There are several: examine, study, assay, investigate.
To overlook or observe closely
A word meaning to work closely with someone is collaborate.
consider, speculate, observe, study
Scrutinise.
compacted
Quite nearly
watch, observe, see
surveillance
Metonymy is the process of replacing one word with another, closely related word. For example, in the sentence "The orders came from the White House", it can be assumed that the orders came from the President because of the close relationship between the two words.
Observe
followed closely /under observation
If you closely notice you will observe that their products are always equal to the wave speed.