for different finite values of
. This series of fractions has no maximum, and it is clear that the segment which it defines (in the whole series of fractions in order of magnitude) is the class of all proper fractions. Or, again, consider the prime numbers, considered as a selection from the cardinals (finite and infinite) in order of magnitude. In this case the segment defined consists of all finite integers.
Assuming that
is serial, the "boundary" of a class
will be the term
(if it exists) whose predecessors are the segment defined by