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In science and especially in mathematical studies, a **variational principle** is one that enables a problem to be solved using calculus of variations, which concerns finding functions that optimize the values of quantities that depend on those functions. For example, the problem of determining the shape of a hanging chain suspended at both ends—a catenary—can be solved using variational calculus, and in this case, the variational principle is the following: The solution is a function that minimizes the gravitational potential energy of the chain.

The history of the variational principle in classical mechanics started with Maupertuis's principle in the 18th century.

Felix Klein's 1872 Erlangen program attempted to identify invariants under a group of transformations.

- Ekeland's variational principle in mathematical optimization
- The finite element method
- The variation principle relating topological entropy and Kolmogorov-Sinai entropy.

- The Rayleigh–Ritz method for solving boundary-value problems in elasticity and wave propagation
- Fermat's principle in geometrical optics
- Hamilton's principle in classical mechanics
- Maupertuis' principle in classical mechanics
- The principle of least action in mechanics, electromagnetic theory, and quantum mechanics
- The variational method in quantum mechanics
- Hellmann–Feynman theorem
- Gauss's principle of least constraint and Hertz's principle of least curvature
- Hilbert's action principle in general relativity, leading to the Einstein field equations.
- Palatini variation
- Hartree–Fock method
- Density functional theory
- Gibbons–Hawking–York boundary term
- Variational quantum eigensolver

- The Feynman Lectures on Physics Vol. II Ch. 19: The Principle of Least Action
- Ekeland, Ivar (1979). "Nonconvex minimization problems".
*Bulletin of the American Mathematical Society*. New Series.**1**(3): 443–474. doi:10.1090/S0273-0979-1979-14595-6. MR 0526967. - S T Epstein 1974 "The Variation Method in Quantum Chemistry". (New York: Academic)
- C Lanczos,
*The Variational Principles of Mechanics*(Dover Publications) - R K Nesbet 2003 "Variational Principles and Methods In Theoretical Physics and Chemistry". (New York: Cambridge U.P.)
- S K Adhikari 1998 "Variational Principles for the Numerical Solution of Scattering Problems". (New York: Wiley)
- C G Gray, G Karl G and V A Novikov 1996,
*Ann. Phys.*251 1. - C.G. Gray, G. Karl, and V. A. Novikov, "Progress in Classical and Quantum Variational Principles". 11 December 2003. physics/0312071 Classical Physics.
- Griffiths, David J. (2004).
*Introduction to Quantum Mechanics (2nd ed.)*. Prentice Hall. ISBN 0-13-805326-X. - John Venables, "The Variational Principle and some applications". Dept of Physics and Astronomy, Arizona State University, Tempe, Arizona (Graduate Course: Quantum Physics)
- Andrew James Williamson, "The Variational Principle -- Quantum monte carlo calculations of electronic excitations". Robinson College, Cambridge, Theory of Condensed Matter Group, Cavendish Laboratory. September 1996. (dissertation of Doctor of Philosophy)
- Kiyohisa Tokunaga, "Variational Principle for Electromagnetic Field". Total Integral for Electromagnetic Canonical Action, Part Two, Relativistic Canonical Theory of Electromagnetics, Chapter VI
- Komkov, Vadim (1986) Variational principles of continuum mechanics with engineering applications. Vol. 1. Critical points theory. Mathematics and its Applications, 24. D. Reidel Publishing Co., Dordrecht.
- Cassel, Kevin W.: Variational Methods with Applications in Science and Engineering, Cambridge University Press, 2013.