is less than some fixed value \(E_r\) corresponds to the set of points contained in the ellipse \(E=E_r\) centered at the origin of the \(x,p\) plane, and the indicator is the function that is 1 on the points inside (and on the boundary) of the ellipse and 0 outside. Its negation, the property that the energy is greater than \(E_r\), corresponds to all the points lying outside this ellipse.