where formula_17 is Euler's number. Solutions of the second equation depend on the potential energy function, formula_7, and so cannot be solved until this function is given. It can be shown using quantum mechanics that the number formula_15 is actually the energy of the system, so these separable wave functions describe systems of constant energy. Since energy is constant in many important physical systems (for example: an electron in an atom), the second equation of the set of separated differential equations presented above is often used. This equation is known as the Time independent Schrödinger Equation, as it does not involve formula_4.