! :-)
 
OK, you're trying to use the fact that $\Bbb Z$ is a closed subspace of $\Bbb R$ which is a complete space, hence so is $\Bbb Z$ in the induced metric. Sure, that's fine.
Better to do the hands-on proof 0celo7 said though.
 
My proof does not use the closedness of $\Bbb R$
I should learn representation theory.
 
I should understand the correlation between the homological Lefschetz number and the smooth one.
 
Do you want my proof of equivalence?
 
5:26 PM
@0celo7 so if i say let (x_n) be a Cauchy sequence in $(\Bbb Z,|.|)$ then $\forall\varepsilon>0, \exists n_0\in \Bbb N, \forall p,q\geq n_0, |x_p-x_q|<\varepsilon$
 
I defer to @BalarkaSen , currently in a topology lecture.
 
but how to explain your idea mathematicaly ?
 
@0celo7 You gave it to me already. I have to put more effort in understanding that.
But I was looking for a more direct way using simplicial homology.
 
I wrote another one.
Which explains the link between intersection theory and Poincare duality
 
I know the link.
 
5:28 PM
The Lefschetz thing is an application
@BalarkaSen hmm
Good luck.
Bredon explains it the way you want.
 
I'll think about it and ask for your write-up when I can't figure it out.
 
Ok.
 
 
2 hours later…
7:31 PM
@BalarkaSen Let $F$ be the function described in GP. Then $dF_{(x,a)}$ is surjective (the matrix contains the $k\times k$ identity matrix so it always has full rank). It is also injective, so $F\pitchfork\{p\}$ for any $p\in\Bbb R^k$. In particular, $F\pitchfork\{0\}$. But at $0$, $F(x,0)=df_x$, so $df_x\pitchfork \{0\}$
wtf
that popped up again in the text box
???
 
lol
Hi @Danu.
 
7:52 PM
Howdy
 
8:07 PM
Hi guys. Could someone please explain me why I can consider the "natural transformations between $h^A$ and $F$ " to be a set/inside Set?

Building up on that, presuming I have something called A that isn't necessarily a set, does it even make sense to say something like {A} is a set?
 
pozdrav
 
Well in set theory everything is considered to be a set.
 
@Jake1234 I don't understand the question. You have two Set-valued functors - a natural transformation between them is a bunch of morphisms between their values and their hom-sets (both being also sets) indexed over a small category (the domain category)
Everything involved is a set, hence the collection of all natural transformations is clearly a set.
Note that the assumption in Yoneda lemma is that you work on a small category, where the collection of objects and morphisms are actual sets.
 
are there any interesting categories that are small
 
Yes, every interesting category is small.
 
8:14 PM
Is Diff small
 
That would make sense, but I don't see that the domain category is small - there's written it's locally small.
 
Manifolds don't form a set
 
I haven't had the pleasure to study category theory yet – are there any classical books on the topic?
 
Mac Lane
 
most categories are not small
 
8:15 PM
Oh, I am confusing terminologies aren't I.
I meant "locally small" when I said "small"
 
@Velvet yes.
 
@0celo7 Thanks, I'll look it up. :)
 
DIFF is locally small.
 
@BalarkaSen If it's locally small, but the domain category's objects arent' a set, how can I tell that a specific natural transformations between the two functors is a set?
 
8:17 PM
I don't think it's a set in general
 
@Jake1234 A natural transformation is not in general a set, as mercio said.
We're talking about the collection of all natural transformations.
That should be a perfectly fine set, given the values of both the functors lie in Set (whose hom-sets are also sets).
 
Ok, so the elements of that set themselves aren't sets, but it's a still a set hm. I didn't even know that's possible.
 
um no
 
@Jake1234 Elements of a set are rarely sets.
 
it depends on your set theory but usually they are
 
8:23 PM
Well if it's possible at all, then that's not so surprising.
 
don't worry too much about what is a set or what isn't
just be careful to not mix things that live at different "levels"
 
@mercio My set theory is just naive set theory. I am a simple-minded person :)
 
though here it turns out that the proper class of proper classes of natural transformations between 2 functors is in bijection with a real set
 
{cat, bat} is a fine set of objects which are not really sets to me.
 
Huy
{set1, set2} is an interesting set
 
8:26 PM
cat?
do you mean the animalian cat
 
our cat died last saturday
 
or the categorical cat
 
;w;
but we still have a lot of bats
so there's that
 
@mercio urk. sorry to hear.
 
Yeah sorry to hear that.
@BalarkaSen Sorry, I don't really see how that follows. Would you be willing to please try and fill in a bit more of the reasoning?
@mercio I generally try to avoid these questions given my problems with set theory, and me being unable to tell whether it's worth thinking about too much... but this one time I wanted to understand this a bit rigorously, and I got stuck lol.
 
8:33 PM
well yoneda's lemma is particularly weird to understand
but why do you want to know if the "set" of natural transformations can be considered a set
do you want to view it as an object of Set ?
 
Well it's said that they're isomorphic, not just in bijection.
 
well isomorphic means in bijection plus a theorem about whatever it is about
 
Were it not directly said it's isomorphic, I'd be fine with just thinking "ok there's some sort of weird bijection between this set/class-thingy and a set"
I'm not sure what theorem you mean at all.
 
here that would be the naturality things
in fact i don't know what they mean by isomorphic
 
@Jake1234 You should be able to see that the collection of natural transformations between F, G: A ---> Set is a subcollection of the direct product of hom(F(X), G(X)) (indexed over X \in ob(A)) x direct product of hom(hom(X, Y), hom(F(X), F(Y)) (indexed over X, Y \in ob(A) x ob(A)). If you can convince yourself that direct product of sets indexed over a proper class is still a set, you're through. I can't because I don't really do set theory in that level of rigor.
 
8:39 PM
oO
 
@mercio Set-isomorphic, surely, but that's where Jake's trouble comes in: Nat(-, -) might not necessarily be a set.
 
hi @BalarkaSen
 
hey.
 
are you guys talking that Hom(X,Y) is always a set?
 
@Jake1234 Actually, do you realize that the Yoneda lemma actually tells you that Nat(h^A, F) is a set?
 
8:43 PM
I think they mean isomorphic as in when you have an arrow from A to B, it relates natural transformations from h^B to F and natural transformations from h^A to F,
and that has to be the same as the corresponding thing on F(B) and F(A)
and I possibly got that backwards
 
@BalarkaSen Well see that's what I was sort of hoping it says.
 
well in general Nat(F,G) is not a set
and you can't look at Hom_Set(Nat(F,G),Nat(H,K))
or look at the reunion of Nat(F,G)
if that would mean anything
(because then you might be able to construct the set of all sets)
(which is pretty bad)
 
@Jake1234 I think it does. It says Nat(h^A, F) is a set and that Nat(h^A, F) is in 1-1 correspondence with F(A).
 
The preopostion that there is a bijection between a proper class and a set doesn't mean that the proper class is a set
nor that the bijection is a set
nor that the elements of the proper class are sets
 
But I'm just not sure whether it makes enough sense from a rigorous perspective - it shows there's some sort of thing like a bijection, but I don't know it's a set in the first place, so I'm not sure how it would make sense... it seems like you still need to presume that it's a set. I'm just ranting now.. I'll read the thing you wrote higher up though.
 
8:45 PM
(because here, natural transformations are not sets either)
 
@mercio Can you give an example of a locally small category with a Set-valued functor F such that Nat(h^A, F) is not a set for some A?
 
well take the category of Sets
 
I am not really convinced that it's not a set.
 
it is locally small because Hom_Set(A,B) is a set
 
Agree.
 
8:47 PM
it's a subset of P(A*B) or something
 
What's the functor F?
 
well let's take F = id
 
OK, why not.
 
I mean F(A) = A
and if f is a function, F(f) = f
F isn't a set
 
I see the problem. Nat(h^A, id) is not really a set.
 
8:48 PM
if F was a set, you could talk about.. idk.. the domain of F
which contains all the sets
so really you don't want to write F(F) ever
it would mean you're doing something wrong or at least completely irrelevant
 
@Jake1234 You should listen to mercio here than me. I think I agree that "isomorphism" is not really a good choice of word - they should just say a 1-1 correspondence.
 
they say isomorphism because the families of those sets of natural transformations, when the object A varies, have some structure
god yoneda's lemma is always so confusing
 
Not like that structure is preserved by the correspondence Nat(h^A, F) ---> F(A) though.
 
the point is it should be preserved
if you have an arrow from A to B, this gives you an arrow from F(A) to F(B)
 
Ah, ok, gotcha.
 
8:53 PM
because F is a functor to Sets, this arrow is a 'real" function
and when you translate that real function back using those correspondance
you should obtain something natural
 
what is F(A) ?
 
Right, it should give the natural arrow Nat(h^A, F) --> Nat(h^B, F).
 
if you have a natural transformation from h^A to F and an arrow from A to B
there should be an easy way to get a natural transformation from h^B to F
 
Well, there is.
an arrow A --> B gives an arrow hom(--, A) ---> hom(--, B)
 
what's the second arrow ?
I suppose it gives a natural transformation from h^B to h^A ?
and I think I messed up the order again
 
8:56 PM
Edited.
By h^A I meant the covariant hom. Sorry about that.
 
but h^A is hom(A,_)
the natural transformation from h^B to h^A is the map that takes a function g from h^B(X) = hom(B,X), precomposes it with our arrow from A to B, and returns the resulting map in hom(A,X) = h^A(X)
so an arrow from A to B gives a natural transformation from h^B to h^A
 
Yes, the orders are all backwards.
But whatever.
For the one hom you just take an arrow X --> A and compose to get X --> A --> B
 
and this gives a natural correspondance from Nat(h^A,F) to Nat(h^B,F)
again by precomposition
 
Yes, which is what I was saying up there.
 
it's funny we are doing the same thing twice at different levels
and the point is, that when you translate Nat(h^A,F) into F(A)
you obtain F(the arrow)
between F(A) and F(B)
 
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