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MathSofiya
Group Title
Use power series to solve the differential equation.
y'=xy
 2 years ago
 2 years ago
MathSofiya Group Title
Use power series to solve the differential equation. y'=xy
 2 years ago
 2 years ago

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MathSofiya Group TitleBest ResponseYou've already chosen the best response.1
I guess the first step is this \[y'xy=0\]
 2 years ago

experimentX Group TitleBest ResponseYou've already chosen the best response.1
hold on ... just let \[ y = \sum_{n=0}^\infty x^n \]
 2 years ago

experimentX Group TitleBest ResponseYou've already chosen the best response.1
\[ y = \sum_{n=0}^\infty a_n x^n \]
 2 years ago

MathSofiya Group TitleBest ResponseYou've already chosen the best response.1
\[y'=\sum_{n=1}^{\infty} nc_nx^{n1}\] according to my book, same thing I guess
 2 years ago

experimentX Group TitleBest ResponseYou've already chosen the best response.1
yes yes ... \[ \sum_{n=0}^\infty n \; c_n x^{n1} + \sum_{n=0}c_n x^{n+1} = 0\] find the recurrence relation.
 2 years ago

MathSofiya Group TitleBest ResponseYou've already chosen the best response.1
oh ok \[y''=\sum_{n=2}^{\infty} n(n1)c_nx^{n2}\] how do I apply these generalizations to this specific problem
 2 years ago

experimentX Group TitleBest ResponseYou've already chosen the best response.1
hah?? why do you want more trouble? you don't need to find y'' for this.
 2 years ago

MathSofiya Group TitleBest ResponseYou've already chosen the best response.1
really, I was just trying to follow this example in the book
 2 years ago

MathSofiya Group TitleBest ResponseYou've already chosen the best response.1
I meant: really?
 2 years ago

experimentX Group TitleBest ResponseYou've already chosen the best response.1
lol .. this example is of second order differential equation. you need it only for second order DE
 2 years ago

MathSofiya Group TitleBest ResponseYou've already chosen the best response.1
because there is only a y' that makes it first order?
 2 years ago

MathSofiya Group TitleBest ResponseYou've already chosen the best response.1
ok so I only consider this then, right? \[y'=\sum_{n=1}^{\infty} nc_nx^{n1} \]
 2 years ago

experimentX Group TitleBest ResponseYou've already chosen the best response.1
yes ... do you have a software called maple?
 2 years ago

MathSofiya Group TitleBest ResponseYou've already chosen the best response.1
but before solving for it, what do I sub?
 2 years ago

MathSofiya Group TitleBest ResponseYou've already chosen the best response.1
or how do I know what do sub, I think I'm just afraid of sums :S
 2 years ago

MathSofiya Group TitleBest ResponseYou've already chosen the best response.1
why do they have (n+2)(n+1) in the example?
 2 years ago

experimentX Group TitleBest ResponseYou've already chosen the best response.1
from maple i got y(x) = y(0)+(1/2)*y(0)*x^2+(1/8)*y(0)*x^4+O(x^6)
 2 years ago

experimentX Group TitleBest ResponseYou've already chosen the best response.1
Honesty I don't like power series method. here's how you do it. assume the solution has of the form \[ y = \sum_{n=0}^\infty a_n x^n \] so we are putting these vales in your DE and hunting a_n's
 2 years ago

MathSofiya Group TitleBest ResponseYou've already chosen the best response.1
I'm back...
 2 years ago

MathSofiya Group TitleBest ResponseYou've already chosen the best response.1
gtg again...
 2 years ago

experimentX Group TitleBest ResponseYou've already chosen the best response.1
after putting the value of y in your DE, you get \[ \sum_{n=0}^\infty n \; c_n x^{n1} + \sum_{n=0}c_n x^{n+1} = 0 \\ \] since on the left side you don't have negative power of x, put this \( \sum_{n=0}^\infty n \; c_n x^{n1} \) = \( \sum_{n=0}^\infty (n+1) \; c_{n+1} x^{n} \) Also we can make \( \sum_{n=0}^\infty \; c_n x^{n+1} \) = \(\sum_{n=0}^\infty c_{n1} x^{n} \) now we have \[ \huge \sum_{n=0}^\infty (n+1) \; c_{n+1} x^{n} + \sum_{n=0}^\infty c_{n1} x^{n} = 0 \\ \huge \sum_{n=0}^\infty (n+1) \; c_{n+1} x^{n} + c_{n1} x^{n} = 0 \\ \huge \sum_{n=0}^\infty ((n+1) \; c_{n+1} + c_{n1} ) x^{n} = 0 \]
 2 years ago

experimentX Group TitleBest ResponseYou've already chosen the best response.1
since you don't have power's of x on the right side, you have \[ (n+1) c_{n+1} + c_{n1} = 0 \\ \text{ or, } c_{n+1} = { c_{n1} \over n+1}\] this is the recurrence relation you get put n=1, you get dw:1348083846860:dw this way you find the coefficients. hence you have solution.
 2 years ago

experimentX Group TitleBest ResponseYou've already chosen the best response.1
not a nice method ... purely brute. but this works for all non linear differential equations ... even if their solution is not closed, you (put it up) as special function like Airy function, Bessel function, Hermite ... etc. and C_0 is your constant of integration. also you might wanna check Frobenius method. Best of luck with these.
 2 years ago

TuringTest Group TitleBest ResponseYou've already chosen the best response.2
I think you made a slight mistake @experimentX though I'm no fan of power series myself
 2 years ago

experimentX Group TitleBest ResponseYou've already chosen the best response.1
hmm ... could you be more specific? power series is the method that is most prone to error ... I never get it correctly on first attempt (for second order)
 2 years ago

experimentX Group TitleBest ResponseYou've already chosen the best response.1
oh ... i see that. i put up + instead of 
 2 years ago

TuringTest Group TitleBest ResponseYou've already chosen the best response.2
I'm just going to go ahead and give mine a shot, it's hard for me tow see how you ended up with c_{n+1} and c_{n1} though
 2 years ago

TuringTest Group TitleBest ResponseYou've already chosen the best response.2
yeah that
 2 years ago

experimentX Group TitleBest ResponseYou've already chosen the best response.1
EDIT:: \[ \huge \sum_{n=0}^\infty (n+1) \; c_{n+1} x^{n}  \sum_{n=0}^\infty c_{n1} x^{n} = 0 \\ \huge \sum_{n=0}^\infty (n+1) \; c_{n+1} x^{n}  c_{n1} x^{n} = 0 \\ \huge \sum_{n=0}^\infty ((n+1) \; c_{n+1}  c_{n1} ) x^{n} = 0 \]
 2 years ago

experimentX Group TitleBest ResponseYou've already chosen the best response.1
EDIT:: \[ (n+1) c_{n+1}  c_{n1} = 0 \\ \text{ or, } c_{n+1} = {c_{n1} \over n+1} \]
 2 years ago

experimentX Group TitleBest ResponseYou've already chosen the best response.1
so the final solution is \[ y = c_0\sum_{n=0}^\infty {1 \over 2^n n!}x^{2n}\]
 2 years ago

TuringTest Group TitleBest ResponseYou've already chosen the best response.2
okay that was nice, let me see if doing it my way changes anything
 2 years ago

experimentX Group TitleBest ResponseYou've already chosen the best response.1
this is same as \[ \huge y = c_0 e^{x^2 \over 2}\]
 2 years ago

experimentX Group TitleBest ResponseYou've already chosen the best response.1
2 am here. gotta sleep ... i'll be watching this thread tomorrow morning :) gotta be careful with power series. this is quite annoying.
 2 years ago

TuringTest Group TitleBest ResponseYou've already chosen the best response.2
yes your solution seems right @experimentX sleep well, I still am gonna try to brush up myself :)
 2 years ago

TuringTest Group TitleBest ResponseYou've already chosen the best response.2
@MathSofiya I have confirmed @experimentX 's answer in a slightly different way if you would like to go over it, let me know
 2 years ago

TuringTest Group TitleBest ResponseYou've already chosen the best response.2
\[y'=yx\implies y'xy=0\]\[y=\sum_{n=0}^\infty a_nx^n\implies xy=\sum_{n=0}^\infty a_nx^{n+1}\]\[y'=\sum_{n=1}^\infty a_nnx^{n1}\](notice the index starts at n=1 for the series for y' because the first term in the series for y is a constant \(a_n\) and disappears upon taking the derivative) Our problem then becomes\[\sum_{n=1}^\infty a_nnx^{n1}\sum_{n=0}^\infty a_nx^{n+1}=0\]The first thing is to get all the x's to the \(n^{th}\) power with index shifts, so we will start the first series one lower at n=1 (so we add 1 to n in the summand) and the second series we will shift up by 1 (so that n shifts down by 1 in the summand). Note these shifts also change the indices of the constants \(a_n\)\[\sum_{n=0}^\infty( n+1)a_{n+1}x^n\sum_{n=1}^\infty a_{n1}x^n=0\]Next we need to get the indices the same again (make n start at the same number for each series) To avoid doing another index shift that will take us back where we started, we will "strip" the n=0 term out of the first series\[\sum_{n=0}^\infty(n+1)a_{n+1}x^n=a_1+\sum_{n=1}^\infty(n+1)a_{n+1}x^n\]notice all we did was plug in n=1 and explicitly write that term out in front of the rest of the series. They call that "stripping out a term" in case you did not know. So we now have\[a_1+\sum_{n=1}^\infty(n+1)a_{n+1}x^n\sum_{n=1}^\infty a_{n1}x^n=0\]\[a_1+\sum_{n=1}^\infty[(n+1)a_{n+1}a_{n1}]x^n=0\]Here we now need to figure out a formula for any pattern(s) in the constants \(a_n\) First, plugging in \(n=0\) since the series is not defined we get\[n=0:a_1=0\] so now we just have\[\sum_{n=1}^\infty[(n+1)a_{n+1}a_{n1}]x^n=0\]and since exponentials like \(x^n\) can never be zero, and the summation sign will not change the fact that what is within the brackets must be zero we have the "recurrence relation"\[(n+1)a_{n+1}a_{n1}=0\implies a_{n+1}={a_{n1}\over n+1}\]so again we start plugging in values for n starting from n=1\[n=1:a_2=\frac{a_0}2\]\[n=2:a_3=\frac{a_1}3=\frac03=0\]\[n=3:a_4=\frac{a_2}4=\frac{a_0}{2\cdot4}=\frac{a_0}{2^2(1\cdot2)}=\frac{a_0}{2^2(2!)}\]\[n=4:a_5=\frac{a_3}5=\frac05=0\]\[n=5:a_6=\frac{a_4}6=\frac{a_0}{2\cdot4\cdot6}=\frac{a_0}{2^3(3!)}\]\[~~~~~~~\vdots\]so what's the pattern for the constants? There seem to be two.
 2 years ago

MathSofiya Group TitleBest ResponseYou've already chosen the best response.1
I'm back
 2 years ago

TuringTest Group TitleBest ResponseYou've already chosen the best response.2
at a very opportune time too :)
 2 years ago

MathSofiya Group TitleBest ResponseYou've already chosen the best response.1
Sorry, women and hairstylist... I'm reading everything now
 2 years ago

TuringTest Group TitleBest ResponseYou've already chosen the best response.2
Take your time, it's not done... not sure if I should wait for you to try the next step. I will prepare the answer in the meantime anyway :)
 2 years ago

MathSofiya Group TitleBest ResponseYou've already chosen the best response.1
sounds good, It's gonna take me a while to understand it. I'm gonna type questions as I come upon them.
 2 years ago

MathSofiya Group TitleBest ResponseYou've already chosen the best response.1
\[y=\sum_{n=0}^\infty a_nx^n\implies xy=\sum_{n=0}^\infty a_nx^{n+1}\] because \[\sum_{n=0}^\infty a_nx^n x \] since the exponents are added that's why it's \[x^{n+1}\] correct?
 2 years ago

MathSofiya Group TitleBest ResponseYou've already chosen the best response.1
I understand what you did by changing, we start one lower at n=1, \[na_nx^{n1} \implies (n+1)a_nx^n\] but why does that change where the sum starts from n=1 instead now from n=0 \[\sum_{n=1}^{\infty} \implies \sum_{n=0}^{\infty}\] sorry I guess I'm still struggling with the basics of summations
 2 years ago

TuringTest Group TitleBest ResponseYou've already chosen the best response.2
try it both ways, write out the first two or three terms and convince yourself how starting earlier means you need to add to the argument
 2 years ago

TuringTest Group TitleBest ResponseYou've already chosen the best response.2
for\[\sum_{n=1}^\infty na_nx^{n1}\]what is the first term?
 2 years ago

TuringTest Group TitleBest ResponseYou've already chosen the best response.2
not quite, it's \(a_n\)
 2 years ago

TuringTest Group TitleBest ResponseYou've already chosen the best response.2
what about the first term of\[\sum_{n=0}^\infty(n+1)a_nx^n\]?
 2 years ago

TuringTest Group TitleBest ResponseYou've already chosen the best response.2
see how by starting our index at n=0 we had to add 1 everywhere to keep the terms the same try it for more terms until you get the feel if you still doubt
 2 years ago

MathSofiya Group TitleBest ResponseYou've already chosen the best response.1
does this mean that we're trying to manipulate the way we write the sum without changing the value of it?
 2 years ago

TuringTest Group TitleBest ResponseYou've already chosen the best response.2
yes if we started from zero the first term would be zero, because it has n as a coefficient the next term would have a coefficient of 1, the one after of 2 etc. if we add 1 then we get the correct coefficients the whole way through because we made up for changing the starting point of the count
 2 years ago

MathSofiya Group TitleBest ResponseYou've already chosen the best response.1
ok so we change the starting point and made up for it. Why are we changing the starting points, what is our eventual goal....ahhh, we want to be able to write the two sums as one sum?
 2 years ago

TuringTest Group TitleBest ResponseYou've already chosen the best response.2
exactly :)
 2 years ago

TuringTest Group TitleBest ResponseYou've already chosen the best response.2
we can only combine the sums once we have their indices at the same starting point and as you will see, we also need the powers on x to be the same, customarily x^n....
 2 years ago

TuringTest Group TitleBest ResponseYou've already chosen the best response.2
these are not short problems :P
 2 years ago

MathSofiya Group TitleBest ResponseYou've already chosen the best response.1
indices...
 2 years ago

MathSofiya Group TitleBest ResponseYou've already chosen the best response.1
{n=..... } are those the indices? I'm kinda rusty on the terminology
 2 years ago

TuringTest Group TitleBest ResponseYou've already chosen the best response.2
yes exactly, n is the index of a sum with n=a under it, and "a" would be the starting point of the index changing the starting point of an index is called an index shift, and requires you to alter the summand (the thing under the summation sign) as I have just tried to describe.
 2 years ago

TuringTest Group TitleBest ResponseYou've already chosen the best response.2
that should say "we explicitly plugged in x=0..." *
 2 years ago

TuringTest Group TitleBest ResponseYou've already chosen the best response.2
I mean n=0 :/
 2 years ago

MathSofiya Group TitleBest ResponseYou've already chosen the best response.1
by taking an \[a_1\] out of the summation how did that change the indices? nothing else seems to have been changed. \[\sum_{n=0}^\infty(n+1)a_{n+1}x^n=a_1+\sum_{n=1}^\infty(n+1)a_{n+1}x^n\]
 2 years ago

TuringTest Group TitleBest ResponseYou've already chosen the best response.2
because we wrote out the first term explitily
 2 years ago

TuringTest Group TitleBest ResponseYou've already chosen the best response.2
explicitly*
 2 years ago

MathSofiya Group TitleBest ResponseYou've already chosen the best response.1
ohh...so either way the first answer would be a_1 is the point again I guess
 2 years ago

TuringTest Group TitleBest ResponseYou've already chosen the best response.2
we don't need to start from n=0 anymore because we already wrote it out and took it out of the series, so repeating the n=0 term would add an extra term
 2 years ago

MathSofiya Group TitleBest ResponseYou've already chosen the best response.1
on either side of that equal sign I mean.
 2 years ago

TuringTest Group TitleBest ResponseYou've already chosen the best response.2
what is important right now is that we get the index to start at n=1 like the other term does
 2 years ago

TuringTest Group TitleBest ResponseYou've already chosen the best response.2
if you have a series that starts at n=0, and you want to write it starting at n=1 you can either do an index shift (which we already did to get x^n on both series) or write out the n=0 term explicitly.
 2 years ago

MathSofiya Group TitleBest ResponseYou've already chosen the best response.1
got it. I am down to this line now \[a_1+\sum_{n=1}^\infty[(n+1)a_{n+1}a_{n1}]x^n=0\] and I do understand how you came to this
 2 years ago

MathSofiya Group TitleBest ResponseYou've already chosen the best response.1
I see a pattern something like this \[\frac{a_0}{2^n(n!)}\]
 2 years ago

MathSofiya Group TitleBest ResponseYou've already chosen the best response.1
well I probably shouldn't use "n"
 2 years ago

TuringTest Group TitleBest ResponseYou've already chosen the best response.2
yes! that is exactly right for half the n
 2 years ago

MathSofiya Group TitleBest ResponseYou've already chosen the best response.1
oh so I can use "n"
 2 years ago

TuringTest Group TitleBest ResponseYou've already chosen the best response.2
well no, we need to differentiate between the two cases for n n only equals what you wrote under what condition for n ?
 2 years ago

TuringTest Group TitleBest ResponseYou've already chosen the best response.2
a_n only equals what you wrote for which n ?
 2 years ago

MathSofiya Group TitleBest ResponseYou've already chosen the best response.1
when n=0
 2 years ago

MathSofiya Group TitleBest ResponseYou've already chosen the best response.1
no no no
 2 years ago

TuringTest Group TitleBest ResponseYou've already chosen the best response.2
actually the n in your formula is outright wrong sorry but look at it another way so you can figure how to fix it... for which values of n is a_n zero?
 2 years ago

MathSofiya Group TitleBest ResponseYou've already chosen the best response.1
durrr...all the even numbers
 2 years ago

MathSofiya Group TitleBest ResponseYou've already chosen the best response.1
haha it took me while to see that :P
 2 years ago

TuringTest Group TitleBest ResponseYou've already chosen the best response.2
yes, but I screwed up the question again, sorry I must phrase more carefully
 2 years ago

TuringTest Group TitleBest ResponseYou've already chosen the best response.2
for which *subscripts on a* is a_n zero ?
 2 years ago

MathSofiya Group TitleBest ResponseYou've already chosen the best response.1
all the odd subscripts
 2 years ago

TuringTest Group TitleBest ResponseYou've already chosen the best response.2
right :) so and we can write any odd number as n=2k+1 with k=0,1,2..., right? so any term with a coefficient we can write as \(a_n=a_{2k+1}=0\) therefore our formula is only concerned with the even number subscripts since the odd ones will drop out
 2 years ago

TuringTest Group TitleBest ResponseYou've already chosen the best response.2
so we only need to think about the terms with n=2k by the way we skipped a set; plugging in n=0 to find what a_1 is (we can't do it with our formula because our series does not have a_1 defined) to back up just for a moment, what does a_1=
 2 years ago

TuringTest Group TitleBest ResponseYou've already chosen the best response.2
yes, how did you know?
 2 years ago

MathSofiya Group TitleBest ResponseYou've already chosen the best response.1
part of the pattern
 2 years ago

TuringTest Group TitleBest ResponseYou've already chosen the best response.2
that does not illustrate the proper reasoning, and I think you know that ;)
 2 years ago

TuringTest Group TitleBest ResponseYou've already chosen the best response.2
that reasoning will certainly not always work and will in fact often fail. the real reason is as follows:\[a_1+\sum_{n=1}^\infty[(n+1)a_{n+1}a_{n1}]x^n=0\]plug in n=0 and notice something: the series is not defined at n=0, only from n=1 on, so we just wind up with the whole series not in the equation\[a_1=0\]
 2 years ago

TuringTest Group TitleBest ResponseYou've already chosen the best response.2
hence the problem we are now dealing with is really just\[\sum_{n=1}^\infty[(n+1)a_{n+1}a_{n1}]x^n=0\]
 2 years ago

MathSofiya Group TitleBest ResponseYou've already chosen the best response.1
interesting, yes
 2 years ago

TuringTest Group TitleBest ResponseYou've already chosen the best response.2
so where were we....?
 2 years ago

MathSofiya Group TitleBest ResponseYou've already chosen the best response.1
one more thing
 2 years ago

MathSofiya Group TitleBest ResponseYou've already chosen the best response.1
so you're saying that I could drop the \[a_1\] because it is not defined at n=0 yes...ok
 2 years ago

TuringTest Group TitleBest ResponseYou've already chosen the best response.2
a_1 is defined at n=0 (a_1=0), but the series is not
 2 years ago

MathSofiya Group TitleBest ResponseYou've already chosen the best response.1
oh ok the series is not defined at n=0 and this is left \[\sum_{n=1}^\infty[(n+1)a_{n+1}a_{n1}]x^n=0\]
 2 years ago

MathSofiya Group TitleBest ResponseYou've already chosen the best response.1
my goodness this is exhausting!
 2 years ago

TuringTest Group TitleBest ResponseYou've already chosen the best response.2
\[a_1+\cancel{\sum_{n=1}^\infty[(n+1)a_{n+1}a_{n1}]x^n}^{\text{not defined}}=0\]\[a_1=0\]leaving\[\sum_{n=1}^\infty[(n+1)a_{n+1}a_{n1}]x^n=0\]and since \(x^n\neq0\) then the coefficients must be zero\[(n+1)a_{n+1}a_{n1}=0\]and we get the recurrence relation we were doing
 2 years ago

TuringTest Group TitleBest ResponseYou've already chosen the best response.2
and yes, it is extremely exhausting I think most people who do DE's whould agree, but they handle nonconstnat coefficient problems well
 2 years ago

MathSofiya Group TitleBest ResponseYou've already chosen the best response.1
so would this finally be the solution \[\sum_{n=1}^\infty[(n+1)a_{n+1}a_{n1}]x^n=0\] I doubt it...seems like it's just the beginning...now it's on to solving the DE
 2 years ago

TuringTest Group TitleBest ResponseYou've already chosen the best response.2
I'm afraid we have ways to go dear, I would understand if you want a break or something... I am mainly practicing, and I have a final solution from beginning to end prepared on a word doc, but it would be better to guide you of course.
 2 years ago

TuringTest Group TitleBest ResponseYou've already chosen the best response.2
that solution above is not an explicit solution for\[ y=\sum_{n=0}^\infty a_nx^n\]which is what we want for an answer
 2 years ago

MathSofiya Group TitleBest ResponseYou've already chosen the best response.1
Wow an example they did in my book took up 2 pages. yes. I don't think I need a break yet...but when we're done with this problem I'm watching an episode of the Big Bang Theory and just laugh at all of their stupid jokes. continue.....
 2 years ago

TuringTest Group TitleBest ResponseYou've already chosen the best response.2
we need a formula for a_n in terms of the index, that's how we get our answer we are mostly there actually, I just want to make sure you understand each step
 2 years ago

MathSofiya Group TitleBest ResponseYou've already chosen the best response.1
"index"?
 2 years ago

TuringTest Group TitleBest ResponseYou've already chosen the best response.2
you will see what I mean: that the index n (which don't get attached to cuz we're gonna change the name of it in a second) is going to give an explicit formula for \(a_n\) in terms of the only constant we can't determine \(a_0\) so you need a formula for all even \(a_n\) in terms of n
 2 years ago

TuringTest Group TitleBest ResponseYou've already chosen the best response.2
you almost had it, but you wrote\[a_n=\frac{a_0}{2^nn!}\]but check the numbers and you can see that's not quite right
 2 years ago

MathSofiya Group TitleBest ResponseYou've already chosen the best response.1
what I screwed up on earlier by calling it \[\frac{a_0}{2^n(n!)}\]
 2 years ago

MathSofiya Group TitleBest ResponseYou've already chosen the best response.1
ok I'll fix it...let's see here...
 2 years ago

TuringTest Group TitleBest ResponseYou've already chosen the best response.2
hint: We are only talking about even numbers, the odd subscripts are all zero. All even numbers can be written as...?
 2 years ago

MathSofiya Group TitleBest ResponseYou've already chosen the best response.1
\[a_n= \frac{a_0}{2^{2k}(2nk!)}\] If this is right.....
 2 years ago

MathSofiya Group TitleBest ResponseYou've already chosen the best response.1
woops hold your horses
 2 years ago

MathSofiya Group TitleBest ResponseYou've already chosen the best response.1
\[a_n= \frac{a_0}{2^{2k}(2k!)}\]
 2 years ago

TuringTest Group TitleBest ResponseYou've already chosen the best response.2
but 2k=n.... the power on 2 is not the same as the subscript on a, check again the relation
 2 years ago

MathSofiya Group TitleBest ResponseYou've already chosen the best response.1
\[a_{2k}=\frac{a_0}{2^{2k}(2k!)}\] That just looks odd
 2 years ago

TuringTest Group TitleBest ResponseYou've already chosen the best response.2
plug in k=2 and check if that's true based on the table I made way back up there
 2 years ago

MathSofiya Group TitleBest ResponseYou've already chosen the best response.1
that gives me a_1 instead of a_0
 2 years ago

MathSofiya Group TitleBest ResponseYou've already chosen the best response.1
am I loosing it?
 2 years ago

TuringTest Group TitleBest ResponseYou've already chosen the best response.2
? k=2 n=2k=4 check a_4
 2 years ago

MathSofiya Group TitleBest ResponseYou've already chosen the best response.1
oh I see what you mean
 2 years ago

MathSofiya Group TitleBest ResponseYou've already chosen the best response.1
\[n=3:a_4=\frac{a_2}4=\frac{a_0}{2\cdot4}=\frac{a_0}{2^2(1\cdot2)}=\frac{a_0}{2^2(2!)}\] uhm? what do you want me to check? the fact that it is not zero?
 2 years ago

MathSofiya Group TitleBest ResponseYou've already chosen the best response.1
which was our goal
 2 years ago

TuringTest Group TitleBest ResponseYou've already chosen the best response.2
no, I want you to check if that fits your formula\[a_{2k}=\frac{a_0}{2^{2k}(2k)!}\]or if not, how to fix it :)
 2 years ago

MathSofiya Group TitleBest ResponseYou've already chosen the best response.1
divide it by half because it seems like double the answer so \[a_{2k}=\frac{a_0}{2^{k}(k)!}\]
 2 years ago

MathSofiya Group TitleBest ResponseYou've already chosen the best response.1
now is that my y_p or something?
 2 years ago

TuringTest Group TitleBest ResponseYou've already chosen the best response.2
and since these are the only a_n we care about we can now write\[a_n=a_{2k}=\frac{a_0}{2^kk!}\]and no, it's much better than our y_p...
 2 years ago

MathSofiya Group TitleBest ResponseYou've already chosen the best response.1
it's my "y" final answer
 2 years ago

TuringTest Group TitleBest ResponseYou've already chosen the best response.2
basically, yes it is the key to the final answer
 2 years ago

TuringTest Group TitleBest ResponseYou've already chosen the best response.2
first notice that the above is true for \(k=0,1,2,...\) and we already made the tacit assumtion that our answer is\[y=\sum_{n=0}^\infty a_nx^n\]now since all odd will give zero terms for \(a_n\), there will be only even n in our formula so we can change all n in the series to 2k, start our series from k=0, and substitute the above formula for \(a_n\) and that will be our final answer
 2 years ago

MathSofiya Group TitleBest ResponseYou've already chosen the best response.1
\[y=\sum_{k=0}^\infty \frac{a_0}{2^kk!}x^{2k}\]
 2 years ago

TuringTest Group TitleBest ResponseYou've already chosen the best response.2
you got it :)
 2 years ago

TuringTest Group TitleBest ResponseYou've already chosen the best response.2
and just to make things crystal clear that we are in fact right...
 2 years ago

TuringTest Group TitleBest ResponseYou've already chosen the best response.2
separation of variables gives y'=yx dy/y=xdx ln y=x^2/2+C y=Ce^(x^2/2) the Tayore xpansion for e^y is\[e^y=1+\frac y{1!}+\frac{y^2}{2!}+...=\sum_{n=0}^\infty\frac{y^n}{n!}\]let \[y=\frac12x^2\]and plug that into the taylor series and you get the same answer
 2 years ago

TuringTest Group TitleBest ResponseYou've already chosen the best response.2
with \(C=a_0\) of course
 2 years ago

TuringTest Group TitleBest ResponseYou've already chosen the best response.2
experimentX was right too, but I thought I would try to practice through explanation myself :)
 2 years ago

MathSofiya Group TitleBest ResponseYou've already chosen the best response.1
Amazing! I owe you half of my paychecks when I begin working as civil engineer. Dude, you are awesome. Thanks for helping me.
 2 years ago

MathSofiya Group TitleBest ResponseYou've already chosen the best response.1
patience is a virtue...you have an abundance of that...oh yeah, sorry I forgot you don't like compliments :P
 2 years ago

TuringTest Group TitleBest ResponseYou've already chosen the best response.2
Haha, don't tempt me I could use them. And thanks as always :) For posterity's sake I wanna post my full explanation without interruption, just because I already had it prepared and it may be easier to read should you decide to review this. It will slow down the page though...
 2 years ago

MathSofiya Group TitleBest ResponseYou've already chosen the best response.1
Thank you. I'm off to watch an episode of the Big Bang Theory. Is that silly? LOL
 2 years ago

TuringTest Group TitleBest ResponseYou've already chosen the best response.2
Makes more sense than doing more series solution DE's to me! I'm off to do something unproductive too, see ya!
 2 years ago

TuringTest Group TitleBest ResponseYou've already chosen the best response.2
gotta take it away from experimentX... which I'll do, he has enough sorry @experimentX ;)
 2 years ago

TuringTest Group TitleBest ResponseYou've already chosen the best response.2
\[y'=yx\implies y'xy=0\]\[y=\sum_{n=0}^\infty a_nx^n\implies xy=\sum_{n=0}^\infty a_nx^{n+1}\]\[y'=\sum_{n=1}^\infty a_nnx^{n1}\](notice the index starts at n=1 for the series for y' because the first term in the series for y is a constant \(a_n\) and disappears upon taking the derivative) Our problem then becomes\[\sum_{n=1}^\infty a_nnx^{n1}\sum_{n=0}^\infty a_nx^{n+1}=0\]The first thing is to get all the x's to the \(n^{th}\) power with index shifts, so we will start the first series one lower at n=1 (so we add 1 to n in the summand) and the second series we will shift up by 1 (so that n shifts down by 1 in the summand). Note these shifts also change the indices of the constants \(a_n\)\[\sum_{n=0}^\infty( n+1)a_{n+1}x^n\sum_{n=1}^\infty a_{n1}x^n=0\]Next we need to get the indices the same again (make n start at the same number for each series) To avoid doing another index shift that will take us back where we started, we will "strip" the n=0 term out of the first series\[\sum_{n=0}^\infty(n+1)a_{n+1}x^n=a_1+\sum_{n=1}^\infty(n+1)a_{n+1}x^n\]notice all we did was plug in n=0 and explicitly write that term out in front of the rest of the series. They call that "stripping out a term" in case you did not know. So we now have\[a_1+\sum_{n=1}^\infty(n+1)a_{n+1}x^n\sum_{n=1}^\infty a_{n1}x^n=0\]\[a_1+\sum_{n=1}^\infty[(n+1)a_{n+1}a_{n1}]x^n=0\]Here we now need to figure out a formula for any pattern(s) in the constants \(a_n\) First, plugging in \(n=0\) since the series is not defined we get\[n=0:a_1=0\] so now we just have\[\sum_{n=1}^\infty[(n+1)a_{n+1}a_{n1}]x^n=0\]and since exponentials like \(x^n\) can never be zero, and the summation sign will not change the fact that what is within the brackets must be zero we have the "recurrence relation"\[(n+1)a_{n+1}a_{n1}=0\implies a_{n+1}={a_{n1}\over n+1}\]so again we start plugging in values for n starting from n=1\[n=1:a_2=\frac{a_0}2\]\[n=2:a_3=\frac{a_1}3=\frac03=0\]\[n=3:a_4=\frac{a_2}4=\frac{a_0}{2\cdot4}=\frac{a_0}{2^2(1\cdot2)}=\frac{a_0}{2^2(2!)}\]\[n=4:a_5=\frac{a_3}5=\frac05=0\]\[n=5:a_6=\frac{a_4}6=\frac{a_0}{2\cdot4\cdot6}=\frac{a_0}{2^3(3!)}\]\[~~~~~~~\vdots\]so what's the pattern for the constants? There seem to be two; one for even subscripts and one for odd. Even numbers can be written as \(n=2k\) and odd as \(n=2k+1\) for \(k=0,1,2,...\) That means that all the \(a_n\) for even n can be written as \(a_{2k}\) and have the form\[a_{2k}=\frac{a_0}{2^kk!}\]and for all odd n we have \[a_{2k+1}=0\]therefor all those terms will disappear from the series. That lets us replace all the n's in the summand with 2k and make a new representation of the series based on that. This way we don't include the odd n terms, which are zero anyway. We can now rewrite \[a_n=a_{2k}=\frac{a_0}{2^kk!}~~~~\text{where}~~~~k=0,1,2,3...\]and our guess for y becomes\[y=\sum_{n=0}^\infty a_nx^n=a_0\sum_{k=0}^\infty\frac{x^{2k}}{2^kk!}\]
 2 years ago

experimentX Group TitleBest ResponseYou've already chosen the best response.1
you guys have patience of a saint ... usually i don't write more than lines.
 2 years ago
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