i am trying to follow the following tutorial. I keep seeing $\delta$ over functions such as $\delta F(x)=F(x)-\langle F(x)\rangle_t$ (Eq 14.4) in this and in other tutorials and questions here. What does the $\delta$ represent here?
Also, is it correct to say that the autocorrelation of such a function $\delta F(x,t)$ Is given by $\langle \delta F(x,t),\delta F(x,t+\tau)\rangle$? If so, how do I develop this? Is that the covariance somehow?
Googled it and could not find anything.