# Solutions for diff equation in form of series

1. May 4, 2014

### Jhenrique

Given the following diff equation: $(1 - D)y(x) = f(x)$, being D = d/dx, the "implicit" solution is: $y(x) = \frac{1}{1-D}f(x)$, so, for "explicit" the solution is necessary to expand the fraction 1/(1-D) by identity: $\frac{1}{1-x}=\sum_{0}^{\infty}x^n \Delta n$, but this infinity series is true only for |x|<1, for |x|>1 is necessary utilize the following series: $\frac{1}{1-x} = -\sum_{-\infty}^{-1} x^n \Delta n$. Happens that D isn't a number for I say that |D| is less or greater than 1. So, how can I interprate this form of solution correctly?

PS, this ideia from Operational Calculus (http://www.latp.univ-mrs.fr/~chaabi/ARTICLES%20IMPORTANTS/Autres%20articles%20interessant/H.-J.%20Glaeske,%20A.%20P.%20Prudnikov,%20K.A.%20Skornik%20-%20Operational%20Calculus%20and%20Related%20Topics%20%28Analytical%20Methods%20and%20Special%20Functions%29%20%28Chapman%20,2006%29.pdf [Broken])

Last edited by a moderator: May 6, 2017
2. May 4, 2014

### Staff: Mentor

No, this makes no sense. You are treating 1 - D as if it were multiplying y(x) - it isn't, any more than (d/dx)y means d/dx times y. Here 1 - D is an operator that operates on y(x). It does NOT multiply y(x).

Last edited by a moderator: May 6, 2017
3. May 4, 2014

### Jhenrique

You are being much formal, look:

And ditto mark for my 1st question more one times...

4. May 4, 2014

### Xiuh

Just a recommendation for you based on reading your other posts.. It seems that you have a lot of misconceptions. Maybe it wouldn't hurt if you mastered basic math before trying to do more advanced stuff :)

There is indeed something similar to what you're thinking. Suppose you have a bounded operator $\textit{T}$ on a normed space $\textit{X}$. If the series $\sum\limits_{k=0}^\infty T^k$ converges, then $(1 - T)$ is invertible and then
$$\left(1 - T\right)^{-1} = \sum\limits_{k=0}^\infty T^k$$
If $\textit{X}$ is Banach and $\| T \| < 1$ (in operator norm) the convergence is guaranteed. The thing is, you have to define the function space you are working with. If you are working with polynomials then convergence is trivial (it is only a finite sum). On the other hand, if you are considering $\frac{d}{dx}: C^1([0,1]) \rightarrow C([0,1])$ the operator is not bounded.

Last edited: May 4, 2014
5. May 4, 2014

### lurflurf

There are ways to know in advance if the method will work, but it is often easier to go ahead with it and check the answer for correctness later. That is use the method to produce a possible solution to be verified afterward.

$$(1-\mathrm{D})\mathrm{y}(x)=\mathrm{f}(x) \\ \mathrm{y}(x)=(1-\mathrm{D})^{-1}\mathrm{f}(t) \\ \mathrm{y}(x)=-\mathrm{D}^{-1}(1-\mathrm{D}^{-1})^{-1}\mathrm{f}(t) \\ \mathrm{y}(x)=-\sum_{k=1}^\infty\mathrm{D}^{-k}\mathrm{f}(t) \\ \mathrm{y}(x)=-\mathrm{D}^{-1}\sum_{k=1}^\infty\frac{(x-t)^{k-1}}{(k-1)!}\mathrm{f}(t) \\ \mathrm{y}(x)=-\mathrm{D}^{-1}e^{x-t}\mathrm{f}(t) \\ \mathrm{y}(x)=-e^{x}\mathrm{D}^{-1}e^{-t}\mathrm{f}(t)$$
where
$$\mathrm{D}^{-1}\{\cdot\}=\int_a^x \! \{\cdot\} \, \mathrm{d}t$$
$$(1-\mathrm{D})\mathrm{y}(x)=\mathrm{f}(x) \\ (1-\mathrm{D})e^x e^{-x}\mathrm{y}(x)=\mathrm{f}(x) \\ -e^x\mathrm{D}e^{-x}\mathrm{y}(x)=\mathrm{f}(x) \\ \mathrm{y}(x)=-e^x\mathrm{D}^{-1}e^{-t}\mathrm{f}(t)$$

6. May 4, 2014

7. May 4, 2014

### Staff: Mentor

How are you going from the 2nd line above to the third?

8. May 4, 2014

### Staff: Mentor

One other thing: if you cite a paper that is 400+ pages long, at least say what page or section you're referring to.

Last edited by a moderator: May 6, 2017
9. May 4, 2014

### lurflurf

^distributive property

$$(1-\mathrm{D})^{-1}=[(-\mathrm{D})(-\mathrm{D}^{-1})+(-\mathrm{D})(1)]^{-1} \\ =[-\mathrm{D}(-\mathrm{D}^{-1}+1)]^{-1}=(-\mathrm{D})^{-1}(1-\mathrm{D}^{-1})^{-1}=-\mathrm{D}^{-1}(1-\mathrm{D}^{-1})^{-1}$$
In operators 1 should be considered the identity of the operator algebra and not a number.
Maybe it would be better to use I
For example D(1-D)=D-D^2 not -D^2

10. May 8, 2014

### Jhenrique

Sorry, kkkkkkkkkk, the theory about operational calculus is in the first pages...

I know that the following expression works, $\frac{1}{1-D}=\sum_{0}^{\infty}D^n \Delta n$, and that the following too, $\frac{1}{1-D} = -\sum_{-\infty}^{-1} D^n \Delta n$. So, my question is: given a diff equation like: $y(x) = \frac{1}{1-D}f(x)$, which of the two I must to use, and why?

11. May 8, 2014

### Staff: Mentor

IMO it's misleading to write $\frac 1 {1 - D}$ instead of (1 - D)-1; i.e., as a fraction instead of an inverse.

12. May 8, 2014

### Jhenrique

But that difference this make?

PS: this link has more example of "operational calculus": http://rip94550.wordpress.com/2012/08/27/heavisides-operational-calculus/

So, again I reiterate my question:

Last edited: May 8, 2014
13. May 9, 2014

### Jhenrique

I didn't undertand...