# Find a Solution using Green's Function

Find a solution to the given initial-value problem using Green's functions

$$y"+3y'+2y= \frac{1}{1+e^x}; y(0)=0,y'(0)=1$$

So I have figured out that

$$y_c=e^{-x}-e^{-2x}$$

Now I am having issues figuring out what $y_p$ is. This is what I have so far:

$$y_1=e^{-x}, y_2=e^{-2x},$$

so

$$W(y_1, y_2)=-e^{-3x}.$$

$$G(x,t)=\frac{e^{-t}e^{-2x}-e^{-x}e^{-2t}}{-e^{-3x}} = -e^{-t}e^{x}+e^{-2t}e^{2x}$$

$$y_p=\int^x_0 G(x,t)f(t)dt$$

$$y_p=-e^x \int^x_0 \frac{e^{-t}}{1+e^t}dt+e^{2x} \int^x_0 \frac{e^{-2t}}{1+e^t}dt$$

So

$$y_p=-\ln(e^{-x}+1)(e^x+e^{2x})-\frac{e^{2x}}{2}+\frac{1}{2}+(e^{x}+e^{2x})\ln(2)$$

Is this correct?

-
I am now beginning to doubt that :) –  math101 Jun 18 '13 at 21:04
Yes I have used them –  math101 Jun 18 '13 at 21:06
Oh yup. My mistake there. Thanks for pointing that out:) –  math101 Jun 18 '13 at 21:10
@Amzoti what about $y_p$ –  math101 Jun 18 '13 at 21:21
Is that better? –  math101 Jun 18 '13 at 21:46

We are given:

$$y''+3y'+2y= \dfrac{1}{1+e^x}, ~ y(0)=0, ~y'(0)=1$$

The homogeneous (complementary) solution is given by: $y_c=e^{-x}-e^{-2x}$

For variation of parameters, we take: $y_1=e^{-x}, ~~y_2= -e^{-2x}$

Thus, the Wronskian $= W(y1, y2) = W(e^{-x},-e^{-2x}) = e^{-3x}$, so

$\displaystyle G(x,t)= \frac{y_1(t)y_2(x) - y_1(x)y_2(t)}{W(x)} = \frac{e^{-t}(-e^{-2x}) - e^{-x}(-e^{-2t})}{e^{-3x}} = e^{2x}e^{-2t} - e^xe^{-t}$, thus:

$\displaystyle y_p=\int^x_0 G(x,t)f(t)dt = -e^x \int^x_0 \frac{e^{-t}}{1+e^t}dt+e^{2x} \int^x_0 \frac{e^{-2t}}{1+e^t}dt$

We have:

• $\displaystyle -e^x \int^x_0 \frac{e^{-t}}{1+e^t}dt = -e^{-x}\left[\ln(e^{-t}+1)-e^{-t}\right]$ evaluated over $(t, 0, x)$, yields: $\displaystyle-e^{-x}\left[\left(\ln(e^{-x} + 1)-e^{-x}\right) - \left(\ln(2) - 1\right)\right]$
• $\displaystyle e^{2x} \int^x_0 \frac{e^{-2t}}{1+e^t}dt = e^{2x}\left[-\frac{e^{-2t}}{2}+e^{-t}-\ln(e^{-t} +1) \right]$, evaluated over $(t, 0, x)$, yields: $\displaystyle e^{2x}\left[\left(-\frac{e^{-2x}}{2} + e^{-x} - \ln(e^{-x}+1)\right) - \left(-\frac{1}{2} + 1 - \ln 2\right) \right]$

Do you see the issues now over those limits? Can you now do the algebra to combine all like terms and find:

$$y = y_c + y_p?$$

The final answer should be (after simplifications) and you can compare to the other answer:

$$y(x) = \left(e^{-x}+e^{-2 x}\right) \ln\left(\dfrac{1}{2} (e^x+1)\right)$$

-
Nice work (indeed messy!)!! (+) –  amWhy Jun 19 '13 at 1:40
@Amzoti Wow, Thanks so much. I really appreciate that. Now I see where I went wrong :) –  math101 Jun 19 '13 at 1:51

You are doing fine except that you should delay finding the constants untill you construct the final solution

$$y = y_c + y_p = \ln \left( {{\rm e}^{x}}+1 \right) { {\rm e}^{-x}}+\ln \left( {{\rm e}^{x}}+1 \right) {{\rm e}^{-2\,x}}-{ c_1{\rm e}^{-2\,x}}+c_2{{\rm e}^{-x}}\longrightarrow(1).$$

You should have the following answer

$$y \left( x \right) = ({{\rm e}^{-x}}+{\rm e}^{-2x} )\ln \left( {{\rm e}^{x}}+1 \right)-{{\rm e}^{-2\,x}}\ln \left( 2 \right) -{{\rm e}^{-x}}\ln \left( 2 \right).$$

Added: Here is how you advance. Using the first initial condition, subs $x=0$ in $(1)$ and equate it tozero gives

$$c_1-c_2=2\ln(2).$$

Diff. $(1)$ and using the second initial condition yields the second equation

$$2c_1-c_2=3\ln(2).$$

Now, just solve the two equations to find $c_1,c_2$ and subs back in $(1)$ to get the desired answer.

-
I am not all that knowledgeable in this area but in my textbook we find the constants in the beginning. Also have you used Green's Function since I am not sure how you got a totally different $y_p$ than I have. Did you simplify the answer? –  math101 Jun 18 '13 at 21:14
@math101: As I said you were doing fine. You found the solution of the homogenous ode and the particular solution using Green's function technique. Note that, you are not solving a homogenous ode with initial condition instead you are solving a non homogenous ode with initial conditions and I already pointed out how you should have advanced. –  Mhenni Benghorbal Jun 18 '13 at 21:54
But How did you get $y_p=\ln(e^x+1)(e^{-x}+e^{-2x})$ –  math101 Jun 18 '13 at 22:29
@math101: Make sure you got the right $y_p$. I did not have a close look at your Green's function technique. However there are other techniques to find $y_p$. –  Mhenni Benghorbal Jun 18 '13 at 22:34
Thanks for helping me :) I really appreciate it –  math101 Jun 18 '13 at 22:34