That is a complex function of two variables x and y, so it should be F(x,y) not F(x). |sin(x)| is not differentiable at np However since cos(-y) = cos(y), it is differentiable everywhere. So F(x,y) = |sin(x)| + i*cos|y| is differentiable everywehere except where x = np, where n is an integer. Edwin