Much insight in quantum mechanics can be gained from understanding the closed-form solutions to the time-dependent non-relativistic Schrödinger equation. It takes the form
H ^ ψ ( r , t ) = [ − ℏ 2 2 m ∇ 2 + V ( r ) ] ψ ( r , t ) = i ℏ ∂ ψ ( r , t ) ∂ t , {\displaystyle {\hat {H}}\psi {\left(\mathbf {r} ,t\right)}=\left[-{\frac {\hbar ^{2}}{2m}}\nabla ^{2}+V{\left(\mathbf {r} \right)}\right]\psi {\left(\mathbf {r} ,t\right)}=i\hbar {\frac {\partial \psi {\left(\mathbf {r} ,t\right)}}{\partial t}},}
where ψ {\displaystyle \psi } is the wave function of the system, H ^ {\displaystyle {\hat {H}}} is the Hamiltonian operator, and t {\displaystyle t} is time. Stationary states of this equation are found by solving the time-independent Schrödinger equation,
[ − ℏ 2 2 m ∇ 2 + V ( r ) ] ψ ( r ) = E ψ ( r ) , {\displaystyle \left[-{\frac {\hbar ^{2}}{2m}}\nabla ^{2}+V{\left(\mathbf {r} \right)}\right]\psi {\left(\mathbf {r} \right)}=E\psi {\left(\mathbf {r} \right)},}
which is an eigenvalue equation. Very often, only numerical solutions to the Schrödinger equation can be found for a given physical system and its associated potential energy. However, there exists a subset of physical systems for which the form of the eigenfunctions and their associated energies, or eigenvalues, can be found. These quantum-mechanical systems with analytical solutions are listed below.