P. 571, line 21

Exercise 11.4, number 4 should be: $\alpha =2,L=20$, and

<math>u(x,0) = \left\{\begin{array}{lc} 0, & 0<x\leq 3\\ \sin 2 \pi x, & 3 < x \leq \frac{7}{2} \\ 0, & \frac{7}{2} < x \leq 20. \end{array} \right.