Reading different topics of physics I ended with the fascinating topic of functionals, functional derivatives (In the context of Euler-Lagrange equations) and functional integral (From Feynman's path integrals) so I want to know If is there a branch that relates both, maybe in an analogous way that fundamental calculus does in dealing with functions.
I read some lectures of variational calculus, but they are more centred on aplication and optimization problems. What I think it's fine since that is what variational calculus cares.
If this exist, also I'm looking for some introductory (or not high difficulty level) resources for understanding and gain some rigourosity in treating this topics, what I think it's important (Preferently online, like some pdf).
Also resources of any of this topics separately is welcome. Any recomendations will be apreciated. Thanks in advance.