P. 571, line 21

Exercise 11.4, number 4 should be:

$\alpha =2,L=20$, and

$u(x,0)=\{\begin{array}{lc}0,& 0<x\le 3\\ \sin 2\pi x,& 3<x\le \frac{7}{2}\\ 0,& \frac{7}{2}<x\le 20.\end{array}$

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