« first day (19 days earlier)      last day (28 days later) » 

00:41
@dydxx Differentiation under the integral strikes again!
0
A: Indefinite integral : $\int \frac{dx}{(1+x^{2})^{\frac{3}{2}}}$

Simply Beautiful ArtWe have $t>0$. Recall that $$\int\frac1{(t^2-x^2)^{1/2}}\ dx=\arcsin\left(\frac xt\right)+c_1$$ Take the derivative of both sides with respect to $t$ to get $$\int\frac{-t}{(t^2-x^2)^{3/2}}\ dx=\frac{-x}{|t|\sqrt{t^2-x^2}}+c_2$$ Setting $t=1$ and simplifying, we reach $$\int\frac1{(1-x^2)^{...

2
A: Integration by parts with a pre-defined integral

Simply Beautiful ArtYou could try differentiation under the integral sign: $$\frac d{dt}\int F(xt)\ dx\bigg|_{t=1}=\int xf(x)\ dx$$ So you'll need to solve $\int F(x)\ dx$ to continue further.

00:53
@dydxx Actually, here's a nice PFD that may interest you: math.uconn.edu/~kconrad/blurbs/analysis/diffunderint.pdf
oo
thanks
Are you familiar with linear algebra and planes?
Nope
lol, do tell
im just confused what transposing does to a matrix
geometrically
with an "ELI5" explanation
01:06
well
I got nothing
lol
its ok
are you on discord often?
$$\int_0^1\frac{x^t−1}{\ln(x)}~dx=\ln(t+1)$$
Oh, whoops
I try to be
But I can't be everywhere
I tried to add you on discord
but the link never worked
for some reason
Can you find the "add friend" button?
@dydxx Or just give me your name
dydx#3879
01:11
Ok, I sent
Why dydx?
Usually I use dydx
but on stackexchange dydx was taken
01:34
@BrevanEllefsen I'm interested in some wild examples of differentiation under the integral sign
Define wild
@BrevanEllefsen Not raised in a farm
Fair enough
btw
it appears that the lerch Transcendent simplifies for integer S nicely
which means I can take care of that general case
it's a recursive formula as far as I can tell
Might be able to get a few closed forms for it when $s$ and $n$ are related or are both integers or one goes to infinity
01:41
33 mins ago, by Simply Beautiful Art
$$\int_0^1\frac{x^t−1}{\ln(x)}~dx=\ln(t+1)$$
I really liked the proof of this integral, and you might too
Ah, I've seen that one before
link?
I can't remember where I saw it
was on some big-list iirc
50 mins ago, by Simply Beautiful Art
@dydxx Actually, here's a nice PFD that may interest you: http://www.math.uconn.edu/~kconrad/blurbs/analysis/diffunderint.pdf
hmmm
I've seen it on this site before
I really want seperate big list for all known, common Hypergeometric identities
all interesting Differentiation under Integral Sign proofs
01:44
I've probably seen one
3 mins ago, by Simply Beautiful Art
33 mins ago, by Simply Beautiful Art
$$\int_0^1\frac{x^t−1}{\ln(x)}~dx=\ln(t+1)$$
This one was nice
I know I saw mention of creating one for $_1F_4(\cdot)$
but nothing came of it
I see lots of cool identities proved on this site
that aren't stored elsewhere
That integral comes by differentiating with respect to $t$
Whereupon it becomes trivial
lol, I would imagine so based on the RHS
anytime your answer is a logarithm
DUIS is a good idea
Differentiation Under Integral Sign
and since RHS is purely in terms of $t$
it's the way to go
But I don't think I would've seen it without DUIS
now, coming up with these proofs without the answer is true gold
01:47
Hehe, yeah
0
A: Indefinite integral : $\int \frac{dx}{(1+x^{2})^{\frac{3}{2}}}$

Simply Beautiful ArtWe have $t>0$. Recall that $$\int\frac1{(t^2-x^2)^{1/2}}\ dx=\arcsin\left(\frac xt\right)+c_1$$ Take the derivative of both sides with respect to $t$ to get $$\int\frac{-t}{(t^2-x^2)^{3/2}}\ dx=\frac{-x}{|t|\sqrt{t^2-x^2}}+c_2$$ Setting $t=1$ and simplifying, we reach $$\int\frac1{(1-x^2)^{...

I came up with this XD
Nothing tooo fancy
Not bad
Not bad at all
I propose a Big-List of cool DUIS proofs
I want to see that
Lemme see about that then...
non-trivial ones preferably
so the RHS of every equation can't be a logarithm or the LHS can't just be a quick way to avoid Integration by Parts
Though I suppose some trig integrals have classy proofs using Euler's Identity
and those could be solved by IBP
so there would need to be rules for what goes into the list
You know
I've never really used IBP
really???
You've used EMSF
but not IBP?
0_0
Euler McLaurin Series Formula
or EMF if you prefer
Ah
I've used IBP
But like, never in a necessary way
It makes proving $\int \log(x) dx$ a breeze if you don't know the rules for integrating inverses
I've used it quite a bit actually
Nah
Don't use that
Want to see me do without?
I can do it without
but the key is, I don't have to
IBP makes life simpler
in some cases
How many ways can you do it without ;)
idk
that's a good question
depends on how many integral transformations I can recall off the top of my head XD
sub $x = e^u$
Inverse integration rules
Any series expansions valid everywhere
I could come up with a few ways at least
Series expansion can't count unless you prove the series without the integral
:P
Most series expansions can be proved pretty easily
for elementary functions
Well yeah of course
anyways, go on?
Heck. An attack with EMSF might work
I haven't seen it
but it might work
01:59
Some horrible series expansions you gonna get
You end up with the log gamma function
I could combine multiple complex logarithms and calculate integral of arctangent
then split
Ooops XD I have a meeting 15 minutes away that just started 0_0
got too caught up in integrals
sigh
off to life
02:00
Oh, sorry XD
Bye man!
0
Q: Interesting examples of differentiation under the integral sign?

Simply Beautiful ArtI was recently looking through integration techniques when I came upon differentiation under the integral sign (DUIS). It seems to be pretty powerful, for example: $$f(t)=\int_0^1\frac{x^t-1}{\ln(x)}\ dx\implies f'(t)=\int_0^1x^t\ dx=\frac1{t+1}\\\implies f(t)=C+\ln(t+1)\\f(0)=0\implies C=0\\\i...

 
3 hours later…
05:32
@SimplyBeautifulArt Haha, I see you adopted my notation of DUIS. It's so much shorter.
@SimplyBeautifulArt I'll add a few examples to that list tomorrow
 
1 hour later…
06:44
@BrevanEllefsen Can you give a clever example of using EMSF to solve an integral
 
5 hours later…
11:56
@dydxx Faulhaber's formula
 
1 hour later…
13:24
@S.C.B. I'd rather talk here btw
0
Q: How to take $\int \frac{x^4-2x^2+2}{x^3-2x^2-x+2}dx$?

M.MassThe integral itself is: $$\int \frac{x^4-2x^2+2}{x^3-2x^2-x+2}dx$$ After long division I got: $$\int \Big(x+2+\frac{3x^2-2}{x^3-2x^2-x+2}\Big)dx$$ And after simplifying the denumerator I got: $$x^3-2x^2-x+2 = x^2(x-2)-1(x-2) = (x^2-1)(x-2)$$ But I am not sure about $A$ and $B$ values Shoul...

@S.C.B. I think we can prove Frullani with DUIS
And why was my answer accepted?
@SimplyBeautifulArt Hello.
Let u speak here.
@SimplyBeautifulArt Yeah, I only learned Frullani today.
XD
Wow, my integral knowledge is apparently advanced?
What do you mean?
@SimplyBeautifulArt
I've known Frullani for a while
We don't learn it at school.
I learn integration randomly and spontaneously
On the net.
And integration isn't my main interest.
13:33
Same...?
True
Evaluate $$ \large \lim_{n\rightarrow \infty} \, \sqrt[ \large n^{2}+n]{\binom{n}{0}\binom{n}{1}\binom{n}{2}\cdots \binom{n}{n}} $$
Hm, tricky limit it looks like...
lol, it's a bit difficult, and I posted a solution on another site.
I never really expected that solution to be popular...
Lol
1
A: Find a reduction formula formula for this definite integral

Simply Beautiful ArtInteresting problem! Let's start with the following integral: $$f(t)=\int_0^1e^{(2x-1)t}e^{x-x^2}\ dx=\frac12\sqrt\pi e^{t^2+\frac14}\left[\operatorname{erf}\left(\frac12-t\right)+\operatorname{erf}\left(\frac12+t\right)\right]$$ It follows obviously that $$I_0=f(0)$$ $$I_1=f'(0)$$ etc. ...

DUIS strikes again?
What are you, the next Feynman (with only the integration knowledge)?
13:40
I suppose
DUIS is fun to do
You should try it out sometime
Have any idea on the limit?
Never guessed this question would be popular.
4
A: How is " point " in geometry undefined?

S.C.B.Note that it says on Wikipedia that [...] in Euclidean geometry, a point is a primitive notion upon which the geometry is built. Being a primitive notion means that a point cannot be defined in terms of previously defined objects. That is, a point is defined only by some properties, called a...

lol
The value of spontaniety.
Oh, I butchered that spelling.
Shoot.
This site just makes me angry
Hm, not my area where I can understand
Lol
That's why I don't listen to politics
Well, see ya later
14:24
@SimplyBeautifulArt But the limit?
15:10
@S.C.B. I'm very familiar with Frullani, but I've never used it XD
@SimplyBeautifulArt you know that integral the other day from your Big-List of Faulhaber's formula solutions?
@SimplyBeautifulArt that I said could be used to solve Basel Problem?
@SimplyBeautifulArt it just occurred to me that it looks almost exactly like something that Frullani could handle with a bit of rearranging. Hmm... I'll take a look at it later. Might be a fun new proof to add to the bag.
I don't think @SimplyBeautifulArt's hear right now ㅠㅠ. @BrevanEllefsen
15:32
@S.C.B. @BrevanEllefsen hi
Hi.
@SimplyBeautifulArt Do you know what the limit is?
Have you tried anything yet?
No, busy @S.C.B.
What are you doing?
@SimplyBeautifulArt
 
2 hours later…
17:37
School @S.C.B.
 
2 hours later…
19:08
Lol anyone here?
19:46
@dydxx sorry, was reading chat drama
Which chat?
20:20
also I was wondering is the gaussian integral solvable
through DUTIS
@dydxx hm
@dydxx I don't see anything obvious
@s
@SimplyBeautifulArt ah ok
@SimplyBeautifulArt what up?
You can derive the gamma function from DUIS
How would you do that
$$\int_0^\infty e^{-xt}\ dx$$
You can go from there
20:32
Ah okay
I see
Mhm
@dydxx you got any interesting integrals?
@SimplyBeautifulArt You're the expert here ;)
@SimplyBeautifulArt In all honesty, I'm very beginner level.
@SimplyBeautifulArt Yes really.
@SimplyBeautifulArt That's why I enjoy looking through the forums and these chats to learn something!
Hm...how do you feel about your integrals after this chat room?
20:43
@SimplyBeautifulArt Differentiation under the integral sign is so cool! However contour integration is still confusing
That's ok. It's not at all simple
DUIS is definitely more understandable XD
@dydxx I have a problem:
$$\int_0^\infty x^2\ln(x)e^{-x^2}\ dx$$
There we go
Yo I'm gonna be out for a few hours so will attempt to solve it when I come back
Reading off the latex what would we add as our parameter t?
I find the hard part for DUIS is trying to find where to put the parameter
@dydxx actually, you should find where the $t$ came from to begin with...
4 mins ago, by dydxx
I find the hard part for DUIS is trying to find where to put the parameter
That is always the problem.
 
2 hours later…
22:49
room topic changed to Simply Beautiful Art's realm of integrals: [integration]
@amWhy I suppose it is more private here
and those that come here will likely not care much
23:09
@SimplyBeautifulArt Indeed. Seriously, I've never encountered what happened earlier, ever. And it could have happened anywhere, I'm guessing, when users feel "off the radar", so I'm absolutely sure it's not a matter of your chat room as it is a matter of the specific user in question. I really do commend @S.C.B, who quickly disengaged with the user in question.
O.O
I didn't even know @S.C.B. was online!
certainly not his usual hours of business
Well, a total of 101 comments were sent to trash....mostly due to one user (not @S.C.B.)
23:28
Already brought that to the mod's attention, who did nothing, hence my Trumpoon Baffoon username, and such, to prove a point.

« first day (19 days earlier)      last day (28 days later) »