Why are some functions denoted by capital letters? For example, an ODE is represented by the equation $F(t, y, y',\ldots, y^n) = 0$. Here, what I understand is that F is actually a multivariate function. Similarly, for $dy/dx=f(x,y)$ if we consider $f(x,y)=-M(x,y)/N(x,y)$ we get separable equation. Why represent $M$ and $N$ with capital letters? Are they different from the functions represented by small letters?