If a system's position is defined by one independent variable, it has one degree of freedom. Its potential energy function can then be plotted against that variable. When the system reaches equilibrium, the slope of the function at that position is zero. The stability of the equilibrium configuration can be investigated using the second derivative of the potential energy function with respect to the variable. If the second derivative is positive, the potential energy is minimum, and the system is in a stable equilibrium configuration. Conversely, if the second derivative is negative, the potential energy is maximum. This implies an unstable equilibrium configuration. If the second derivative is zero, higher-order derivatives must be evaluated. If the first non-zero derivative is positive and has an even order, the equilibrium configuration is stable. Alternatively, if the first non-zero derivative is negative and has odd or even order, it is unstable. If all the higher-order derivatives are zero, the system has achieved neutral equilibrium. Here, the potential-energy function remains constant around the equilibrium position.