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Fourier series of square wave: f(x)=1 for 0<x<pi, -1 for -pi<x<0. f is odd so all an=0. bn = (2/pi) integral 0 to pi of sin(nx)dx = (2/npi)(1-cos(n pi)). For odd n: bn=4/(n pi); even n: 0. So f(x) = (4/pi)(sin x + sin3x/3 + sin5x/5 + ...).