# Picard's Method

1. Jun 21, 2011

### trojansc82

1. The problem statement, all variables and given/known data

y' = -y , y(0) = 1

2. Relevant equations

Picard's method

3. The attempt at a solution

I found y1 as 1 + 1/2x2, however y1 is really 1 + x

Last edited: Jun 21, 2011
2. Jun 21, 2011

### LCKurtz

Neither of those is correct for the equation you wrote. Show us what you did so we can find your mistake(s).

3. Jun 21, 2011

### lanedance

you should show you method, so we can see where you're going wrong - what was your first approximation? starting with a constant should integrate to a constant times x and you should have the solution after one iteration

4. Jun 21, 2011

### lanedance

also as LCKurtz points out neither is correct, you should alway check by substituting into the original DE

5. Jun 22, 2011

### trojansc82

Sorry I meant y1 is really 1 -x. It was a typo.

6. Jun 22, 2011

### HallsofIvy

Staff Emeritus
I think LCKurtz and lanedance are misunderstanding your notation. Neither $y= 1+ x^2/2$ nor $y= 1+ x$ is a solution to the equation but you are not claiming it is.

Picard's method of solving the differential equation y' = f(x,y), with initial condition $y(0)= y_0$ is an iterative method. Taking $y_0$ to be the initial value, $y_1= \int f(x, y_0)dx$ is the first iteration, then $y_2= \int f(x, y_1(x))dx$, $y_3= \int f(x,y_2(x))dx$, etc.

For the problem as given, the first iteration of Picard's method is, indeed, $y_1(x)= x+ 1$. But no one can show you where you went wrong until you show us what you did.

7. Jun 22, 2011

### LCKurtz

Nor are we claiming that.

No, it isn't.

8. Jun 22, 2011

### HallsofIvy

Staff Emeritus
Oh, I completely misread the equation! It is y'= -y, not y'= y as I was seeing!

My apologies to both LCKurtz and lanedance.

trojansc82, I still don't see how you got "1+ x^2/2". Please show us what you did.

9. Jun 22, 2011

### trojansc82

Again, I apologize, I mistyped the answer.

The answer for y1 = 1 - x

10. Jun 22, 2011

### LCKurtz

Is that the answer you got? In your original post you said you got something else. Have you figured it out? You never did show us what you did...

11. Jun 22, 2011

### trojansc82

Within the integral I multiplied -1 (since y was -y) by t. I ended up integrating -t, which came to -1/2 x2.

I have trouble within the integral.

12. Jun 22, 2011

### LCKurtz

What integral? We aren't mind readers. My guess is you have the integral wrong in the first place. Show us your work.