Let and
be two classes of positive
integers. Let
be the number of integers in
which are less than or equal to
, and let
be the number of integers in
which are less than or equal to
. Then if
and
are said to be equinumerous.
The four classes of primes ,
,
,
are equinumerous. Similarly, since
and
are both of the form
, and
and
are both of the form
,
and
are also equinumerous.