Feynman slash notation

In the study of Dirac fields in quantum field theory, Richard Feynman invented the convenient Feynman slash notation (less commonly known as the Dirac slash notation). If A is a covariant vector (i.e., a 1-form),

using the Einstein summation notation where γ are the gamma matrices.

Using the anticommutators of the gamma matrices, one can show that for any ${\displaystyle a_{\mu }}$ and ${\displaystyle b_{\mu }}$,

where ${\displaystyle I_{4}}$ is the identity matrix in four dimensions.

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