Group Abstract Group Abstract

Message Boards Message Boards

[WSG21] Daily Study Group: Differential Equations (begins November 29)

A new study group devoted to Differential Equations begins next Monday! A list of daily topics can be found on our Daily Study Groups page. This group will be led by one of our outstanding Wolfram certified instructors, Luke Titus, and will meet daily, Monday to Friday, over the next three weeks. Luke will share the excellent lesson videos created by him for the upcoming Wolfram U course "Introduction to Differential Equations". Study group sessions include time for exercises, discussion and Q&A. This study group will help you achieve the "Course Completion" certificate for the "Introduction to Differential Equations" course after you complete the course quizzes.

Sign up: Study group registration page
POSTED BY: Devendra Kapadia
170 Replies
Sort By:

Thank you, Rohit.

You wrote: "If you are trying to reproduce the image in the plot_dashes.nb notebook ..." No, I do not. Sorry for ambiguity. I am solving a Diff. Eq. and making a plot of its solutions. In that plot, I define thickness and color of the lines, say PlotStyle-> {{Red, Thick},{Blue,Thick}}. In my plot, however, In addition to the required features, lines contain dots and dashes which are unwanted. To demonstrate, I sent a plot of Sin[] functions, which are also supposed to be just colored, but also contain unwanted dashes. I do not know how to to get rid of dashes.

Thank you. M
POSTED BY: Michael Partensky

Perhaps we can help more if you show the complete code, including the differential equation.

/Hakan
POSTED BY: Hakan Kjellerstrand

That was my original plan. However, after discovering that it has nothing to do with the code and shows up with a set of simplest function, I posted the simple version. Hope things are going well with you, and we will meet again soon.

Best. Mo
POSTED BY: Michael Partensky
Rohit Namjoshi
Rohit Namjoshi
Posted 4 years ago

Hi Michael,

If you are trying to reproduce the image in the plot_dashes.nb notebook you need to add a dash specification to the PlotStyle. Something like

dashStyles = {Dashing[Medium], Dashing[{0, Small}], 
  Dashing[{0, Small, Small, Small}], Dashing[{0.03, 0.01}], 
  Dashing[{0.02, 0.01}]}

Plot[Evaluate[Table[Sin[v s], {v, 0.1, 0.7, 0.15}]], {s, 0, 15}, 
  PlotStyle -> Table[{dashStyles[[i + 1]], Hue[1 - 0.15 i], Thickness[0.01]}, {i, 0, 4}], 
  AspectRatio -> 0.75, 
  PlotRange -> {{0, 15}, {-1.2, 1.2}}, 
  Frame -> True, 
  GridLines -> Automatic]

You will have to experiment with the Dashing to exactly reproduce the plot.
POSTED BY: Rohit Namjoshi

Thank you, Hakan. Yes, I checked all the "usual suspects". And the version is 12.3.1. My actual question is how to control the Plot (line) style to set up the output exactly as desirable. There is definitely a way to do so. With mutual effort it will be discovered!

Thank you again. Mo
POSTED BY: Michael Partensky

Hi, Hakan! Great to hear from you! I almost sure that it did not happen before. In the list plots it can be addressed by Joined->True., but here it does not work. Thank you! M

Attachments:
plots_dashes.nb
POSTED BY: Michael Partensky

Ah, now I see what you mean. Have you tried to reset the session, e.g. with Quit[]? What version of Mathematica do you use?

/Hakan
POSTED BY: Hakan Kjellerstrand

Unwanted Dashes

Hi! I hope you are still around :) I have a strange issue trying to plot solutions of equation. To demonstrate it, I use a plot of Sin[]:

Plot[Evaluate[Table[Sin[v s], {v, 0.1, 0.7, 0.15}]], {s, 0, 15}, PlotStyle -> Table[{Hue[1 - 0.15 i], Thickness[0.01]}, {i, 0, 4}], AspectRatio -> 0.75, PlotRange -> {{0, 15}, {-1.2, 1.2}}, Frame -> True, GridLines -> Automatic];

How/why do the dashes appear and how to get rid of them? Thanks!
POSTED BY: Michael Partensky

Hi Michael.

When I run this program (in v13.0 and v12.3.1) I don't see any dashes. Would you mind showing it in an image?

/Hakan
POSTED BY: Hakan Kjellerstrand
Richard Bryant
Richard Bryant
Posted 4 years ago

There appear to be a number of problems with Quiz 8. For instance, it marks me incorrect when I say the matrix {{0,4},{0,0}} is not diagonalizable but DiagonalizableMatrixQ[{{0, 4}, {0, 0}}] yields false.
POSTED BY: Richard Bryant
Doug Beveridge
Doug Beveridge
Posted 4 years ago

I am checking out the next study group "Quantum Computation Framework"

I have just installed Mathematica 13 so I can run this paclet.

I have attempted both methods to run the paclet below but get an error 

PacletInstall::notavail: No paclet named Wolfram__QuantumFramework is available for download from any currently enabled paclet sites.

enter image description here
POSTED BY: Doug Beveridge
POSTED BY: Artin Hatzikioseyian

Hi Artin, I confirmed that both day's recordings are available on the BigMarker study group page. If you still continue to face issues watching these, please email us at wolfram-u@wolfram.com.
POSTED BY: Abrita Chakravarty

Thank you Abrita. Maybe it was my mistake that I had not seen the videos.
POSTED BY: Artin Hatzikioseyian

I hope that everyone will stay hooked for a while. There are still some interesting questions to discuss. In addition, it will be painful to part with a group of such wonderful and productive individuals. I am not well familiar with the social media, but there should be a way to stay in touch (?) Thanks to Wolfram Research and the Wolfarm-U team for making this possible.
POSTED BY: Michael Partensky

Thanks for a really great and interesting Study Group and Course! I've learned a lot on this fascinating topic and ordered the Boyce & DiPrima book ("Elementary Differential Equations", the 2017 version) that Devendra talked about, so I can continue to learn more.

Again, thanks!
POSTED BY: Hakan Kjellerstrand

Suggestion. I think, next time you should include a derivation of the equations used by Ernest Rutherford and his colleagues to describe the processes of radioactive birth and decay. Why? A graduate of your course, unaware of Rutherford's work is likely to explain / model a birth and decay process by the method of integration by parts or alternatively by some expression including the Heaviside step function. See: Rutherford E et al (1951) Radiations from Radioactive Substances. (Chapter I pages: 1 - 37) https://archive.org/details/radiationsfromra0000ruth_a2v4 I learned a lot. I enjoyed this course very much. Thanks.

Attachments:
Rutherford1951 C...pdf
POSTED BY: Alan White

POSTED BY: Hakan Kjellerstrand
POSTED BY: Michael Partensky
POSTED BY: Hakan Kjellerstrand

Hi, Hakan. Thanks for your insightful contributions. Trying to digest them, I had issues with creating local copies (possibly, because I do not have Math. One account). Could you please attach the NB file to a couple of most recent, dealing with Root reduce.

Thank you and hope to meet you in new classes. Best. M
POSTED BY: Michael Partensky

Hi Michael.

Below the post there is a button "Make Your own Copy" which should show a Wolfram Cloud version of the note book. Here's the explicit link: https://www.wolframcloud.com/env/c0022b5b-98c8-4085-a1a8-c3c83cd0818b .

Does that work for you? If not, I've also attached my original notebook, "laplacetodiffeq.nb".

I've registered for the two upcoming Wolfram U Study Groups: "Introduction to Cryptography" and "Quantum Computation Framework", and the Wolfram U Webinar Series "New in Wolfram Language 13". Perhaps we'll meet again in the Study Groups? It will be a busy January and February...

All the best and thanks for your great contributions here.

Hakan

Attachments:
laplace_to_diffeq.nb
POSTED BY: Hakan Kjellerstrand

Great thanks, Hakan. Thank you for being our Forty Niner! Will probably see you in one of the classes u've mentioned. I am choosing between QC and Crypto. What do you think? Which on would be more conceptual rather than (mostly) package-learning? Is their content available? Best. Michael
POSTED BY: Michael Partensky

Hi Michael.

(I first thought that 'Forty-Niner' is a baseball/football term, and had to google it. :-) Thanks.)

What I understand, the Quantum Study Group is a study group, but not a course (https://www.bigmarker.com/series/daily-study-group-quantum-computation-framework/series_details). The Framework is presented here: https://community.wolfram.com/groups/-/m/t/2416125 . It's is over my head for the moment, and that is why I would like to learn more.

OTOH, the Cryptography study group (https://www.bigmarker.com/series/daily-study-group-intro-to-cryptography/series_details?utm_bmcr_source=wolfram-u ) is both a Study Group and a course and seems to be more conceptual - it's over 3 weeks - and with much Wolfram Language coding :-). Here is a Wolfram Technology Conference talk on the Cryptograhy course: https://www.youtube.com/watch?v=MATcvnsfSS8 .
POSTED BY: Hakan Kjellerstrand

Thanks again, Hakan. - very valuable information. Frankly, I did not even recognize difference between the study group and the course.

Hope to TTYL in the classes to come.
POSTED BY: Michael Partensky
Wissam Barakat
Wissam Barakat
Posted 4 years ago

A remark on a final exam question: The question involving the oscillating spring needs a specification of the positive direction of the coordinate system, is it up or down?
POSTED BY: Wissam Barakat
POSTED BY: Michael Partensky
POSTED BY: Alan White

Thanks Alan. After reading for a while, I now seem to understand what you mean (I just didn't find it in the lecture). I see the spikes on the curves of AFs e.g. in this article: https://en.wikipedia.org/wiki/Airy_function. The reading also reminded me that my first exposure to AF came in a basic QM course - solving the Schrödinger equation for a triangular barrier. Thanks for the push.

Best, M
POSTED BY: Michael Partensky
Updating Name
Updating Name
Posted 4 years ago
POSTED BY: Updating Name

Sorry, "boring" was a joke. The function is amazing, and even more amazing is that such a simple equation could make such a big contribution in almost everything.
POSTED BY: Michael Partensky
POSTED BY: Alan White
Graham Gyatt
Graham Gyatt
Posted 4 years ago

Small suggestion for improvement to the videos (and examples) - pause for a few seconds at the final result so the student has a chance to read and digest the result. Thanks.
POSTED BY: Graham Gyatt
POSTED BY: John Burgers

Hi John,

Right now there is no way to navigate from your Wolfram Cloud home directory to the Wolfram U courses. This is something we will consider for the future.

The easiest way to get to all Wolfram U courses right now is from the Wolfram U site https://www.wolfram.com/wolfram-u/all-classes-courses/

Of course once you have taken some of the quizzes and exercises from a course, copies of these quiz and exercise notebooks will start appearing in your "Copied Files" folder within your Wolfram Cloud home directory. enter image description here
POSTED BY: Abrita Chakravarty

Hi Luke, One suggestion for Lesson 17 Slide 11. As a reminder for some students would it be worth to state the relation Gamma[n+1] = Factorial[n] for integer n > 0 ?
POSTED BY: John Burgers
POSTED BY: Michael Partensky

I have a question that has a duplicate answer I am taking the Calculus Final Exam for Wolfram U and this is a question enter image description here
POSTED BY: Peter Burbery

Wow guys! It develops faster than the speed of thought! I watch it as a thrilling (and thought-provoking) show, holding my breath. Thanks to all the participants! M
POSTED BY: Michael Partensky

I have a question that does not have any possible choices on the final exam. Is it okay for me to post a question with no answers on the final exam, or should I not post it?
POSTED BY: Peter Burbery
POSTED BY: Hakan Kjellerstrand
POSTED BY: Zbigniew Kabala

Hi again, Zbigniew.

Your equation, or rather the initial values is not the same as mine. I have {y'[0] -> 0, y[0] -> 1} while you have {y'[0] -> 1, y[0] -> 0}. When I use your initial values SolveValues[] returns 0, as does InverseTransformTransform[], which would indicate that there is no solution?

In version 12.3.1 both variants behaves as your example. So there seems to be a version issue.

But now Luke starts to talk...

Best,

Hakan
POSTED BY: Hakan Kjellerstrand

Since I don't know how to get the nice copyable outputs as Zbigniew's, here's a notebook showing the different experiments using Mathematica version 13.0 (in Mathematica Online).

The notebook is available for further experiments here: euler equation laplace transform.nb
POSTED BY: Hakan Kjellerstrand

That is absolutely awesome, Hakan! I will definitely download and install Mathematica 13.
POSTED BY: Zbigniew Kabala

Hello Hakan et al.,

Long time (6 days) no see. I finally got Mathematica 13 and was able to return to the old problem of solving Euler's equation with the Laplace Transform.

The crucial point that we left hanging is the ability of SolveValues to solve the equation generated by Hakan and denoted leqn in his post. To make life simple I imbed my notebook here and remind us of the dilemma (I also attach it--see below).

By the way, Hakan, it would be nice to have your contact info and keep in touch. Please post it or send it to my email at Duke: kabala@duke.edu.

Cheers,

Zbigniew

Attachments:
Hakan_s_Euler_eq...nb
POSTED BY: Zbigniew Kabala
Updating Name
Updating Name
Posted 4 years ago
POSTED BY: Updating Name

Hi Hakan,

I'm not sure how long this forum would stay open beyond the quiz and final exam submission deadline of Dec 27. So thanks for your contact info.

I'm also a math enthusiast, although I have a math-related job at Duke University (teaching fluid mechanics and other courses). We tend to trust systems like Mathematica, but they are all products made by humans, so they are prone to mistakes. This was an interesting exercise and a reminder about the need to be vigilant and skeptical.

Best,

Zbigniew
POSTED BY: Zbigniew Kabala

Hi again, Zbigniew.

Unless they change the policy for some reason, these Study Group discussions are not closed. The study group discussions from the last year are still open.

Best,

Hakan
POSTED BY: Hakan Kjellerstrand

Good to know, Hakan!

I wish they made it clear from the beginning and, especially at the end of the course. I find the discussions stimulating and can see myself returning to these groups over and over again.

Best wishes, Zbigniew
POSTED BY: Zbigniew Kabala
POSTED BY: Zbigniew Kabala

POSTED BY: Hakan Kjellerstrand

Very interesting. Thank you, Hakan, for sharing. M
POSTED BY: Michael Partensky

[Comment removed by the author since he was confused.]
POSTED BY: Hakan Kjellerstrand
Richard Sweney
Richard Sweney
Posted 4 years ago

Will the participants in this course have online access to the course lessons and exercises after 12/27?
POSTED BY: Richard Sweney

Yes, this course will be available for everyone on the Wolfram U site once it has been released. We have shared a pre-release version of the course with the participants of this particular study group.
POSTED BY: Abrita Chakravarty
POSTED BY: Hakan Kjellerstrand
POSTED BY: Peter Burbery
Rohit Namjoshi
Rohit Namjoshi
Posted 4 years ago

Don't know what is in the quiz but the code in your question is invalid, should be

DSolve[{-3 y[x] - x y'[x] + y''[x] == 0, y[0] == 0, y'[0] == 1}, y[x], x]
(*
{{y[x] -> (
   E^(x^2/2) Sqrt[\[Pi]] + E^(x^2/2) Sqrt[\[Pi]] x^2 - 8 HermiteH[-3, x/Sqrt[2]])/(2 Sqrt[2])}}
*)
POSTED BY: Rohit Namjoshi

Is there a way to convert a series into summation notation? For example, is there a way to convert the following?

Series[Exp[x],{x,0,10}]

into x^n/n! in summation notation?
POSTED BY: Peter Burbery

Use the Normal function to do that. Sometimes you need also the Activate function, but not in the example below:

     In[4]:= Series[Exp[x], {x, 0, 6}] // Normal
     Out[4]= 1 + x + x^2/2 + x^3/6 + x^4/24 + x^5/120 + x^6/720

     In[5]:= % /. x -> 1.5
     Out[5]= 4.47754
POSTED BY: Zbigniew Kabala

I have a question about downloading Wolfram University notebooks that does not relate to this course directly but more to Wolfram University's course Visual Explorations in Data Science. I have had a lot of trouble downloading the notebooks from the course page with the link at the bottom. Is there another place I can download the notebooks for Visual Explorations in Data Science?
POSTED BY: Peter Burbery

Please let us know where on Wolfram U is this course with its quizzes, lectures, and recordings ...

I tried to find it but to no avail: when I googled for "wolfram u differential equations course" I got the following link (somehow the link can't be copied, so please repeat the search to see the issue):

       [Differential Equations, Interactive Online Video Course][1]
       A comprehensive introduction to fundamental concepts and solution methods 
       for differential equations, including video lessons and interactive notebooks. Follow ...

and below is the screenshot of what I get when I click the link, "Comming Soon." Again, where is the course? enter image description here
POSTED BY: Zbigniew Kabala

Thank you everyone for reporting the errors. We are looking into this and will hopefully be able to share a resolution at tomorrow's session.

-Wolfram U Team
POSTED BY: Abrita Chakravarty

Just wanted to post an update: We have received quiz submissions from 39 users for at least one or more of the 8 quizzes and 36 successful submissions (with score >- 60%) for at least one or more quizzes.

We are still looking into what might be causing the errors posted in the screenshots above.
POSTED BY: Abrita Chakravarty
Luis Marin
Luis Marin
Posted 4 years ago
POSTED BY: Luis Marin
Luis Marin
Luis Marin
Posted 4 years ago

UPDATE: Everything seem to be working now, thanks!
POSTED BY: Luis Marin

Guys, No need to post ALL your error messages ... We got it. The quizzes are messed up, and we have to wait until they are fixed.
POSTED BY: Zbigniew Kabala

Awsome, Zbigniew. Welcome to the team! I will send you some background information and a sort of initial proposal. Later, I am sure, it will expand from both sides :)

PS I do not want to overload this forum, and will send directly to your email.

POSTED BY: Michael Partensky

Hello Wolfram U team,

I'm not able to submit Quiz answers without errors. And all answers are incorrect, no matter which I choose.

For Quiz 1, the web site reports error messages, see attached image, similar to those reported above by Fabio and Luis.

John Burgers.

Attachment

Attachments:
2021-12-13 17_01...png
POSTED BY: John Burgers
Fabio Alcaro
Fabio Alcaro
Posted 4 years ago

Good morning,

I tried to submit Quiz 1 of the on-line course but I get an error during the grading (btw, it looks like no answer is correct!). I attach the image of the reported errors. Please, let me know if you have any ideas on what's going wrong.

Regards - - - Fabio

enter image description here
POSTED BY: Fabio Alcaro
Luis Marin
Luis Marin
Posted 4 years ago

Same here enter image description here
POSTED BY: Luis Marin

In lesson 20, Special Functions, on slide 12, you whetted our appetites with the following statement:

   It can be shown that given a mass initially displaced at an angle theta_o on a pendulum, its
   period of oscillation will follow the hypergeometric function:  2F1[1/2, 1/2, 1, sin(theta_o)^2]

Well, it can be shown? SHOW US, please! Below, I comment on this statement.

Attachment

Attachments:
pick1.jpeg
POSTED BY: Zbigniew Kabala

Again, I would like to know how equation (1), in lesson 20, Special Functions, on slide 12 (see also my previous note), was derived.

Below is my attempt at it, in which I ended up with an expression involving the EllipticF function.

Wow! Surprisingly, it turns out that this expression is actually equivalent to the one involving the hypergeometric function. By the way, I'm amazed by the symbolic power of Mathematica in finding the integral and even more in finding that the two expressions are equal to each other. If I were to do it by paper and pencil, it would be ...

But the question still remains: how did you come up with (1)?

Attachment

Attachments:
pick2.jpeg
POSTED BY: Zbigniew Kabala
POSTED BY: Luke Titus

Thanks, Luke, I really appreciate the pointer. Cheers, Zbigniew
POSTED BY: Zbigniew Kabala

Our College math professor called HGF "The mother of all bombs". And he really meant it, being in his youth a part of the "Russian Manhattan Project". I should also mention Dr. Eric Weinstein and his MathWorld (a part of WR) - the outstanding Encyclopedia of Math, which (in conjunction with Wolfrfam Demo projects) allows to test many concepts using the Mathematica.

PS: Morse and Feshbach, in my view, is still a great resource to be kept in the vicinity of your desk :)
POSTED BY: Michael Partensky

Thank you Zbigniew for this insightful post.

POSTED BY: Michael Partensky

Thanks, Michael for your pointers and kind words. I appreciate them. Also, I'm fascinated by your lab. I can't wait till the next Wolfram conference, hopefully in person, and am looking forward to meeting you. Cheers, Zbigniew
POSTED BY: Zbigniew Kabala
POSTED BY: Michael Partensky

Michael,

Awesome idea, and I'm game. Definitely! I'll be mostly free after I give and grade the final exam in my undergraduate Fluid Mechanics, i.e., probably in two days. I would love to get involved in such a project. I'm not sure how long this discussion board will be available. So for this kind of collaboration, I propose that we deal with each other directly via email and/or skype or zoom. My email address is below.

Best wishes, Zbigniew

Giles and West House Faculty in Residence

919-667-5150 (c)

Zbigniew J. Kabala, PhD, PE, Associate Professor

Duke University

Department of Civil & Environmental Engineering

E-mail: kabala@duke.edu

http://www.cee.duke.edu/faculty/zbigniew-j-kabala
POSTED BY: Zbigniew Kabala
POSTED BY: Hakan Kjellerstrand

Hi Hakan, Please email wolfram-u@wolfram.com directly about any feedback related to the course.
POSTED BY: Abrita Chakravarty

[WSG21] Daily Study Group: Differential Equations (begins November 29)
POSTED BY: Michael Partensky

Hi Michael, Oh, I have missed this one. Thanks for the pointer. Best, Zbigniew
POSTED BY: Zbigniew Kabala

Even though I'm registered for this study group, I stopped receiving notifications about the upcoming meetings as well as the follow-ups about their recordings. I would appreciate receiving those announcements.
POSTED BY: Zbigniew Kabala

Hi, Zbigniew. The mist important was about opening the course at the Wolfram-U site, with all the lectures, Quizzes and Exam posted there. With this you do not need to get the weekly invites (well, they will still send them as reminders). Did you get that one?

If you don't, please contact the team. I was cited a few times for sending the links through this forum.
POSTED BY: Michael Partensky

Hi Zbigniew, Could you please email wolfram-u@wolfram.com from the email you used to register for the study group? We can check to make sure you receive the reminders.
POSTED BY: Abrita Chakravarty

Section 5, Exercise 2 The radius of convergence can be calculated using SumConvergence:

In[2]:= SumConvergence[n (2 x + 1)^n, n, Assumptions -> x [Element] Reals]

Out[2]= -1 < x < 0

Therefore, the series converges whenever -1<= x <= 0, so the radius of convergence is 1/2.

In the previous statement the original inequality -1 < x < 0 is substituted by -1<= x <= 0*. However, they are not equivalent, because the Sum [n,{n,0,Infinity }] does not converge.

To avoid the confusion, the "=" sign should be dropped.

  • The situation is slightly different in Exercise 1. Addressing this difference would be helpful for revealing a nuance in the definition of the radius of convergence.
POSTED BY: Michael Partensky

Thanks for pointing that out. I'll make the change in the notebook.

POSTED BY: Luke Titus

I tried to post it before, but for some reason it did not work

Lesson 17, slide 1.

"When the coefficients of a linear second-order differential equation become polynomials, the equation appears in the form:

P(x) y''(x)+Q(x)y'(x)+R(x)y(x)=0 (1)"

This statement is correct but also slightly confusing. It may sound as if the form (1) relies on P, Q, and R being polynomial (while each of then can me (almost) any function of x). In my view, it would be better to say

"When the coefficients P, Q and R in the equation 

  P(x) y''(x) + Q(x)y'(x) + R(x) y(x) =0

are the polynomial functions of x, the solution can be found by Taylor series expansion of y(x) ....".

Well. it's just about wording, sorry for being a nag.
POSTED BY: Michael Partensky

That was just to simplify the discussion. The method works when they are general analytic functions, but the term analytic wasn't defined at that point.

POSTED BY: Luke Titus

Thank you, Luke. Just double-checking: If the functions are polynomial, then, in order to find the coefficients in the series expansion of y(x), you can use them (the functions) "as is". If they are analytic but not polynomial, you need fist to expand them and then to find the (coefficients in the expansion of ) y(x). Is it correct?

Thank you. M
POSTED BY: Michael Partensky

That's right. The polynomial assumption at the beginning was just to guarantee that we are working with analytic functions so we wouldn't have to worry about any of that.

POSTED BY: Luke Titus
POSTED BY: Luke Titus

How to copy a Quiz in the local NoteBook?
POSTED BY: Michael Partensky

Log into your Wolfram Cloud account. Navigate to the "Copied Files" directory within your Home directory. you will find your copy of the quiz notebooks there. enter image description here
POSTED BY: Abrita Chakravarty

OK, that's why I really need to get an account ! (?) Now I will follow your and Arben's advice and write Wolfram-U a letter :)

PS Just did it.
POSTED BY: Michael Partensky

Hi Abrita,

I did not receive any quizz on my Wolfram account. Furthermore, I am waiting for the link to the videos of prior sessions. Please help.
POSTED BY: Jürgen Kanz

Hi Jürgen, We will address this more details at today's session to clarify all questions about accessing the lesson videos and the quizzes within the online course.
POSTED BY: Abrita Chakravarty

[WSG21] Daily Study Group: Differential Equations (begins November 29)
POSTED BY: Michael Partensky

Is there a Level 1 Certification available for this study group?
POSTED BY: Peter Burbery

A Level 1 certification will be available by taking the Final Exam available in the course. We hope to share the pre-release version of the course today with all our study-group attendees.
POSTED BY: Abrita Chakravarty

Congratulations and many thanks for the pre-release version of the course. Excelent!

Greetings from Barranquilla Colombia. Jorge O

POSTED BY: Jorge O. Garcia L

Thank you, Luke. I knew there must be a reason. I'm glad to know it now. Thanks again!
POSTED BY: Zbigniew Kabala

In lesson 11, slide 12, the solution of the problem in Example 1 is defined by means of the ASSIGNMENT, "=", whereas in slide 14, the analogous solution of the problem in Example 2 is defined by means of the DELAYED ASSIGNMENT, ": =". Traditionally, functions in Mathematica are defined via the delayed assignment. Is there a reasonstudentfor the cited difference, or it simply doesn't matter? Please advise!

Attachment

Attachments:
Delayed_OR_NOT.jpeg
POSTED BY: Zbigniew Kabala

In my notebook "Lesson 11", slide 12, the first sol you mentioned is also a delayed assignment:

Now you have almost found the same solution as that found by DSolveValue:

sol[x_] := c1 y1[x] + c2 y2[x]

Perhaps you have accidentally changed that := to = and the saved the notebook?

POSTED BY: Hakan Kjellerstrand

No, I didn't do it. But, an excellent hypothesis, Hakan! It prompted me to investigate this matter further. I originally downloaded the materials before the meetings started. Then additional materials were added, so after one week into the course, I downloaded all the materials again (and I keep both zip files). It is there where this change took place. I downloaded the materials again a few minutes ago, and the "error" is there--you can confirm it yourself.

My question stands: is it an error or a correction? Or, perhaps, it doesn't matter, as both versions seem to work equally well.
POSTED BY: Zbigniew Kabala

Ah, this is interesting. I downloaded the files early as well, before they was zipped into a file. And I can confirm your findings: The new/current Lesson 11 (dates 2021-12-02 in the zip file) has indeed sol2 = ....

I also noted that the notebook has been converted to presentation notebooks, not the "continuous notebook" as I had earlier. (I tend to prefer the non continuous versions since they are easier to browse.)

Thanks for pointing this out. Now I've updated the notebooks.
POSTED BY: Hakan Kjellerstrand

Thank you, Hakan for confirming my observation.
POSTED BY: Zbigniew Kabala

Hi Zbigniew. Using the Set vs SetDelayed (sol2[x]= vs sol3[x]:=) is just for presentation reasons. Using just = will display the output while using := will not display the output. They are both accomplishing the same task though.

POSTED BY: Luke Titus

I'm looking at the overview of Lesson 15. There is a nice working Manipulate implementation of the oscillating spring and a block attached to it. The code is hidden very well--I can't find it. In fact, the cell is marked as though there is no code associated with it. Where is the code for this Manipulate function? Please advise.

Attachment

Attachments:
Spring-Manipulate.jpeg
POSTED BY: Zbigniew Kabala

Just above the output cell bracket you will see a tiny little cell bracket. Select that, then (on Windows) go to the menu Cell -> Cell Properties then select "Open"
POSTED BY: Luke Titus

Wow! I would have never guessed it. So far I have been closing the code cells by double-clicking their output cell bracket. I've been a Mathematica user for a long time, but I didn't know that there was another option to do the same by selecting the code cell and then going to Cell >> Cell Properties and clicking or unclicking the box next to "Open" (in Linux). Again, I find it very interesting and, I bet, so would many other users. If I may, I suggest you point out this option during the Friday review. Many thanks for this pointer!
POSTED BY: Zbigniew Kabala

I'll comment on that at the beginning of the session today.

POSTED BY: Luke Titus

Sorry for asking, but where should we look for the Quizzes? Is it at the usual "amoeba site" or anywhere else? Thank you.
POSTED BY: Michael Partensky

Unfortunately we have not been able to release the quizzes yet. We hope to release the quizzes on Friday along with the beta version of the entire course (including all the videos and the final exam). We'll clarify further at tomorrow's session.
POSTED BY: Abrita Chakravarty

https://www.bigmarker.com/series/daily-study-group-intro-to-differential-equations/series_details
POSTED BY: Michael Partensky
KRISHAN SHARMA
KRISHAN SHARMA
Posted 4 years ago

I joined the study group today. How to download the previous lecture series ?
POSTED BY: KRISHAN SHARMA

Welcome to the study group. The reminder emails for the study group sessions will contain all the links you need for this study group--including the link to download all the notebooks and the links to the recordings.

Also you can access the previous recordings by clicking on the "Start this section" links for each day at the link posted by Michael above: https://www.bigmarker.com/series/daily-study-group-intro-to-differential-equations/series_details
POSTED BY: Abrita Chakravarty

Thank you. Still, it does not look pretty. It would be good to tune up the action of CoefficientList, so that it considers only non-zero elements. Am I asking too much ? :)

PS: It can be done by using If[], but it would not look neat either.
POSTED BY: Michael Partensky

Exercise 13(1). Just for practice, I tried to do it without "manual labor" while still reproducing all the steps of defining the "undetermined coefficients" . However, the implementation is sloppy. For instance , using "DeleteCases" to remove a bunch of 0s produced on the previous step, seems redundant. Apparently, I did not find the right version of the "CoefficientList[]" Please, advise.

eq =y''[x] + 2 y'[x] + y[x] - x^2 - 3 Sin[x] == 0;

parSol[x_]:= a Sin[x] + b Cos[x] + c x^2 + d x + e;

collConst = Collect[eq[[1]] /. y -> Function[x, parSol[x]], {x, Sin[x], Cos[x]}]

Out[39]= 2 c+2 d+e+(4 c+d) x+(-1+c) x^2+2 a Cos[x]+(-3-2 b) Sin[x]

In[40]:=
Solve[DeleteCases[Flatten[CoefficientList[collConst, {x, Sin[x], Cos[x]}]], 0] == Table[0, {5}], {a, b, c, d, e}]

Out[40]= {{ a->0, b->-(3/2), c->1, d->-4, e->6 }}
POSTED BY: Michael Partensky

I think the way you are doing it is probably the best way simply. Deleting the zeros will be necessary since CoefficientList will consider all combinations of powers of the terms. The combinations of powers that do not appear will be given the value of zero, and those will need to be deleted.

POSTED BY: Luke Titus

Thank you, Luke. I agree that exp (rx) is the decisive factor, but in our case we are not comparing exp (r x) with x, but rather x with 1.
POSTED BY: Michael Partensky

Exercise 12: Repeated characteristic roots.

Slide 5 ("exercise 4"), after the equation

y (x) = c1 exp (r x) + c2 x exp (rx),

it is stated: "Thus, we can conclude that the solution behaves like C exp (r x) at x-> Infinity". I guess this is a typo: it would be wrong in general to neglect the (dominant) contribution ~ x exp (r x) in comparison to ~ exp(r) in the limit of large x.
POSTED BY: Michael Partensky

Hi Michael. Yes, while the exponential term dominates the long term behavior, the factor of x should still be kept. I will fix that in the notebook for that exercise.

POSTED BY: Luke Titus

In reviewing Lesson 7 on Exact Equation, Example 1, Slide 9, I'm confused about a particular step.

After integrating "M" & "N", expressions, we are told that the result of combining them to create the auxiliary function is: [Phi](x,g(x)) = 2 g(x) ln(x) + 3x^2 - 2g(x).

However, when you integrate "M", you get: 3 x^2 + g Log[x]; when you integrate "N", you get: - 2 g + g Log[x].

In the lesson, Luke indicates that, when combining "M" & "N", you consider the term that is common to them, g Log[x], only one time. This would give you the expression above for PHI.

Including it would give an expression with a "2" in front of the g(x) Ln(x) term: [Phi](x,g(x)) = 2 g(x) ln(x) + 3x^2- 2g(x). Using that expression, I got a slightly different answer for g[x] that had a "2" in front of the Log[x] term in the denominator.

My question: What allows us to ignore the second instance of g Log[x] when combining "M" & "N" to create PHI?

POSTED BY: William Matthews

Sorry, the original expression in the lesson is: [Phi](x,g(x)) = g(x)ln(x)+3x^2-2g(x).
POSTED BY: William Matthews

Hi William. Thank you for your question. It is an important point.

What we are doing in that step is identifying the terms terms that compose Phi(x,g). Notice that Phi(x,g) will consist of three terms. The first is a function of x and g, the second is a function of only x, and the third is a function of only g.

To calculate M, we take the partial derivative of Phi with respect to x. Therefore the part of Phi that is a function of only g will drop out because that partial derivative will be zero. Therefore, when we integrate M with respect to x we only get back the function dependent on g and x and the function dependent on x.

Similarly for N, that is calculated by taking the partial derivative of Phi with respect to g. Therefore, the part of Phi that depends only on g drops out when taking that partial derivative, so after integrating we only get back the function dependent on g and x and the function dependent on g.

We used integration to identify the parts of the function Phi. These are: a function dependent on g and x, a function dependent on only x, and a function dependent on only g. To obtain Phi, we do not add the results of the integrals, we just pick out these functions from the integrals.

POSTED BY: Luke Titus

Luke,

Thanks!

That made it a lot clearer!

By the way, great course! Well-developed, well-executed!

William
POSTED BY: William Matthews

Thanks William! I really appreciate it.

POSTED BY: Luke Titus
Doug Beveridge
Doug Beveridge
Posted 4 years ago

I second that , The best WolframU course so far ......
POSTED BY: Doug Beveridge

Exercise 10(4)

y1[x_] := x^(1/2 (1 - Sqrt[5]));
y2[x_] := x^(1/2 (1 + Sqrt[5]));

What are the advantages of using [29]-[30] instead of [38]-[39]?

n[29]:= (x^2 y''[x]-y[x]==0)/.**y->Function[x,y1[x]]**//Simplify

Out[29]=True
In[30]:= (x^2 y''[x]-y[x]==0)/.**y->Function[x,y2[x]]**//Simplify

Out[30]=True

In[38]:= (x^2 y''[x]-y[x]==0)/.**y->y1**//Simplify

Out[38]=True

In[39]:= (x^2 y''[x]-y[x]==0)/.**y->y2**//Simplify

Out[39]=True

And similarly with the exercise 9(3) Thanks
POSTED BY: Michael Partensky

Both ways will get the job done. However, using Function is a little more versatile as it can handled pretty much any form of the function. For example, if you had defined y1[x] instead as y1 = x^(1/2 (1 - Sqrt[5])), then y -> y1 wouldn't work and you would need to use y -> Function[x, Evaluate[y1]].

POSTED BY: Luke Titus

From what you said, I take that using the "Function" is a more secure, bullet-proof approach. It is good to know! Thank you.
POSTED BY: Michael Partensky
Richard Sweney
Richard Sweney
Posted 4 years ago

I am getting an error message when I active DSolveValue for the verifying the solution in Exercise 4 of the second order ODE Exercise 9 set: "DSolveValue::deqn: Equation or list of equations expected in the first argument." The equation is clearly displayed. My example notebook is attached. Please advise how to fix this. Thanks.

Attachments:
Exercise 9.4.nb
POSTED BY: Richard Sweney

The problem is that you have assignments (=) instead of equatity (==) for y[0] and y'[0]. This should work:

DSolveValue[{4*y''[t] + 6*y'[t] + 2*y[t] == 0, y[0] == 5, y'[0] == 1/2}, y[t], t] // Expand

Note, however, that after you have evaluated the wrong DSolveValue expression, then Mathematica has saved the two assignments (y[0] = 5 and y'[0] = 1/2) and will interfere when evaluating the corrected expression.

One way to fix this is to restart the kernel after fixing the expression (e.g. using the Quit[] command).
POSTED BY: Hakan Kjellerstrand
Richard Sweney
Richard Sweney
Posted 4 years ago

My oversight, thank you.
POSTED BY: Richard Sweney

In the Lesson 7,

the first equation 5 e^y(t)+5 t e^y(t) dy/dt =0 is equivalent to dy/dt = - 1/t; which is trivially solved in one line: y(t) = - ln(t) + c What is the purpose of multiplying both sides by "5 e^y(t)" and making the simple solution more complicate? Is it done for purely *didactic * reasons?
POSTED BY: Michael Partensky

Hi Michael. That term is there mainly to demonstrate the concept of an exact equation and the methods to solve them. You could definitely cancel that term and solve the equation much easier, but it wouldn't demonstrate how exact equations can be solved. Since you can cancel that term to get a simpler equation, it is a nice way to check the result obtained when using the methods of exact equations.

POSTED BY: Luke Titus

OK. I suspected that this was a didactic tool. Thanks for confirmation.
POSTED BY: Michael Partensky

From Lesson # 3, Verify a Solution: Why do we need to use Functon & Evaluate:

solution = 90 + C1 Exp[1/3 t];

verify = (p'[t] == 1/3*p[t] - 30) /. p -> Function[t, Evaluate[solution]]

Why do the following two statements FAIL?

1) verify = (p'[t] == 1/3*p[t] - 30) /. p -> Function[t, solution]

OR

2) verify = (p'[t] == 1/3*p[t] - 30) /. p -> solution

OK, the first one fails, perhaps, because of the HoldAll attribute for Function. But why does the second one fail when there is no HoldAll attribute there?
POSTED BY: Zbigniew Kabala

Hello Zbigniew,

The second one fails because Derivative requires a Function input as shown in In[3] below.

In[1]:= solution = 90 + C1 Exp[1/3 t];

In[2]:= (p'[t] == 1/3*p[t] - 30) /. p -> solution

Out[2]= Derivative[1][(90 + C1 E^(t/3))][t] == -30 + 1/3 (90 + C1 E^(t/3))[t]

In[3]:= f'[t] // InputForm

Out[3]//InputForm=
Derivative[1][f][t]
POSTED BY: Devendra Kapadia

Thank you, Devendra,

I didn't realize that all the different ways of expressing derivatives in Mathematica are based on the Derivative[n, m, ...][f][x, y, ...] function. All this makes perfect sense now.

Best, Zbigniew
POSTED BY: Zbigniew Kabala

Is it fair to say that DSolveValue [...] is simply equivalent to DSolve [...] [[1,1,2]]?
POSTED BY: Michael Partensky

Hello Michael,

Yes, that is correct for single ODEs but for a system you could use [[1,All,2]], as shown below.

In[1]:= DSolve[y'[x] == 1, y[x], x][[1, 1, 2]]

Out[1]= x + C[1]

In[2]:= DSolveValue[y'[x] == 1, y[x], x]

Out[2]= x + C[1]

In[3]:= DSolve[{y'[x] == 1, z'[x] == 2}, {y[x], z[x]}, x][[1, All, 2]]

Out[3]= {x + C[1], 2 x + C[2]}

In[4]:= DSolveValue[{y'[x] == 1, z'[x] == 2}, {y[x], z[x]}, x]

Out[4]= {x + C[1], 2 x + C[2]}
POSTED BY: Devendra Kapadia
Updating Name
Updating Name
Posted 4 years ago

Thank you, Devendra. Very nice: With "All", it displays the second element of all the solutions/assignments. M

PS It seems to me that in the early versions there was a spot reserved for the arrow "->" , so that {a -> 3.}[[1,2]] would return "->". Am I mistaken?
POSTED BY: Updating Name

Hello ! For some reason the GeneratedParameters function does not work for me the GeneratedParameters function does not work

Could you help me ? thank you

POSTED BY: Aiken Kosherbayeva

Great! Thank you, Devendra, for opening the hood. Also, I did not realize that "body" is the second argument. Apparently, I had in mind a function of several variables. All the best. M
POSTED BY: Michael Partensky
Rohit Namjoshi
Rohit Namjoshi
Posted 4 years ago

If there is just one argument the { } can be omitted. For multiple arguments

f = Function[{x, y}, x*y];
f[3, 4]
(* 12 *)
POSTED BY: Rohit Namjoshi

Thank you, Devendra. When you said "Function does not evaluate its second argument automatically" was it a general statement, or one related to a particular example? What would the "second argument" mean in general? M
POSTED BY: Michael Partensky
Doug Beveridge
Doug Beveridge
Posted 4 years ago

Yes, I also find it strange that the "Function does not evaluate its second argument automatically". When I first ran into the problem I tried a number of alternatives and the error will occur without using "Function" . I cannot fathom the reason for Wolfram doing this ....

enter image description here
POSTED BY: Doug Beveridge

Hello Michael,

In Function[x, body], the variable 'x' is the first 'argument' and the expression 'body' is the second 'argument'.

The variable 'x' is effectively a 'local' (Module) variable inside Function and hence, as shown below, the function evaluation does not depend upon its name.

IIn[1]:= f = Function[x, x^2];

In[2]:= g = Function[a, a^2];

In[3]:= f[5]

Out[3]= 25

In[4]:= g[5]

Out[4]= 25

In[5]:= f[x]

Out[5]= x^2

In[6]:= g[x]

Out[6]= x^2

Also, technically, Function has the attribute HoldAll:

In[7]:= Attributes[Function]

Out[7]= {HoldAll, Protected}

In practice, this means that the second argument 'body' is not evaluated by Function, so Evaluate is required below (since 'body' is defined outside Function).

In[8]:= body = x^2;

In[9]:= Function[x, body][5] (*incorrect*)

Out[9]= x^2

In[10]:= Function[x, Evaluate[body]][5] (*correct*)

Out[10]= 25

Hope this helps.
POSTED BY: Devendra Kapadia

Could you please remind how to send questions about the quizzes? Thanks. M
POSTED BY: Michael Partensky

Please email wolfram-u@wolfram.com
POSTED BY: Abrita Chakravarty

OK. Thanks. Btw, yesterday I sent there message about the MathOne , but did not hear back yet. Best. M
POSTED BY: Michael Partensky

Thank you. I wonder which is a better way to avoid a naming conflict: localizing p[x] using , say, Module, or by using 

/. {p -> Function[{x}, Evaluate[solution]]}

Could you please provide an alternative example? Thanks. M
POSTED BY: Michael Partensky
Doug Beveridge
Doug Beveridge
Posted 4 years ago

Hi

I am trying to resolve this trivial problem trying to verify a solution . In the first run I get the incorrect answer but in the second run if I substitute the solution directly into the 4th line I get the correct answer . Any suggestions ?

enter image description here
POSTED BY: Doug Beveridge

Function does not evaluate its second argument automatically, so you could use Evaluate as shown below to achieve the desired result in the first case:

In[1]:= sol = DSolveValue[p''[x] + 2 p'[x] - 3 p[x] == 0, p[x], x]

Out[1]= E^(-3 x) C[1] + E^x C[2]

In[2]:= solution = sol /. {C[1] -> 1, C[2] -> 0}

Out[2]= E^(-3 x)

In[3]:= verify = 
 p''[x] + 2 p'[x] - 3 p[x] == 
   0 /. {p -> Function[{x}, Evaluate[solution]]}

Out[3]= True
POSTED BY: Devendra Kapadia

I notice for the form y'(x)=f(x,y(x))

we can use VectorPlot[{1,rhs}...] as direction field, how to derive this? thanks,
POSTED BY: vincent feng

ok, the first list is slope, got it,
POSTED BY: vincent feng

Thanks, Vincent. Could you please explain what's the meaning of "1" in the list. Is it the module (length) of the vector which is, in fact, variable (but this is apparently handled by color coding) or something else? The lecture said that the "1" is D[x,x] , but why are we taking this derivative?

Thank you.
POSTED BY: Michael Partensky
Doug Beveridge
Doug Beveridge
Posted 4 years ago

In the email giving a link to the first recording. a link to the Series Landing Page is given as

Link to the Series Landing Page

https://wolfr.am/YxPcS9oQ

This is the landing page for the previous session ("Introduction to Notebooks")

What is the correct landing page ?
POSTED BY: Doug Beveridge

Here is the link to the Differential Equations series: https://www.bigmarker.com/series/daily-study-group-intro-to-differential-equations/series_details
POSTED BY: Abrita Chakravarty
D. R. Grimes
D. R. Grimes
Posted 4 years ago

Where are the mathematica notebooks for the sessions stored?
POSTED BY: D. R. Grimes

A link to the download folder containing all the lecture notebooks (as well as any additional materials used during the sessions) is included in the reminder email you receive from BigMarker. It will also be shared in the live session everyday of the study group series.

POSTED BY: Abrita Chakravarty
D. R. Grimes
D. R. Grimes
Posted 4 years ago

Thank you very much!
POSTED BY: D. R. Grimes

When does today's session start? Thanks.
POSTED BY: Michael Partensky

Friday sessions for this series will start at 10:30 AM CT with the review session for the week running from 10:30-11:00 and the day's lesson beginning at 11:00 AM CT.
POSTED BY: Abrita Chakravarty
Updating Name
Updating Name
Posted 4 years ago

Lesson 17, slide 1.

"When the coefficients of a linear second-order differential equation become polynomials, the equation appears in the form:

P(x) y''(x)+Q(x)y'(x)+R(x)y(x)=0 (1)"

This statement is correct but also slightly confusing. It may sound as if the form (1) relies on P, Q, and R being polynomial (while each of then can me (almost) any function of x). Im my view, it could be better to say

"When the coefficients P, Q and R in the equation 

      P(x) y''(x) + Q(x)y'(x) + R(x) y(x) =0

      are the polynomial functions of x, the solution can be found by Taylor series expansion of y(x) ....".

Well. it's just about wording, sorry for being a nag.

POSTED BY: Updating Name
Reply to this discussion
Community posts can be styled and formatted using the Markdown syntax.
Tag limit exceeded
Note: Only the first five people you tag will receive an email notification; the other tagged names will appear as links to their profiles.
Reply Preview
Attachments
Remove
or Discard