Recently, I was reading a paper and stumbled upon something for which I don't find the formal definition unfortunately in any textbook that have in my hands. It's the notion of the endomorphism algebra of a functor $\mathcal{F}$, of course I can imagine that there are some restrictions for that functor but I am completely unaware! Can you help me?
So can you give me the formal definition of the algebra $End({\mathcal{F}})$?