# Yang–Mills equations

In physics and mathematics, and especially differential geometry and gauge theory, the **Yang–Mills equations** are a system of partial differential equations for a connection on a vector bundle or principal bundle. They arise in physics as the Euler–Lagrange equations of the **Yang–Mills action functional**. They have also found significant use in mathematics.

Solutions of the equations are called **Yang–Mills connections** or **instantons**. The moduli space of instantons was used by Simon Donaldson to prove Donaldson's theorem.

The principle of least action dictates that the correct equations of motion for this physical theory should be given by the Euler–Lagrange equations of this functional, which are the Yang–Mills equations derived below:

In addition to the physical origins of the theory, the Yang–Mills equations are of important geometric interest. There is in general no natural choice of connection on a vector bundle or principal bundle. In the special case where this bundle is the tangent bundle to a Riemannian manifold, there is such a natural choice, the Levi-Civita connection, but in general there is an infinite-dimensional space of possible choices. A Yang–Mills connection gives some kind of natural choice of a connection for a general fibre bundle, as we now describe.

Assuming the above set up, the Yang–Mills equations are a system of (in general non-linear) partial differential equations given by

In this sense the search for Yang–Mills connections can be compared to Hodge theory, which seeks a harmonic representative in the de Rham cohomology class of a differential form. The analogy being that a Yang–Mills connection is like a harmonic representative in the set of all possible connections on a principal bundle.

The Yang–Mills equations are the Euler–Lagrange equations of the **Yang–Mills functional**, defined by

The moduli space of ASD instantons may be used to define further invariants of four-manifolds. Donaldson defined rational numbers associated to a four-manifold arising from pairings of cohomology classes on the moduli space.^{[9]} This work has subsequently been surpassed by Seiberg–Witten invariants.