# Normal ordering

1. ### Catria

118
I know that, given scalar $A_{i}$ fields, the normal order is defined as $:A_{1}...A_{n}: = \sum \prod A^{-}_{i}\prod A^{+}_{j}$ with the $A^{-}_{i}$ being the negative frequency parts, containing creation operators, and the $A^{+}_{j}$ being the positive frequency parts, containing annihilation operators.

But how does one go on to calculate $:A_{1}...A_{n}:A_{n+1}$ if $A_{n+1}$ is also a scalar field with such components?