I'm trying to derive the relation:

$\phi(x)\phi(y)=:\phi(x)\phi(y):+\langle 0|\phi(x)\phi(y)|0 \rangle$

but struggling to see the first few steps I need to make. I've made the substitutions

$\phi(x)=\phi^+(x)+\phi^-(x)$

and the same for $\phi(y)$, but it hasn't got me very far.

I'm then unsure what the it means when it says to compute

$\langle 0|\phi(x)\phi(y)|\bf{k_1},\bf{k_2} \rangle$.

Any help would be great!

This post imported from StackExchange Physics at 2014-05-04 11:10 (UCT), posted by SE-user user13223423