Someone used my tag! : ) : )

@Gigili Thanks! I will : )

Mackey...the one who published the famous proof of cauchy's theorem?

I'm not aware of this. George W. Mackey.

Where W. stands for the horrible name Whitelaw.

Gigili's gravatar fascinates me..but it's not what you think. i wonder things like: what IS that thing in the hair? a barrette? a flower? a strangely-shaped ear?

@DavidWheeler a watch.

I don't know, do you call that bracelet or wrist-band or whatever?

It's my hand and watch, yes. But how is this relevant to George W. Mackey?

I have no idea

@tb @Gigili please don't accuse me of relevance

no worries :)

Asaf, is that you?

(I jest.)

you have a keen and cruel sense of humor. let's go steady!

the only one who manages to come close to the V dream team in terms of legible and comprehensible questions.

Uh-oh.

What a messy question.

wowthatissounintelligibleanditriedtoreaditandicouldn'tsonowidon'tknowwhattodo

reminds me of that

what a mess

It's actually worse than that...

@Gigili What's messy about the question? Sure, the OP went off on a completely wrong track, but that's OK. It's nice to know what they have and haven't already tried.

(I love RRs, BTW)

@tb I didn’t even realize that anything was going on until I saw your comment.

@DavidWallace There were wrong dollar signs everywhere, but he edited it.

Oh, OK. Might not have been the OP who edited it.

Besides, not everyone is born fluent in LaTeX.

I didn't blame him, I just failed to understand what's being asked.

I’m surprised at the number of people who drop into $\LaTeX$ only for things that they can’t do otherwise, so that they end up breaking up formulas into little pieces. That’s what happened to that poster, only he did it worse than most.

@BrianMScott yes, the notification that someone else edited didn't show up for me.

That's why we now have 5 edits instead of just one.

@BrianMScott because when asking a question, one wants to focus on the mathematics and the question itself. The LaTeX is initially a distraction. Until, of course, one discovers that one really needs it.

@DavidWallace The OP was the one, because he can edit the question within two minutes or so which won't be shown in the revision history.

@tb fair enough - I wondered what you had been up to.

@DavidWallace I’d expect complete ignorance rather than that sort of odd patchwork.

I'm guilty of it myself from time to time. I was fluent in LaTeX 20 years ago; these days, I mostly mess it up. So I try to avoid it unless I really, really need it.

I’d never used it until I started playing here last summer.

But LaTeX aside, wasn't this a fairly straightforward recurrence relation to solve?

@BrianMScott "Last summer" means June-July-August or December-January-February?

Very. As I said in my comment, I answered the same question for someone else by giving three easy solutions in considerable detail. (And someone read the comment: I just picked up two upvotes for the old answer!)

Northern hemisphere.

OK, thanks.

But I guess actual courses in RRs are not common. It would be really good to find a nice intuitive primer in solving problems of this kind.

Most of the elementary discrete math books from which I’ve taught go into the solution of homogeneous linear recurrences and the technique of ‘unwinding’ simple non-homogeneous recurrences. Any combinatorics course that gets into generating functions will do more.

Hehe, I guess I've just read all the wrong books, or been to the wrong courses. I wish you had taught at the university that I attended.

@BrianMScott The problem is that many math curricula teach students neither discrete math nor combinatorics, or only optionally.

Our discrete maths / combinatorics lecturer never would have touched on this.

i never got any schooling in them at all...i feel cheated. well, i did take a course in combinatorial topology...but that doesn't count. a poor choice of words, perhaps.

(But this is the same man who defined "proof" in his first lecture as "an argument that convinces". I almost stood up and walked out).

@DavidWallace well what IS a proof?

i've always wanted to know.....

stop, David the blue!

I don't know, exactly, but I felt strongly that this definition was wrong.

Easy counterexample is the original alleged proof of the four colour theorem.

well, in the right kind of context, it could be right...it depends on what you mean by "convince"

But "convince" is even more subjective than "prove".

@tb True. My department had four $200$-level (nominally sophomore) math courses, of which every major had to take at least three; they were discrete math, linear algebra, multivariate calculus, and differential equations. All math majors going for secondary certification had to take discrete math, as did most computer science majors.

language does seem to have a lot of problems of that ilk....

but, sure, i mean if that definition was sufficient, proof by intimidation would be a LOT more common

you MUST believe FLT is true, or i'll beat yer bloody brains out!

I could almost buy "an argument that convinces and is logically sound".

@tb Where's Rob? Have you seen him recently?

So, the original non-proof of 4CT fails because, despite being convincing, it has a big logical flaw.

yeah, one element most people agree on, is some hint of deduction, along with a rich tannin finish

:-)

@BrianMScott When I was studying there was a compulsory course on "theoretical informatics" where we did some analysis of algorithms, sorting algorithms and stuff. There were one or two exercises on generating functions but nothing more. We had a strong education in differential equations and the discrete counterpart would have been very illuminating, I think...

And if I recall correctly, this particular 200-level discrete maths course covered certain classes of RR, but never mentioned the "use your brains and listen to what the numbers are telling you" principle.

that's what i need, for sure! better algorithms. really flexible ones. maybe twins.

@Gigili I think I last saw him on Wednesday or so, but I'm not sure.

The way that guy kicked him out ... I'd never come back if I was him.

@DavidWallace I always insisted that a useful first step was to calculate some values!

and you're not him! see, it all works out fine :)

@BrianMScott The voice of sanity speaks.

well...2 is prime, 3 is prime, 5 is prime...based on my first few calculations...everything is prime

But in this particular case, it's more than that. You see that each value is roughly double the one before, and you know straight away that it's going to be ( fudge $\times 2^n + $ fudge ).

@DavidWheeler Feel first, prove afterwards.

oh, you're going all mersenne on us, now are you?

@Gigili It wasn't nice, yes. On the other hand both parties are to blame, I think...

@tb For several years I had the pleasure of teaching a $400$-level course from Concrete Mathematics; I concentrated on finite calculus and generating functions.

finite calculus? like difference equations?

@DavidWheeler Hey, mathematics is not a mechanical process. If it were, computers could do it.

@BrianMScott pardon my ignorance, but I don't know what those level numbers mean. The higher the more advanced?

@tb x00 means x'th year at a tertiary institution.

the way i do mathematics leaves me with with lots of wasted paper, and pencils with no erasers left

so, for example, abstract algebra might be 325

@tb In theory.

I see. Thanks!

Actually, within one hundred-block the ordering was pretty random, and there wasn’t much difference in level between $300$ and $400$. But the $100$-level courses were definitely the most elementary.

@Gigili Asaf, huh?

my one remaining ambition in life is to write a book on group theory for 10 year olds

@DavidWheeler I would so help you with that!

what's actually bogging me down is the beginning

@tb Right. At least his account was suspended for 30 minutes but the other one is playing around happily.

@DavidWheeler Start from the end.

there's notational issues, like set builder notation, and what i can expect they know about "counting numbers"

@DavidWallace Hum? Asaf what? No, my enemy!

@DavidWheeler You gotta do set theory first; basic set operations. Then to go to groups, all you have to do is define an arbitrary + on some arbitrary collection of bits and pieces.

@Gigili Oh, I see.

well, i was thinking of trying to get to the geometry first....S3 is much easier to understand by looking at a triangle

@DavidWheeler then what happens with groups that don't fit any nice geometry?

@DavidWheeler actually, on second thoughts, that's a fantastic idea.

@Gigili I guess the only thing that you can infer from that is that people are more willing to overlook some kind of behavior from people who actually try to contribute something.

dihedrals aren't much harder....and i want to talk about these earlier, to de-emphasize "habitual commutativity"

Because you can do groups of transformations on various spaces.

the way i see it, if kids can pick up an object...and SEE what i'm writing about...it's even better than a written proof

Sure, kids don't do written proofs.

@tb Painfully true.

I was at least 13 before I even understood what a proof (as normally presented) really was.

there are some parts about groups that are actually EASIER, then say, calculus, which has a reallllllly long learning curve

Oh, of course.

I think that to grok calculus, you've got to have some kind of mental picture that relates to rates of change and stuff.

not only that, but you have to have a well-developed spatial sense, and the formalism is just atrocious

I used to teach calculus by talking about the speed of a tennis ball thrown in the air; but when I had some students with poor spatial skills, this stopped working for me.

epsilon-delta proofs are just like pulling teeth, and they obscure the underlying ideas

@DavidWheeler I used to teach that you had to understand why the limit was what it was first, then try to express that with elspeth and delilah.

and limits...they're a sophisticated concept....they're not really numbers....

i like the concept of "nearness" i wish it was more widely accepted

x near A implies f(x) near f(A) is so much better than epsilon delta

I see a Galois connection.

@DavidWheeler Sure, so there are two steps to any problem involving limits. Understand whether or not "x near A implies f(x) near f(A)"; then do the mechanics of converting that understanding into an epsilon-delta argument.

but i doubt topology will ever get taught before calculus...too bad. look at what a dismal failure "new math" turned out to be

These two parts require completely different thought processes form one another.

what "i" mean is nearness can be defined rigorously, and it's equivalent to epsilon/delta, and the thought process isn't so byzantine

@DavidWheeler So once you've done group theory for ten year olds, you can do topology for eleven year olds?

Is the rigorous definition of nearness that you require to do limit problems any easier to formulate than an argument with delta and epsilon?

the way it is now, first you look at a restricted range, pull that back to the domain to find a suitable restrcited domain that lands you in the established restricted range, it's a serpentine process

@DavidWallace see: en.wikipedia.org/wiki/Kuratowski_closure_axioms

@tb Yikes! I'll stick with Elspeth and Delilah, thanks all the same.

a function is continuous if and only if $x \in \overline{A}$ implies $f(x) \in \overline{f(A)}$. Nearness is the same thing as belonging to the closure.

right, but i don't like the language of open/closed...people get the wrong ideas

@DavidWallace However, it’s possible in a calculus course to develop the notion of nearness without explicitly talking about closure.

... and then the student looks in their textbook, and it's all completely different.

so we need new textbooks, that's ok

Not if you’re teaching from notes specifically designed for that approach. I’ve not done it, but they do exist.

And there’s at least one freshman calc text, by Jerry Keisler, that does calculus with legitimate infinitesimals. Then you can meaningfully say that $f$ is continuous at $a$ if for every $x$ infinitely close to $a$, $f(x)$ is infinitely close to $f(a)$.

i guess what i mean is it doesn't seem fair to hit students 18-20 with, ok, now we're really going to get into topology in a real analysis or complex analysis or vector analysis course

American people look like Chinese people, Chinese people look like Japanese people. Does it imply that American people look like Japanese people?

Is it an equivalence relation?

continuity isn't THAT involved an idea....and when people first start seeing metric spaces they should be like: oh, yeah, i have a base of e-balls.

@DavidWheeler a little out of left field you mean?

@Gigili No, I don't think "look like" is transitive.

@Gigili My version is ‘green triangles are like green squares, and green squares are like red squares, so green triangles are like red squares?!’

well "like" implies a certain sort of similarity...if the "likeness" conditions are suitable defined, it COULD be an equivalence relation

Some people say I look like my sister. Some people say I look like my father. Some people say my sister looks like my mother. Nobody says my father looks like my mother.

My main problem with all those ideas is that there may well be very elegant and nice approaches to all kinds of things in basic math. In the long run I think it's a disservice to remove all kinds of obstacles in the student's educations. In other words, I do believe in social Darwinism in education.

The good ones will survive no matter what.

well, no matter HOW you present it...certain problems will STILL have an unavoidable complexity

yes.

and i'm not talking about "dumbing it down" i'm talking about not sticking with the historical recapitulation of math as learned by mankind

@BrianMScott Yes, I see your point. But it doesn't apply to my question, does it? Because Americans do look like Japanese people, IMO.

@Gigili Really?

Umm, I don't get it.

Seems to me that it depends on the coarseness of one’s looks like filter.

@BrianMScott right, and what features are captured by "looks like"

Yellow looks like orange. Orange looks like scarlet. Scarlet looks like crimson. Crimson looks like purple. Yellow does not look like purple.

for example if "looks like" means bipedal, two hands, and a head....

@tb: Did you see the fellow who wants to teach high school math but hates proofs?

Exactly so. They have a similar nose to Japanese people.

@BrianMScott I found that a bit scary.

@BrianMScott was that the tl;dr question?

@DavidWallace Yellow does not look like orange.

@DavidWheeler tl;dr?

OK, yellow looks like gamboge, gamboge looks like orange - the argument still works.

tl;dr = "too long, didn't read" internet slang for "wall of text"

Might have been; it is pretty long.

yeah, that's the one

Ah, that one. I was still suffering from the "math in muisc" question so I figured I shouldn't open it.

I must have missed that one. Just as well, I take it?

When the daughter of two of my Chinese friends was a baby, she liked playing with my nose. Her mother said it was because my (Caucasian) nose was a completely different shape from the (Chinese) noses of everyone in her family. OK, I'm not American, and my friends are not Japanese; but surely American noses don't resemble Japanese noses.

i see a lot of people posting on the internet things like: "i've always been good at math, and i've had THREE YEARS of calculus, but now i'm taking linear algebra and i have to prove stuff and i can't stand it, and boo hoo hoo aaaaiiieeeee"

@BrianMScott I don't want to keep it from you...

@tb: I just found a question to answer, I need to lock it so that other users can't answer it and the pop-up message "1 new answer" doesn't show up or else I shoot.

runs and hides

@tb Bless your heart! :-) That is ... quite something. And he signs himself Dr., too!

@Gigili i so need to answer that question, too. link plz.

@DavidWallace Your argument doesn't work at all, since you're a NZer!

@DavidWheeler Never! neener neener

"Euler got I worng, boys"...priceless

@DavidWheeler I always warned my discrete math students at the beginning of the term that many of them would find this the hardest math course they’d ever taken, because unlike the most of the courses that they’d had, it wasn’t primarily computational.

@BrianMScott well, i wish i would have taken it. i HATE computation. i'm an idea man.

We’ve had some gems today, including Rudy Toody and his ‘half of all squarefree integers are multiples of any given prime $p$’.

the thought of a pristine, luscious idea, ripe for the picking...ooh, it makes me quiver!

@Gigili Japanese people don't look like me. Americans look like me (well, not Japanese Americans, African Americans, assorted others). So if you contend that Japanese people look like Americans, then you're contending that it's not an equivalence relation.

What? who told you that you look like Americans?

everyday...*sigh* i scour the internet searching for something that will light my mental fuse....today, all i found was: prove if a is a unit and b^2 = 0, that a+b is a unit. how utterly depressing.

well, that was that universal property of projections question....but tb only gave me 3 out of 5 stars, for lack of incisiveness...and that was b4, it's dead to me now

@DavidWallace Anyways, if $p$ is false then it implies every $q$. That is, when your assumptions are wrong, so your conclusion might be correct.

@BrianMScott I'm having kind of a dry spell answer-wise and also questions-that-spark-my-interest-wise. The answers I give these days are not terribly good and on topics that don't draw a lot of attention or are too advanced. Or then I mess up horribly and the latest instance was more or less verbatim the same answer posted three minutes before I finished up.

Or whatever.

@tb on the plus side, you've surely raised my IQ at LEAST half a point

That makes me proud :)

@tb I think that you tend to be a little too critical of your own answers: I’ve seen you delete several perfectly good ones.

Hi guys

'Ello.

Okay, someone answered the question.

Farewell.

i.minus.com/ibyjbOzyAs1dn5.png Can somebody please help me with this proof? In this case $\phi$ does not have $x$ as a free variable. I understand that it should work, but I don't see $x$-free specific things. So it should not work if we just replace $\phi$ with $\phi(x)$ (i.e. $x$ is a free var in $\phi$), but I don't see where it breaks.

If you still want to provide definitions of the stuff you're talking about we can always reopen.

@tb Sounds like you're getting old. : )

@BrianMScott Thanks. Yes, there are various reasons for that. But I don't like to post answers that are more or less duplicates of those already posted and I like to leave easy ones to others who may want to contribute.

@tb I rather enjoy doing thorough explanations of some kinds of elementary material, especially if I have the impression that the OP is seriously confused, so I’ll post some easy answers even if there are competing briefer ones.

Like for example the quotient space answer yesterday : ) I plus-one-d it.

@MattN: the Fourier series question you commented on is a classic and it must be $\frac{\sin{(nx)}}{\log{n}}$. The fun of it is that the cosine version of that is a Fourier series of an integrable function. Your first interpretation doesn't really make sense (as the resulting function isn't periodic)

@MattN I’m trying to remember: did you have an answer to that one?

@BrianMScott Yep. I assumed I got plus-one-d by you after I plus-one-d you.

Sounds right.

@tb Ok. But if it's a homework question it could've been the first and then he could've written on his sheet that it can't be a Fourier series because it's not periodic.

That would be one silly homework question...

I don't think so.

@BrianMScott Oh, yes, and I enjoy reading those (that's one reason why I like André's answers a lot).

@tb Me too. And some of Arturo’s are in that category as well.

@Gigili Did you see this? (You're doing computer science, right?)

That poster’s going to run out of breath in a hurry if he talks the way he writes.

run forrest run!

@BrianMScott : )

@Gigili Or (easier), this.

Oh great.

Aren't they OT?

No, I think they're ok here.

@BrianMScott Are your sinuses clear again?

@BrianMScott And another question: do you agree that writing "an holomorphic function" is wrong English?

In spoken English, I think it's okay. I heard often enough: an homology theory, an homotopy group.

By a Brit?

@MattN My sinuses are clear, and I’ve almost stopped having coughing fits. Each stage lasted almost exactly a week: (1) fever, (2) flooding sinuses, and (3) $\text{cough}^\omega$.

(by American native speakers)

@MattN I consider it incorrect.

@BrianMScott Ok, thank you : )

@BrianMScott So have you tried any of the chocolates?

finding a primitive root mod 22...3 and 5 didn't work, bit of bad luck, there

@MattN I thought that I’d wait one more day $-$ that’s exactly three weeks. Call it theobromic numerology. :-)

: D

lhf and it smells like a dupe.

bbl

@BrianMScott I'm confused: is the aspiration of the h any stronger in holomorphic than in homology or homotopy?

(or historic, habitual, etc.)

I never learned such a rule but I sort of figured that in spoken American English it was okay to use an in front of an only mildly aspired h.

@tb No, it’s not. But unless it’s actually silent, as in hour, the rule is to use a. In some British writing, especially older writing, you’ll find an hospital and the like, largely because for a great many speakers the h was silent. This is now something of an affectation, and it’s limited to writing.

@tb Prescriptively it’s an error in U.S. English. I’m surprised that you’ve heard it.

just checking to see if this is right: if i am taking discrete logs (mod 22) then $\log_7(5) = 2$?

@BrianMScott I believe you, but it's strange. I know I heard it used several times by various native speakers, because I was surprised by that.

an heir to the throne a hair on my head

@tb At a guess, they dropped or nearly dropped the h and used the form of the article corresponding to what they actually said without thinking about the formal pronunciation of the word.

if the aitch is aspirated, the enn is omitted...probably a holdover from those silly french kuhniggets

@BrianMScott Yes, I believe you're right. Hence my "theory" of the mildly aspirated h.

Thanks :)

@DavidWheeler yourrr mosa was a hamsterrr

and your father smelt of elderberries

I give up :)

i can haz answer for my discrete log question?

Not from me. It's for lack of ability, not for lack of want.

well, it's just that today is the first time i've ever calculated one, so i may misunderstand how they work

@DavidWheeler $7^2\bmod 22=5$, so it looks okay.

Brian, sorry to bother you with something else. You mentioned $G_\delta$-topologies in connection with that $\Delta$-system-type lemma I asked you about. Where could I read a little on that?

it's the inverse isomorphism from Z10 to U(22), right?

@tb Let me think about that for a little; it’s been years since I worked with anything like that.

or maybe i have that backwards...put in products, spit out sums is what i mean

@BrianMScott thanks in advance!

wow, apparently computing discrete logs is a "hard" computational problem

i did not know that

That's what a lot of crypto rests on!

see one-way functions

@DavidWheeler so is computing square roots (modulo some number).

someone told me finite fields are actually used for checksums to make sure UPC's are scanned correctly at cash registers

@Srivatsan hey, Srivatsan!

hi tb

(I am here only for a few minutes. There's a scheduled power outage in 3 mins. :))

i already knew about RSA crypto based on factoring

@Srivatsan Oh, then I wish you a pleasant day and hopefully see you soon!

@Srivatsan hi srivatsan whose name is hard to type

@DavidWheeler I imagine it this way: if computing square roots is hard, then logarithm should be even harder, no?

@tb I don’t think that I’m going to be able to come up with any specific references, but in searching it may help to know that spaces in which $G_\delta$ sets are open are sometimes called $P$-spaces. Unfortunately, there is a completely unrelated notion of $P$-space due to Morita and the completely unrelated $p$-spaces of Arkhangel’skiĭ, and I think that there’s another unrelated notion of $p$-space as well to foul up the works.

@DavidWheeler hi David :) (I will let my parents know of your complaint about my name :=))

hi Brian

Long time. I saw in the transcript that you were/are not too well. Take care.

@BrianMScott then there's also PSPACE-completeness to complete the confusion. Thanks for the hints!

Hullo, Srivatsan.

it sounds hindustani, but i could be wrong

@DavidWheeler Indian, yes. The name translates to "son of Sri" (assuming this helpsto "parse" it :)).

as in sri ganesha?

Time for me to call it a night; I’ll see you folks later.

Take care @Brian

I missed your time here... :(

Hi all of you...

Bye @Brian, thanks for the guidance on $G_\delta$-spaces, I found some things that look somewhat promising.

@tb BTW, just finished that one problem that bugged me... I thought I had the solution long back in the morning...I started to write down and it broke down! Now, finally, I approached my instructor to show him what I had. He suggested an approach that worked.

okay, nice to hear. I haven't thought about it any more, sorry.

And, that sequence thing, I cooked up another argument. I did not get Rohatgi's argument as yet.

What doesn't kill you, hurts!

@Gigili lol, orly?

Really.

so...come here often?

Come here often?

Yeah, explain yourself, blue David.

you guys! pshh!

pshh-es

@Gigili so what kind of math do you do?

Nice typo: "if somebody could shade some light on this". I sincerely hope I didn't shade the light.

wipes away pshh

is shading some light like taking a gift?

@DavidWheeler I don't do math. I'd rather do something else.

One definition in thefreedictionary is "To change or vary by slight degrees"

@Gigili why do you hang out here, then, if i may ask?

@DavidWallace If DavidW is blue, You're purplish brown?

@DavidWheeler I used to come here during the preparation for the university entrance exam.

Pinkish brown?

Brownish pink.

they made you take an exam to see if your door-opening skills were up to snuff? how odd.

heh. blue David. i like it. makes me sound as if i'm all moody and shizzle.

@DavidWheeler Indeed but I had no choice.

Yeah, and you're kind of grey.

that implies a certain...moral ambiguity....

I am green.

we were discussing politics?

Oh?

kermit's my homeboy

'sup frawg?

I just downvoted the long question. I'm not sure there was a question in there.

you BEAST!

i'll leave it up to you to decide if that is a compliment, or an insult.

I lost it when he wrote "the hands of the clock must have the same center or centers related by a vector"

rut roh...henning's here. i don't think he likes me.

@DavidWallace oh i know...i think he overindulged in a schedule I substance or something

Maybe it belongs on writers.SE. That's where @RegDwight moves stuff like this.

@DavidWheeler Huh? I don't remember writing anything that ought to have given that impression.

@HenningMakholm lol, it's all good.

Whew, I was hoping not to have to witness another chat-fight today.

@DavidWallace :)

@DavidWallace sorry to disappoint. maybe next time.

I think my attempt to get the two gentlemen on English.SE to kiss and make up was a dismal failure.

How disgusting.

@Henning: What do you think about this edit? I think it was Mathematica source (or something similar) before the edit. Should I roll back and bring it into code block form or just leave it as is?

Umm, again my edit.

It's a beautiful day to save lives. Let's start with my own.

@Gigili What's disgusting?

@Gigili your life needs saving? gimme a minute, i'll go get my sword...

@DavidWallace Imaging two gentlemen kissing each other

@Gigili I find that very narrow-minded of you.

@tb Hmm... I approved because it looked not obviously wrong and seemed just to polish the presentation. I you can see a reason to restore the original formulation, by all means go ahead.

@DavidWheeler No time to lose.

@DavidWallace Thank you!

@HenningMakholm Okay, I'll try and keep the best of both. It might be useful to have the copy-pasted Mathematica thingie for those who want to play with it (if there are any.

as a matter of fact, many members of my family are gay. it doesn't matter as much as you think it might.

@tb The more important thing to figure out might be whether the question is, "how do I get Mathematica to do this for me?" or "how can I do this numerically (with a general-purpose programming language)?".

@tb Close to namesake and Gravatars are in some sense similar...

I have an odd feeling that the automatically generated gravatars may be based on email address. If two people have email addresses that are very similar, they may just end up with very similar gravatars.

I wonder whether this is to help the moderators detect sock puppets.

That's right--they are generated from the hash code of an email address @DavidWallace

Well maybe it's not the greatest hashing algorithm in the world then.

It's MD5. So technically you're right -- MD5 is not the greatest hashing algorithm in the world -- but that doesn't mean that similar email addresses ought to come out with similar hashes.

So if two people have email addresses that are very similar, they'll actually end up with completely different gravatars.

If I can guess what one's email address might be, so I should register with that and check if my avatar is similar to it?

Yes. Or, with the expected small probability, with gravatars that look vaguely alike.

@Gigili You're devious. I love how your mind works.

but by "similar", you actually mean "identical".

@Gigili You don't need to register -- just compute the hash yourself as described at en.gravatar.com/site/implement/hash and request the gravatar for that hash in your browser.

i wanted to use my real life pic for my gravatar, but i was worried people might get me and David Wallace mixed up

@DavidWheeler Huh? Sounds like you both look alike!

with a suitably coarse filter of alike....sure....

@HenningMakholm I edited and left a comment to that end.

Thanks.

(If DavidWallace looks like his gravatar here, of course... or you know DavidWallace IRL.)

My gravatar is indeed a photograph of me.

I have never met Blue David, so I can't comment on whether he looks similar to me.

@DavidWallace If he's American ...

@Gigili here are some instructions for that.

then of course i look japanese!

@tb May I ask why you deleted "\times"?

For all I know, David Wheeler is black.

Is he?

@tb Great. Sounds exciting.

Thank you.

@Gigili To save space. People don't usually write multiplication signs anymore.

I doubt it, since he's concerned that people would get the two of us confused if he used his real photo.

@Gigili "For all I know" usually means "I have no information either way".

@tb Multiplication signs?

@Gigili what else should it be? it's an ODE on a real interval.

@HenningMakholm I thought he meant "surely". But it's English anyway, it usually means the opposite of what you think.

I do know that he's older than me.

you know, i never knew before today that i was black...AND japanese! i tell you, you people are really on to something with this here mathematics thingie. yessirree bob.

@DavidWallace He's 36.