Schwinger's quantum action principle is a variational approach to quantum field theory introduced by Julian Schwinger.

FABIA utilizes well understood model selection techniques like variational approaches and applies the Bayesian framework.

The naive variational approach does not take advantage of the connected nature of the cluster amplitudes and results in a non-terminating set of equations.

In order to study these strongly correlated systems, variational approaches are a better alternative.

Here, we will use a variational approach.

In practice, approximate techniques, such as variational approaches, could be used.

Particularly variational approaches, such as those relying on Duvaut and Lion's existence and uniqueness theories.

A drawback of the well-founded variational approaches is their large computation times.

The way that Lanczos introduced the tensor originally was as a Lagrange multiplier on constraint terms studied in the variational approach to general relativity.

This short account of the Hartree-Fock method explains why it is called also the variational approach.