21-260: Online assignment #6

The Online Assignment for Week #6 is due at 11:00pm tonight. When I set up the assignment, I neglected to set the time, so it is listed as 9:00am. When I went back into the system to change that today, I got the warning that "gradebook information might be lost." I decided not to risk it! Go ahead and complete the assignment by 11:00pm as usual. It will be marked as late, but I'll sort it out later.

Thursday Office Hour Changes

I will be changing my office hours for some upcoming Thursdays to accommodate the Graduate Student Seminar this week and my trip to the Circus in two weeks. The hours will be:

Oct. 1: 6:35pm - 7:35pm
Oct. 15: 2:00pm - 3:00pm

All other Thursdays will remain unchanged.

21-260: Week #6

THe Reading and Homework assignments for Week #6 have been posted. You can follow the link from the Schedule page.

Presentation Points

Each homework problem is graded on a scale of 7 points for correctness and 3 points for presentation. The purpose of grading on presentation is to encourage students to write solutions to problems in a way that another person who had not done the problem themselves could understand the solution. This is an extremely important skill, not just for mathematicians but in all walks of life where mathematical conclusions need to be explained to supervisors, clients, inspectors, and such.

But it can be pretty subjective if we just say it's a score for "presentation". So I'll explain what my grading philosophy on this is.

All differential-equations-related steps are to be identified in words. Examples of this would be attempting to separate variables, multiplying the equation by an integrating factor, or plugging in the initial conditions to solve for a constant of integration. You will learn other techniques during this course that will have to be identified similarly.

Some calculus steps should also be identified in words, such as u-substitution or trig-substitution to solve an integral, or differentiating to find a maximum or minimum point. You don't have to explain that the derivative of sine is cosine, or that you can pull a constant out of an integral.

Very few algebra or arithmetic steps need to be identified. Finding the roots of a polynomial or doing a partial fraction decomposition are the only ones I can think of off the top of my head.

So this is how points will be awarded:

3: All the above steps are identified using words.
2: A significant proportion of the steps that should be identified using words are not, but it is immediately clear to me what the student is doing.
1: A significant proportion of the steps that should be identified using words are not, and it takes some effort on my part to comprehend what the student is doing, due to minor steps being omitted, extremely unclear or incorrect notation, or the work being scattered around the page rather than flowing from top to bottom and right to left.
0: Vital steps are completely missing from the solution, or the problem is not attempted at all. Note that this may also affect the correctness score. I also reserve the right to give 0 points in special cases at my discretion, for instance if a problem requiring significant mathematical work is answered with an incorrect solution that requires little or no work. I don't want to give out zeros, trust me.

If a problem has multiple parts, with different presentation quality among the parts, the score will be a rough average of the presentation score for each part.

Following the above simple presentation rules gives all students the opportunity to get points that may make up for minor errors in the solutions themselves. Please take advantage of that opportunity.

21-260: Exam#1 Wrapup

Well, Exam #1 has been graded and (mostly) returned to you. The grade cutoffs for the exam are 75 (for an A), 65 (B), 55 (C) and 45 (D). To compute grade cutoffs for your mid-semester and final grades, I'll average the cutoffs for each assignment. That way, for instance, someone who scores just above the A cutoff on each assignment will have a final average that is just above the A cutoff.

I've taken the time to tabulate some statistics for the course, including the current grade cutoffs for cumulative averages.

Section E recitation cancelled, Thursday Office Hours

Just a reminder: since the recitation for section E at 12:30 has been canceled due to G-20 preparations, I will be holding office hours from 12:30-1:20 today, so you can pick up your exams there.

semi-stable points

Before I mentioned that in order to determine if a point is stable, unstable, or semistable you can take the derivative of f(y) and plug the critical points into f '(y). Well, if for some "y" you get that f '(y) = 0, this does not mean that the critical point is semistable, it simply means that you can't use this derivative test, and you have to check the intervals around that critical point. However, if f '(y) > 0, then the point is unstable and if f '(y) < 0, then the point is stable.

Office Hours

Ravi will be holding extra office hours this Sunday from 12 Noon - 1 PM in Wean 6215. You will be able to pick up the homework (Section B), and then I'll probably leave it with the other TA's.

Strange Office Hours, or, How I Learned To Stop Worrying And Love Differential Equations

In view of the upcoming exam on Monday, I will be holding bonus office hours (and missing the Vikings-Lions game) this coming Sunday from 1:00pm to 3:00pm.

21-260: Euler's Method in Dfield

This week's written assignment asks you to use dfield to plot numerical solutions generated using Euler's method. Euler's method is not the default method in dfield, but it can easily be changed.

First, plot the direction field for the equation you want to study. Then in the DFIELD Direction Field Window go to the Options menu. Choose Options > ODE Solver > Euler. This brings up the Dfield Solver Settings Window with a text box to enter the step size. Enter your desired step size and click "Apply". (If you don't click Apply, there will be no change to the step size. This is, for some reason, annoyingly frustrating. But maybe that is just me.)

In order to gain more precision in plotting solutions to initial value problems, you may wish to enter them from the keyboard, rather than clicking in the window. To do this, select Solution > Keyboard Input of Initial Value in the DFIELD Direction Field Window. This brings up the DFIELD Keyboard Input window, which performs just as you would expect.

21-260: Exam #1

The Review page for Exam #1 has been posted. You can follow the link from the Schedule Page.

21-260: Week #4

The reading and homework assignmetns for Week #3 have been posted. You can follow the link from the Schedule page.

21-260: Week #3

The reading and homework assignmetns for Week #3 have been posted. You can follow the link from the Schedule page.

Office Hours

My Office Hours will be on Tuesdays from 10:30-11:30 in Wean 6215.