Mathematics - 2013-11-18 (page 2 of 3)

Pedro Tamaroff

04:00

@TedShifrin Oh, sorry.

Fernando Martin

Ah, I see, it's more or less the same argument posted in that thread

Ted Shifrin

Oh, i took parallel lines joined by a thick bar at the top that (shrinkingly) goes to infinity.

Don Larynx

I don't get what that means @Mike

Can you explain?

or @Ted

the cartesian product of $0$ with a line from $-\infty$ to $n$?

Mike

Embed $\mathbb{R}$ in $\mathbb{R}^2$; delete the bit $\{(-n,n)\}$ from the embedding

Fernando Martin

How's that closed?

Don Larynx

04:02

Right, but does't that make it disconnected

or not because its finite\

Fernando Martin

The intersection is disconnected @Don

Mike

Err, how isn't it closed? I'm talking about the complement of that.

Don Larynx

but $(z, 0) \cup (-z, 0)$ is disconnected!!

Mike

Oh.

Ugh.

That was silly.

Fernando Martin

@Mike: That's easily fixed though

Mike

04:03

Okay, delete an open box that gets progressively wider.

Ted Shifrin

My $C_n = \{0,1\}\times\Bbb R \cup \Bbb R\times [n,\infty)$.

Fernando Martin

Just choose $(-n,n)\times (-1,1)$ and that works

Mike

That's clever, @TedShifrin

I like it better than just cutting the plane in two

Fernando Martin

Indeed, that's nice

Ted Shifrin

I assume @Pedro resolved his quandary and I can go to sleep.

Pedro Tamaroff

04:06

@TedShifrin I think I did.

I can consider the circle instead of the sphere.

Ted Shifrin

What's your $R$?

The circle?

Pedro Tamaroff

@TedShifrin Disk, sorry.

Ted Shifrin

Yes, right. So the answer is ...

Pedro Tamaroff

@TedShifrin I am doing it.

Ted Shifrin

This is not unlike that ant problem in my book I tried to get you to do months ago. :)

NO @Green.

Pedro Tamaroff

04:09

Over the disk, the field is $F(x,y,0)=(-x,y,4-x^2-y^2)$. Well, I do need to change the x ans ys too.

Ted Shifrin

You've already applied Stokes to convert to flux across the disk.

Pedro Tamaroff

@TedShifrin Yes.

Ted Shifrin

What is the integral over the disk?

Don Larynx

How is $R^2 - R$ an answer to my exercise?

It's the union of two separated sets

the negative side and the positive side

Pedro Tamaroff

@TedShifrin Well, I need to do a few calculations.

Ted Shifrin

04:11

What are your connected sets, @Don?

@Pedro: You're not thinking right. What is $dz$ on the disk, so what is the $2$-form?

Pedro Tamaroff

@TedShifrin It is zero.

Ted Shifrin

Right, so the flux across the disk is what?

Pedro Tamaroff

So $(4-x^2-y^2)dx\wedge dy$.

Ted Shifrin

Right ...

Don Larynx

@TedShifrin A = \{y : y > 0\}$ and $B = \{y : y < 0\}$

Ted Shifrin

04:14

@Don, your problem was about a sequence of nested connected sets. What are those?

Mike

I was puzzling over this the other day: What are the periodic points of $e^x$? It's not painfully difficult to see that it has no fixed points, but does it have any periodic points?

Pedro Tamaroff

@TedShifrin I end up with $$2\pi \int_0^R (4-r^2)rdr$$

That cannot be right.

Ted Shifrin

I think not @Mike.

Mike

I don't think so either, but it's not something you can brute-force like the fixed point case.

Ted Shifrin

It's right, @Pedro, but once again you should think rather than compute.

Pedro Tamaroff

04:17

@TedShifrin What should I think?

Ted Shifrin

What $R$ maximizes that integral? What does that integral represent to a beginning calc student?

Pedro Tamaroff

@TedShifrin I don't know.

anon

(when does the integrand become negative)

Ted Shifrin

What is $\int_\Omega f dx\wedge dy$ for $\Omega\subset\Bbb R^2$?

Hi @anon, you spoilsport :)

Pedro Tamaroff

@TedShifrin I know the solution to this, I just don't know where you want to get at with "think and don't calculate".

Ted Shifrin

04:24

What region maximizes the signed volume under $z=f(x,y)$? We discussed this months ago for my ant problem. :)

Pedro Tamaroff

The level zero level set of $f$?

Ted Shifrin

Right!

Well, the zero super-level set :)

Pedro Tamaroff

@TedShifrin I mean... just take where f is zero, the "inside".

Ted Shifrin

$\Omega=\{x:f(x)\ge 0\}$.

Pedro Tamaroff

@TedShifrin Right.

Ted Shifrin

04:28

OK, bedtime for me. Night!

Pedro Tamaroff

@TedShifrin Byes.

@anon Do you have any suggestion to make this more "reader friendly"? It is just some algebraic manipulation, really. Maybe change $x^2x'^2$ to $(xx')^2$ to hint on the binomial?

Mariano Suárez-Alvarez

urgh. Do not write it using pairs.

well, it is too late now :-)

but hiding the fact that numebrs are numbers cannot help!

Pedro Tamaroff

@MarianoSuárez-Alvarez Point taken.

anon

yo @Mariano, question for you

Pedro Tamaroff

I just thought $N(x+\sqrt 2y)$ was a little cumbersome.

And I don't like $\sqrt 2$ to appear. >:(

=P

@MarianoSuárez-Alvarez I couldn't sort out the cyclic vector problem. =/

anon

04:36

over here the constant from minkowski's is $(4/\pi)^{r_2}(n!/n^n)|\Delta_L|^{1/2}$. the bound I am familiar with is $(2/\pi)^{r_2}|\Delta_L|^{1/2}$. does the former one come from a different argument or something? it seems more powerful (esp. for big $n$)

Pedro Tamaroff

Heh, Green's identities are cool looking.

anon

@PedroTamaroff I think it'd be best to just go along with the hint than introduce norm maps

either way writing $x+\sqrt{2} y$ is better than tuples

Pedro Tamaroff

What is $E$ and $H$ in $$\int\limits_C {Eds} = - \frac{1}{c}\frac{d}{{dt}}\iint\limits_S {HdS}?$$

Mike

@anon I'm seeing this formulation in Stewart and Tall:

Pedro Tamaroff

Faraday's Law, it is.

anon

04:41

@Mike yes that's where I got $(2/\pi)^{r_2}|\Delta_L|^{1/2}$ from

Mike

HBT p 190

err

anon, page 190

$|\text{N}(\alpha)| \leq (\frac{4}{t})^t \frac{n!}{n^n} |\Delta_L| \text{N}$ $(\frak{a})$

Pedro Tamaroff

@Mike \frak a

Mike

Thanks

anon

@Mike my page 190 is about kummer theory

Mike

What edition do you have

I have a chapter 10, computational methods

It's theorem 10.2 for me

anon

04:43

ah, thm 10.2 indeed (it's 173 in mine)

Mike

Glad to hear you have it, I didn't want to have to read the argument...

Zibadawa

I haven't had a math book in my possession in years.

Mike

:(

Pedro Tamaroff

@MarianoSuárez-Alvarez Are you there?

Mike

I feel bad without my copy of Atiyah-MacDonald but I can't even imagine not having my mini-library

Zibadawa

04:52

Try being a researcher without it.

Sometimes the internet doesn't have what you're looking for.

Mike

Yikes

What do you study?

Zibadawa

I prefer not to specify.

Mike

Gotcha, sorry for the intrusion.

Zibadawa

No problem, it's a natural question to ask.

Pedro Tamaroff

@Zibadawa Why you don't like to tell?

Mike

04:55

Some people prefer anonymity and specifying your field makes it easier to break that.

Pedro Tamaroff

Do you guys think you can help me with a problem in linear algebra?

Fernando Martin

Let's see

Pedro Tamaroff

I already asked about, but I couldn't finish it.

I have to prove that $f:V\to V$ an endo over a fin dim vector space admits a cyclic vector iff C(f)=K[f].

Admiting a cyclic vector is equivalent to $m_f =\chi_f$.

Don Larynx

Hey guys

I need to know....

anon

you left your keys on your dresser

Don Larynx

05:02

Why is this the last exercise in "Compactness" in Kaplansky: "Let $A$ and $B$ be compact subsets of a metric space, prove $A \cup B$ is compact". It's straightforward isn't it?

Am I missing something?

Mike

It's straightforward, yes

Pedro Tamaroff

@DonLarynx What's your argument?

anon

yes

Don Larynx

Basically this: math.stackexchange.com/questions/187734/… @Pedro

Pedro Tamaroff

@DonLarynx There is a difference between $A+B$ and $A\cup B$.

Don Larynx

05:03

It's essentially the same idea @Pedro

anon

apparently I downvoted an answer over there but have no memory of that thread

Pedro Tamaroff

@DonLarynx Can you make your argument explicit?

anon

take an open cover of A U B, pick finite ones for A and B, union them, finite cover for A U B

Pedro Tamaroff

@anon Happens every time. =)

Don Larynx

@anon: That doesn't answer my question.

Pedro Tamaroff

05:05

@DonLarynx How so?

anon

I was talking to Pedro

Don Larynx

Because it's #17 it should be #1

anon

my answer to your question above is the word "yes," as in yes it is that simple

Pedro Tamaroff

@anon I know the argument, I wanted to hear Don's.

anon

don's was inevitably going to be the same

Don Larynx

05:06

@Pedro it's essentially the same as both @anon and the question i posted

it is really not that hard

Now I have to go through the trouble of typing, etc...

when you never even answered why:
It is #17 instead of #1!!

Focus. @Pedro

Pedro Tamaroff

@DonLarynx What?

Don Larynx

Why is it #17 instead of #1? Am I missing something insightful?

anon

haha

Pedro Tamaroff

@DonLarynx What are you talking about?

Don Larynx

Why is this the last exercise in "Compactness" in Kaplansky: "Let $A$ and $B$ be compact subsets of a metric space, prove $A \cup B$ is compact". It's straightforward isn't it?

Am I missing something insightful?

Pedro Tamaroff

05:09

I don't know,

Mike

It's just a standard exercise.

Jaycob Coleman

Would anyone care to hear a fascinating elementary observation related to perfect numbers?

I think you'd be crazy not to find it fascinating

anon

@Pedro You may find this interesting. My algebraic number theory book decided it wanted to prove that the hypervolume of the parallelotope with edges given by n vectors (same as the dim of the space) is the det of the matrix formed by the vectors. It proved it by writing the vol as an integral of '1' over the parallelotope and then used change of variables.

but the change of variables formula is proved using the fact det(V) is the volume of the parallelotope!

Pedro Tamaroff

►

Ok, what is it?

@anon Did the book collapse into itself?

Mike

@Jaycob feel free, but I'm crazy

Jaycob Coleman

05:12

It's well known that every perfect number is a practical number.

Don Larynx

Numbers drive me crazy @Jaycob

Mike

@anon I maintain that geometry of numbers is boring as heck

Jaycob Coleman

Recently I observed that every multiply-perfect number appears to be practical as well

Pedro Tamaroff

@JaycobColeman Practical meaning...?

Jaycob Coleman

A practical number

Mike

05:13

Yep, I'm definitely crazy by your definition

Fernando Martin

save me a place in the insane asylum

Jaycob Coleman

Consider the sequence of integers $n$ such that $\sigma(n)/n=k/m$, where $\gcd(k,m)=1$ and $m$ is practical.

Then $n$ is practical. This is true for thousands of terms at least.

Mariano Suárez-Alvarez

@PedroTamaroff I am now

Jaycob Coleman

It's a huge generalization of the multiply-perfect numbers

Pedro Tamaroff

@MarianoSuárez-Alvarez Ah! I was almost leaving. It is about the cyclic vector thing. I could not sort it out.

Mariano Suárez-Alvarez

05:15

Ah.

Don Larynx

I think I want to forget all of the number theory I learned this semester.

Mariano Suárez-Alvarez

I'm in the middle of writing a few recommendation letters

let's leave that for next time, for otherwise I 'll never finnish this

(it bores me immensely :-) )

Pedro Tamaroff

@MarianoSuárez-Alvarez OK.

Mariano Suárez-Alvarez

what was the statement you wanted?

Mike

Hey, at least you're writing them!

I still need to harass one of my recommenders

Pedro Tamaroff

05:17

@MarianoSuárez-Alvarez An endo f:V --> V admits a cyclic vector iff its centralizer are the polynomials in f. Here V is finite dimensional.

Jaycob Coleman

It implies the conjecture that every multiply-perfect number is even and much much more

Mariano Suárez-Alvarez

@PedroTamaroff ok

Pedro Tamaroff

@MarianoSuárez-Alvarez I already know the proof for cyclic => C(f)=K[f].

I need the other direction.

Zibadawa

@PedroTamaroff Privacy and paranoia.

Pedro Tamaroff

And I know I can equivalently prove C(f)=K[f] => m_f = X_f

Mike

05:19

I understand that, @Zibadawa

Jaycob Coleman

Here are the terms <10^6.

Mike

Scary story: a friend of mine posts on an internet forum completely unrelated to her academic career. Someone didn't like her, decided to find out her real name and such, and post tons of bad reviews on ratemyprofessor, spam the internet with their name and, well, connections you don't want to have with your name...

She's since pretty much stopped using the internet.

anon

that's sad

Ethan

yes

Mike

yeah.

Ethan

05:26

i like to live my life dangerously, though

Zibadawa

Mind giving me your SSN and credit card info, then?

Jaycob Coleman

Any interest? I want people other than myself know about this sequence, because it is most definitely significant. I'll be adding it to oeis soon enough.

Ethan

it was a joke, a quote from austin powers

anon

"I too like to live dangerously"

in any case, sounds absurd coming from the delet-o-matic!

Ethan

lol

user60887

05:30

hey guys

i asked this question earlier but I want to know if I got the foward direction right. math.stackexchange.com/questions/571393/…

Zibadawa

UC Berkely is in that list twice.

anon

@user60887 yes, I saw your edit and upvoted

user60887

oh ok thanks

anon

you essentially did what I was getting at in my comment

Ethan

@Zibadawa didn't notice, that's pretty stupid lol

Zibadawa

05:33

Several others are duped, too. Don't know why. Master's and Ph.D. programs considered separately?

Mike

I'd love to comment too but with Ethan's deletion spree I can't!

Ethan

grad-schools.usnews.rankingsandreviews.com/…

user60887

are you guys looking at statistics grad schools?

Jaycob Coleman

@PedroTamaroff are you perhaps looking into practical numbers?

Pedro Tamaroff

@JaycobColeman Nope, into something else.

Mike

05:34

@Ethan I've heard from quite a few different people never to bother with USNews rankings for graduate school. Ask a statistics professor about active departments that place their students well, that sort of thing

user60887

are statistics grad schools competitive to get into if you want to get a masters?

Ethan

@user60887 looking for undergrad schools actually, but I assume if there graduate programs are good so would the latter.

Mike

Okay, then, I redact what I said - if you're looking for undergrad schools, USNews rankings are a pretty safe heuristic

Jaycob Coleman

@PedroTamaroff. Shame. Well won't bother everyone further with it, but I recommend looking at the terms in the pastebin link I posted. The sequence is much larger than any known generalization of perfect numbers, which have been studied simply because someone noticed they had even terms. With the idea that the multiply-perfect numbers were practical (a set with density 0 in the integers), I sought out further generalizations with that property, and this is what I found.

user60887

and lastly regarding that same question if I wanted to show a specific had infinite order say $1+2\sqrt 2$ I can do so by the binomial theorem? Since I can show each time I multiply $1+2\sqrt 2$ its increasing?

Don Larynx

05:49

@Jaycob try making your own chat room.

Maybe you'll interest people with similar interests

Jaycob Coleman

@DonLarynx I have found very few people aware of/interested in practical numbers.

Mike

It's fairly rare that a number theorist is interested in that sort of thing

Mariano Suárez-Alvarez

what is a practical number?

Jaycob Coleman

Every integer less than a practical number can be represented as a sum of distinct divisors of the practical number

Pedro Tamaroff

@MarianoSuárez-Alvarez You can write any of its predecessors as a sum of its (the practical number) divisors.

Jaycob Coleman

05:56

Pedro puts it better

Mariano Suárez-Alvarez

I can see why people do not get that interested in them :-)

Pedro Tamaroff

@JaycobColeman Missed distinct factors.

Else every number is practical!

@MarianoSuárez-Alvarez Turns out my problem is a theorem "The Cyclic Vector Theorem." hehehe

Mariano Suárez-Alvarez

sure

everything which is true is a theorem!

and the negation of everything that is false

there are lotss of theorems :-)

Pedro Tamaroff

The proof of the converse uses the decomposition of the transformation into the rational form, if that is the correct name,.

Mariano Suárez-Alvarez

I doubt one needs that

Pedro Tamaroff

05:59

@MarianoSuárez-Alvarez The proof is a little "ugly"

Yes, I think the same.

That is, they use we can write $A$ in blocks as companion matrices.

I'll go now.

Byes.

Jaycob Coleman

@MarianoSuárez-Alvarez The practical numbers are in a sense "dual" to the primes. You should not underestimate their significance.

Mariano Suárez-Alvarez

well, I had never heard of them so I can guess that they are not immensely important

Jaycob Coleman

You would think that, but consider that they were only defined 50 years ago

Not enough is known yet to say

Mariano Suárez-Alvarez

unless something useful has been done with them, it sounds like something very firmly grounded in the area of what is known as recreational number theory

which is not bad n itself, of course

but explains the fact that nunber theorists are not in a rage about them, say

Jaycob Coleman

What do you consider "useful?"

Mariano Suárez-Alvarez

06:05

searching for "practical number" in Mathreviews gives 26 results

going from Srinivasan, A. K. Practical numbers. Current Sci. 17, (1948). 179–180.

to Pollack, Paul; Thompson, Lola On the degrees of divisors of Tn−1. New York J. Math. 19 (2013), 91–116.

the first paper is a one page pager. not many of those.

(3 of the papers have nothing to do with this, really)

of course, it may happen that next week someone shows that the Riemann hypothesis is equivaent to something or another relative to practical numbers and who knows

but the same can be said of impractical numbers, which are those whose digits in binary spell complete sentences of the Bible in ASCII

Mike

you get 26 results in mathreviews?

Mariano Suárez-Alvarez

yup

Mike

i only get 9

Jaycob Coleman

Actually I've conjectured that every highly abundant number other than 3 and 10 is practical, which implies that any counterexample to Robin's inequality is a practical number.

Mike

prove some theorems, write some papers

Mariano Suárez-Alvarez

06:15

@Mike, "Anywhere=(practical number)"

Mike

ahhhh

i was searching by title

yeah, it looks like there are about 3-4 authors that publish on the subject

@Mariano I picked up Halmos, though I won't have a chance to look at it until January. It looks like a friendly exposition!

Mariano Suárez-Alvarez

which halmos

Mike

oh wow

I meant Hilton

Hilton-Stammbach, Homological algebra

Mariano Suárez-Alvarez

ah

H&S is very very nice

it is not cozy, though :-)

there is a certain coldness to it

to get warmth, you should pick Maclane's book Homology and browse it a bit

Mike

Certainly better than Weibel, which I was also considering

Mariano Suárez-Alvarez

06:25

it has tons and tons of motivation, historical information and what not

weibel's is difficult

it is not very introductory in spirit

even if it starts from the beginning

Mike

I know the category and module theory and whatnot needed to at least start it (from Hungerford), but I got turned off by seeing a diagram on every page within the first ten pages

Mariano Suárez-Alvarez

(it has a great thing: it does spectral sequences very very early, and uses them along the way. I think that is a great idea)

diagrams are good, not bad

Mike

I don't like diagram chasing :(

Mariano Suárez-Alvarez

that's a silly objection

Mike

it's still true!

I mean, I know I have to do it nonetheless

Mariano Suárez-Alvarez

06:28

it's like saying I like to write but I do not like drawing the letter h

Mike

it's such a bland letter

Mariano Suárez-Alvarez

:-)

gotta run now

ciao

Mike

See ya

Don Larynx

07:29

I'm alone!

The room is all mine again.

HAHAHA!!!

mesel

07:41

not any more :)

But,we can share it :)

Don Larynx

I'm going to sleep, good night @mesel

mesel

Good night

Actually,here is morning :)

Alec Teal

08:46

Morning!

Oh seriously?

BUMP AGAIN math.stackexchange.com/questions/571161/…

Babibu

09:00

Hello

@mesel good morning

@AlecTeal Can you help me with some thing?

Alec Teal

09:15

Sure

I can try

@Babibu

Babibu

0

Q: Convert 4-sat to 3-sat

I want to know in general how can I convert $4-SAT$ to 3-SAT. And I have a specific case that if you can help me optimize it to 3-SAT it will be greate. I want to do this so I be able to use sat solvers programs. $(C \lor A \lor D) \land (C \lor B \lor D) \land (\lnot C \lor A \lo...

@AlecTeal This is my question

Alec Teal

09:37

@Babibu I probably can't help sorry, I don't know boolean algebra well enough to know even what SAT means

Babibu

oh

ok

tnx any way

Alec Teal

10:02

@DanielFischer please don't gie up

Daniel Fischer

@AlecTeal Who's givin' up?

Alec Teal

I dunno when nothing happens for a few minutes I fear someone just moved on.

Daniel Fischer

@AlecTeal a) I'm slow typing, b) I occasionally have other things to do in between, c) chat.stackexchange.com/rooms/11557/…

Alec Teal

I didn't say I wasn't needy :P

Daniel Fischer

@AlecTeal So what about joining the chat room for that question?

Alec Teal

10:12

Didn't you mean here?

@DanielFischer

Daniel Fischer

No, @Alec, I thought the automatic room would offer less distraction.

Grobber

11:11

Well I was try to self-study affine geometry.

Can you suggest me a good resource to gain momentum?

dfeuer

@Grobber, I don't know any geometry books I can recommend. I had a fabulous geometry teacher back in middle school, but I believe she is retired, elderly, and ill, if still alive.

Grobber

okay thanks

What are its prerequisites?

laovultai

13:10

about question 483912 I asked from one user following: Where do you get this multivariable chain rule? Any references please?

Is there now that user?

Enjoys Math

16:42

Are there any other constructions making use of quotients of free groups like the tensor product?

It seems like a hugely useful way to construct things, but I don't know of any others

Enjoys Math

16:57

O_O

:X

Alizter

17:09

@EnjoysMath You could try lego

Enjoys Math

what's that?

lego my ego?

*eggo

Alizter

Lego

Lego is a popular line of construction toys manufactured by The Lego Group, a privately held company based in Billund, Denmark. The company's flagship product, Lego, consists of colourful interlocking plastic bricks and an accompanying array of gears, minifigures and various other parts. Lego bricks can be assembled and connected in many ways, to construct such objects as vehicles, buildings, and even working robots. Anything constructed can then be taken apart again, and the pieces used to make other objects. Lego began manufacturing interlocking toy bricks in 1949. Since then a g...

Enjoys Math

OH, lol

Chris's sis

Greetings

Alizter

da dum tss

Enjoys Math

17:10

math constructions are way more important than lego constructions

Alizter

@EnjoysMath You asked for other constructions. They aren't necessarily important ;)

Enjoys Math

Oh, the free group on leggos is too hard to understand

Alizter

The fact that it says "manufactured by The Lego Group" cracked me up

Enjoys Math

What would be even better is if you took your legos out to a field

and got bitten by a field mice, a vector

Alizter

you would have to buy a new set

Enjoys Math

17:14

OMG

Alizter

Stealing a lego brick from one set to use in another can be the definition of homomorphism

Enjoys Math

@Chris'ssis sawadi ka

Alizter

I am too distracted. Must revise Economics such that I can go to a good university and then study pure maths yay.

Chris's sis

17:44

hmmm, I'd like to have a beautiful picture to illustrate the distribution of 2 double series on a table like chess table :-(

Alizter

18:58

@Chris'ssis

Enjoys Math

19:26

►

Nimza

Hellollollollollo llollollo llollollo, llollollollollo!

N3buchadnezzar

@Chris'ssis $$ \int_0^{\pi/2} \sin 100x \sin 101x$$

@Alizter Hi

Alizter

Helllo @N3buchadnezzar

N3buchadnezzar

@Alizter A somewhat strange question ^^ Do you remember an interesting integral similar to the trivial one above?

Alizter

Not really no

Charlie

19:48

@Nimza lots of lollos

Nimza

@Charlie it's a famous song

Charlie

@Nimza hmm...

Hi @Chris'ssis

Chris's sis

@Charlie Hi

Charlie

@Chris'ssis how are you?

Chris's sis

@Charlie Not too bad. I'm working on a question. How about you? :-)

Charlie

19:56

@Chris'ssis I'm fine, just a lot dizzy

Chris's sis

@Charlie dizzy? Why?

Charlie

@Chris'ssis I don't know

Chris's sis

@Charlie did you check your blood pressure?

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