P. 571, line 21

Exercise 11.4, number 4 should be:

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

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

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