I am trying to learn Black-Scholes risk-neutral densities with only prior knowledge of fundamental B-S equations (not the derivation). Sorry if this was asked already or if I sound completely clueless.
Basically I do not understand what lowercase x stands for in the integral formula below. It is then subsequently used for the lognormal density. X stands for exercise price of a European call option as usual.
"Let denote the risk-neutral density of . Then
If you have any recommendations on sources to read about this more thoroughly, feel free to link them, thanks.