1/N expansion

How a three gluon vertex would appear in 't Hooft's double index notation. This makes the analogy to a string theory that will appear at large N apparent.
Examples
1	2

In quantum field theory and statistical mechanics, the 1/N expansion (also known as the "large N" expansion) is a particular perturbative analysis of quantum field theories with an internal symmetry group such as SO(N) or SU(N). It consists in deriving an expansion for the properties of the theory in powers of ${\displaystyle 1/N}$, which is treated as a small parameter.

This technique is used in QCD (even though ${\displaystyle N}$ is only 3 there) with the gauge group SU(3). Another application in particle physics is to the study of AdS/CFT dualities.

It is also extensively used in condensed matter physics where it can be used to provide a rigorous basis for mean-field theory.