Looking at the shape of the graph, there is an asymptote about the X and Y axes. An asymptote is an imaginary line where the graph will never touch. This already implies that n must be a negative number. But not all negative numbers will give this shape. Assume n=-2, how will the graph look like? Since even power will never produce a negative number, the graph will always be above x-axis (shown by red graph). Thus, n can only be negative odd number like -1, -3...... (shown by blue graph).