## Linear Algebra and Its Applications, Exercise 1.7.4

Exercise 1.7.4. For the differential equation with $f(x) = 4\pi^2 \sin 2\pi x$ the corresponding difference equation is

$\begin{bmatrix} 2&-1&0 \\ -1&2&-1 \\ 0&-1&2 \end{bmatrix} \begin{bmatrix} u_1 \\ u_2 \\ u_3 \end{bmatrix} = \frac{\pi^2}{4} \begin{bmatrix} 1 \\ 0 \\ -1 \end{bmatrix}$

for $h = \frac{1}{4}$. Solve the above equation for $u_1, u_2, u_3$ and compare their values to the true solution $u = \sin 4\pi x$ at $x = \frac{1}{4}$$x = \frac{1}{2}$ and $x = \frac{3}{4}$.

Answer: We do Gaussian elimination on the difference matrix and the right-hand side:

$\begin{bmatrix} 2&-1&0 \\ -1&2&-1 \\ 0&-1&2 \end{bmatrix} \quad \frac{\pi^2}{4} \begin{bmatrix} 1 \\ 0 \\ -1 \end{bmatrix} \rightarrow \begin{bmatrix} 2&-1&0 \\ 0&\frac{3}{2} &-1 \\ 0&-1&2 \end{bmatrix} \quad \frac{\pi^2}{4} \begin{bmatrix} 1 \\ \frac{1}{2} \\ -1 \end{bmatrix}$

$\rightarrow \begin{bmatrix} 2&-1&0 \\ 0&\frac{3}{2} &-1 \\ 0&0&\frac{4}{3} \end{bmatrix} \quad \frac{\pi^2}{4} \begin{bmatrix} 1 \\ \frac{1}{2} \\ -\frac{2}{3} \end{bmatrix}$

Solving for $u_3$ we have

$\frac{4}{3}u_3 = \frac{\pi^2}{4}(-\frac{2}{3}) \rightarrow u_3 = \frac{3}{4} \frac{\pi^2}{4} (-\frac{2}{3}) = -\frac{\pi^2}{8} \approx -1.23$

Solving for $u_2$ we have

$\frac{3}{2}u_2 - u_3 = \frac{\pi^2}{4} \frac{1}{2} \rightarrow \frac{3}{2}u_2 = \frac{\pi^2}{8} - \frac{\pi^2}{8} = 0 \rightarrow u_2 = 0$

Finally we solve for $u_1$:

$2u_1 - u_2 = \frac{\pi^2}{4} \rightarrow 2u_1 = \frac{\pi^2}{4} + 0 \rightarrow u_1 = \frac{\pi^2}{8} \approx 1.23$

For the exact solution $u(x) = \sin 2\pi x$ we have

$u(\frac{1}{4}) = \sin 2 \pi \frac{1}{4} = \sin \frac{\pi}{2} = 1$

$u(\frac{1}{2}) = \sin 2 \pi \frac{1}{2} = \sin \pi = 0$

$u(\frac{3}{4}) = \sin 2 \pi \frac{3}{4} = \sin \frac{3\pi}{2} = 1$

NOTE: This continues a series of posts containing worked out exercises from the (out of print) book Linear Algebra and Its Applications, Third Edition by Gilbert Strang.

If you find these posts useful I encourage you to also check out the more current Linear Algebra and Its Applications, Fourth Edition, Dr Strang’s introductory textbook Introduction to Linear Algebra, Fourth Edition and the accompanying free online course, and Dr Strang’s other books.

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