Hi, I'm trying to understand this. The given equation is y_tt = 4 y_xx 0 < x < pi, t>0 where y_tt is the 2nd derivative with respect to t, y_xx is 2nd wrt x Boundary conditions y(0,t) = 0 and y(pi,t) = 0 And initial conditions y_t (x,0) = 0 = g(x) y(x,0) = sin^2 x = f(x) --- My teacher wrote that F(x) is the odd periodic extension of f(x), and then wrote F(x) = sign(sinx)sin^2 x 1) I assume this is to make it odd but why wouldn't he just write sign(x)sin^2 x? 2) Also, there was a similar question in class but -infinity < x < infinity and no boundary conditions given with one of the initial conditions y(x,0) = 1/(1+x^2) = f(x). In that case, since f(x) is even why isn't it necessary to use the sign function? Thanks for your help!