<math>a_0 = {{4\pi\epsilon_0\hbar^2}\over{m_e e^2}}</math>
where:
It has a value of 5.29189379×10-11 m, or about half an angstrom.
... is real. (Geometrically, this means that the parabola described by the quadratic equation touches the x-axis in a single point.) If the discriminant is positive, ...