College Math Teaching

November 22, 2014

One upside to a topologist teaching numerical analysis…

Yes, I was glad when we hired people with applied mathematics expertise; though I am enjoying teaching numerical analysis, it is killing me. My training is in pure mathematics (in particular, topology) and so class preparation is very intense for me.

But I so love being able to show the students the very real benefits that come from the theory.

Here is but one example: right now, I am talking about numerical solutions to “stiff” differential equations; basically, a differential equation is “stiff” if the magnitude of the differential equation is several orders of magnitude larger than the magnitude of the solution.

A typical example is the differential equation y' = -\lambda y , y(0) = 1 for \lambda > 0 . Example: y' = -20y, y(0) = 1 . Note that the solution y(t) = e^{-20t} decays very quickly to zero though the differential equation is 20 times larger.

One uses such an equation to test a method to see if it works well for stiff differential equations. One such method is the Euler method: w_{i+1} = w_{i} + h f(t_i, w_i) which becomes w_{i+1} = w_i -20h \lambda w_i. There is a way of assigning a method to a polynomial; in this case the polynomial is p(\mu) = \mu - (1+h\lambda) and if the roots of this polynomial have modulus less than 1, then the method will converge. Well here, the root is (1+h\lambda) and calculating: -1 > 1+ h \lambda > 1 which implies that -2 >   h \lambda > 0 . This is a good reference.

So for \lambda = 20 we find that h has to be less than \frac{1}{10} . And so I ran Euler’s method for the initial problem on [0,1] and showed that the solution diverged wildly for using 9 intervals, oscillated back and forth (with equal magnitudes) for using 10 intervals, and slowly converged for using 11 intervals. It is just plain fun to see the theory in action.


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