## ← Nonlinear 1.2 Quiz solution video

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Showing Revision 2 created 04/13/2021 by jnelson42.

1. The first problem on this quiz is an
exercise in evaluating the logistic map,
2. which looks like this.
3. Going back to the problem statement,
we see we're supposed to use
4. r = 2.5 and x_0 = 0.5
5. By my calculator, I get 0.625 for that.
6. To get x_2, we plug that back in, like so.
7. When I plug that into my calculator, I
get approximately 0.586.
8. By the way, what I'm really getting,
because my calculator has seven
9. significant figures to the right of the
decimal place is this
10. And I'm rounding to
three significant figures.
11. To get the third iterate, we plug that
back in; and when I do that I get
12. So the answer I get is x_3 = 0.606.
13. Now this gets pretty painful, so you can
imagine why it might be nice to have
14. a computer program to do this.
15. Here's a simple Matlab
program that does that.
16. I'll post this on the supplementary
materials as well.
17. As is requested in the quiz problem,
it takes three inputs:
18. the first initial condition x_0,
the parameter value r,
19. and the number of iterates n.
20. And it spits out n iterates of the
logistic map from that x_0 using that r.
21. So our task is to compute
the tenth iterate,
22. starting from x = 0.2, with r = 2.6.
23. And I'm going to use my Matlab
program to do that.
24. And here's the tenth iterate,
right there: 0.6157.
26. For the third problem, you'll need
to go to the app that we've written
27. for you to use to
explore the logistic map.
28. You can find that app under the
supplementary materials section
29. of the Complexity Explorer
homepage for this course.
30. Scroll down to the current unit,
which is unit 1, and the current segment,
31. which is segment 1.2, and there's the app.
32. The task was to plot 50 iterates of the
logistic map with r = 2
33. from an initial condition x_0 = 0.2.
34. Remember, you have to hit the Restart
Simulation button to get that to work.
35. The question was, "Does the orbit reach a
fixed point?"
36. and that sure looks like it
reaches a fixed point.
37. So the answer is definitely yes.
38. In question four, we're supposed to raise
r from 2 (where it was in question 3),
39. up to 2.7 and repeat the same
experiment: 50 iterates from x_0 = 0.2.
40. Let's try that.
41. The shape of the graph is a little bit
different, but the orbits do
42. converge to a fixed point.
43. So the answer is again yes.
44. The last question asks us, "If the orbits
in questions 3 and 4 both reached a fixed
45. point, (which is true), is that fixed
point at the same value of x?"
46. So let's look back at the app.
47. Here's r = 2.7; it looks like the fixed
point is above this 0.6 line on the plot.
48. There's r = 2.0; again, still a fixed
point, but the fixed point is at x = 0.5.
49. So the answer is no, because those fixed
points were at different values of x.