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The Schwinger–Dyson equations (SDEs), also known as the Dyson–Schwinger equations, named after Julian Schwinger and Freeman Dyson, are general relations between Green functions in quantum field theories (QFTs). They are also referred to as the Euler–Lagrange equations of quantum field theories, since they are the equations of motion of the corresponding Green's function.
They form a set of infinitely many functional differential equations, all coupled to each other, sometimes referred to as the infinite tower of SDEs.
In his paper "The S-Matrix in Quantum electrodynamics", Dyson derived relations between different S-matrix elements, or more specific "one-particle Green's functions", in quantum electrodynamics, by summing up infinitely many Feynman diagrams, thus working in a perturbative approach. Starting from his variational principle, Schwinger derived a set of equations for Green's functions non-perturbatively, which generalize Dyson's equations to the Schwinger–Dyson equations for the Green functions of quantum field theories.
Today they provide a non-perturbative approach to quantum field theories and applications can be found in many fields of theoretical physics, such as solid-state physics and elementary particle physics.
Equivalently, in the density state formulation, for any (valid) density state ρ, we have
To make the connection to diagramatical techniques (like Feynman diagrams) clearer, it's often convenient to split the action S as S[φ]=1/2 D−1ij φi φj+Sint[φ] where the first term is the quadratic part and D−1 is an invertible symmetric (antisymmetric for fermions) covariant tensor of rank two in the deWitt notation whose inverse, D is called the bare propagator and Sint is the "interaction action". Then, we can rewrite the SD equations as
If F is a functional of φ, then for an operator K, F[K] is defined to be the operator which substitutes K for φ. For example, if
and G is a functional of J, then
then, from the properties of the functional integrals
the Schwinger–Dyson equation for the generating functional is
If we expand this equation as a Taylor series about J = 0, we get the entire set of Schwinger–Dyson equations.
An example: φ4 
To give an example, suppose
for a real field φ.
The Schwinger–Dyson equation for this particular example is:
Note that since
is not well-defined because
is a distribution in
- x1, x2 and x3,
this equation needs to be regularized!
In this example, the bare propagator, D is the Green's function for and so, the SD set of equation goes as
(unless there is spontaneous symmetry breaking, the odd correlation functions vanish)
- F. Dyson (1949). "The S Matrix in Quantum Electrodynamics". Phys. Rev. 75: 1736. Bibcode:1949PhRv...75.1736D. doi:10.1103/PhysRev.75.1736.
- J. Schwinger (1951). "On Green's functions of quantized fields I + II". PNAS 37: 452–459. Bibcode:1951PNAS...37..452S. doi:10.1073/pnas.37.7.452.
Further reading 
There are not many book that treat the Schwinger–Dyson equations. Here are three standard references:
- Claude Itzykson, Jean-Bernard Zuber (1980). Quantum Field Theory. McGraw-Hill.
- R.J. Rivers (1990). Path Integral Methods in Quantum Field Theories. Cambridge University Press.
- V.P. Nair (2005). Quantum Field Theory A Modern Persepctive. Springer.
There are some review article about applications of the Schwinger–Dyson equations with applications to special field of physics. For applications to Quantum Chromodynamics there are
- R. Alkofer and L. v.Smekal (2001). "On the infrared behaviour of QCD Green's functions". Phys. Rept. 353: 281. arXiv:hep-ph/0007355. Bibcode:2001PhR...353..281A. doi:10.1016/S0370-1573(01)00010-2.
- C.D. Roberts and A.G. Williams (1994). "Dyson-Schwinger equations and their applications to hadron physics". Prog. Part. Nucl. Phys. 33: 477. arXiv:hep-ph/9403224. Bibcode:1994PrPNP..33..477R. doi:10.1016/0146-6410(94)90049-3.