Whatever LaTeX does by default
LaTeX: typically let software decide for me, override if it looks bad.
Paper: Too shit at writing to make a consistent choice
B. A only when there is little space
Are those called limits in English? How do you call those things then?
lim x->0 1/x
For integrals, we would say that “b and a are the limits of integration”.
The notation “lim x->0 1/x” would be read as “the limit of 1 over x as x goes to zero.” In general, “lim” is short for “limit” of whatever follows it, with respect to what is below the “lim” symbol. Rarely, I have also seen the notation “l.i.m.” used for the limit in mean, i.e. the limit with respect to the L^2 norm.
Better question: Where do you put the dx?
Immediately after the integral symbol, before the integrand, is also common: https://math.stackexchange.com/questions/1146345/notational-position-of-dx-in-integral
It has a nice “operator” look this way.
