What is meant when an author gives an expression, labeled say $(3.4)$ and then immediately below writes: (cf. eq $3.2$)? i.e. what does the cf. stand for?
Not sure this is completely necessary but for instance I am currently looking at the text Elementary Fluid Dynamics by Acheson and a clear example of this can be found on pg $103$ where the author gives equation $(3.134)$ and then immediately below writes (cf. eqn $(3.117)$).